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Large, highly organized radio structures near the galactic centre
Abstract
A radio map of the galactic centre made at a wavelength of 20 cm with the Very Large Array telescope reveals that most of the radio emission arising within 50 pc of the galactic centre is not associated with ionizing radiation from recently formed stars. Rather, the large-scale geometry and the measured polarization of the radio emission strongly suggest that the nonthermally emitting gas is arranged along magnetic structures indicative of a substantial poloidal component to the magnetic field in the central region of the Galaxy.
... Figure 1a is the integrated intensity map of the SgrAMC in the C 32 S J = 1-0 emission line with the velocity range of V LSR = −30 to 0 km s −1 . The contours in the figure show the Galactic Center Arc (GCA) in the 20 cm continuum emission for comparison (Yusef-Zadeh et al. 1984). The molecular cloud is identified as two curved ridges along the GCA (e.g., Serabyn & Güsten 1987). ...
... A red arrow indicates a candidate of the CCC site; M0.014−0.054. The contours show the 1.44 GHz (20 cm) continuum emission for comparison (Yusef-Zadeh et al. 1984). The contour levels are (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, and 50) × 95 K in T B . ...
... The LTE molecular cloud masses of objects A and B are estimated from the C 32 S and C 34 S emission line (Yusef-Zadeh et al. 1984). The contour levels are (1,2,3,4,5,6,7,8,9,10,15,20,30,40, and 50) × 24 K. (Color online) Downloaded from https://academic.oup.com/pasj/advance-article/doi/10.1093/pasj/psaa095/5934917 by guest on 13 November 2020 observations using the procedure shown in subsection 4.4. ...
We performed a search of cloud–cloud collision (CCC) sites in the Sagittarius A molecular cloud (SgrAMC) based on the survey observations using the Nobeyama 45 m telescope in the C32S J = 1–0 and SiO v = 0 J = 2–1 emission lines. We found candidates abundant in shocked molecular gas in the Galactic Center Arc (GCA). One of them, M0.014−0.054, is located in the mapping area of our previous ALMA mosaic observation. We explored the structure and kinematics of M0.014−0.054 in the C32S J = 2–1, C34S J = 2–1, SiO v = 0 J = 2–1, H13CO+J = 1–0, and SO N, J = 2, 2–1, 1 emission lines and fainter emission lines. M0.014−0.054 is likely formed by the CCC between the vertical molecular filaments (the “vertical part,” or VP) of the GCA, and other molecular filaments along Galactic longitude. The bridging features between these colliding filaments on the PV diagram are found, which are the characteristics expected in CCC sites. We also found continuum compact objects in M0.014−0.054, which have no counterpart in the H42α recombination line. They are detected in the SO emission line, and would be “hot molecular cores” (HMCs). Because the local thermodynamic equilibrium mass of one HMC is larger than the virial mass, it is bound gravitationally. This is also detected in the CCS emission line. The embedded star would be too young to ionize the surrounding molecular cloud. The VP is traced by a poloidal magnetic field. Because the strength of the magnetic field is estimated to be ∼mgauss using the Chandrasekhar–Fermi method, the VP is supported against fragmentation. The star formation in the HMC of M0.014−0.054 is likely induced by the CCC between the stable filaments, which may be a common mechanism in the SgrAMC.
... As the resolution and sensitivity of observations improve, we are increasingly finding that the interstellar medium (ISM) of our Galaxy is filled with filamentary (i.e., long and narrow) structures from a variety of different origins. These filaments are observed in emission across the electromagnetic spectrum including with X-rays (e.g., De Vries & Romani 2022), ultraviolet (UV; e.g., Bracco et al. 2020), optical (e.g., Fesen et al. 2021), infrared (e.g., Green et al. 2022), and radio (e.g., Yusef-Zadeh et al. 1984;Heywood et al. 2022), and there are also long and narrow structures detected through non-emitting tracers such as Faraday rotation (e.g., Campbell et al. 2022) and scintillation of background sources (e.g., Wang et al. 2021). The filaments are associated with many different environments and origins including star-forming regions, supernova remnants (SNRs), bow shock nebulae, and also filaments of uncertain origins (e.g., Jelić et al. 2015;Bracco et al. 2020), and the radiation can come from both thermal and nonthermal (i.e., synchrotron) emission mechanisms. ...
... The timeline depends strongly on the magnetic tension and therefore the magnetic field strength. In the Galactic center, there are many straight and compressed nonthermal filaments (Yusef-Zadeh et al. 1984;Heywood et al. 2022) that are thought to possibly be relics of old SNRs (Sofue 2020). There is also increasing evidence that the neutral hydrogen (H I) component of the Galaxy contains filamentary structures, or "fibres," that align with the Galactic magnetic field (Clark et al. 2014). ...
Using data from the Galactic Arecibo L -band Feed Array Continuum Transit Survey, we report the discovery of two previously unidentified, very compressed, thin, and straight polarized filaments approximately centered at Galactic coordinates, ( l , b ) = (182.°5, − 4.°0), which we call G182.5–4.0. Using data from the Isaac Newton Telescope Galactic Plane Survey, we also find straight, long, and extremely thin H α filaments coincident with the radio emission. These filaments are positioned in projection at the edge of the Orion-Eridanus superbubble and we find evidence indicating that the filaments align with the coherent magnetic field of the outer Galaxy. We find a lower limit on the total radio flux at 1.4 GHz to be 0.7 ± 0.3 Jy with an average linearly polarized fraction of 40 − 20 + 30 % . We consider various scenarios that could explain the origin of these filaments, including a shell-type supernova remnant (SNR), a bow shock nebula associated with a pulsar, or relic fragments from one or more supernova explosions in the adjacent superbubble, with a hybrid scenario being most likely. This may represent an example of a new class of objects that is neither an SNR nor a bow shock. The highly compressed nature of these filaments and their alignment with the Galactic plane suggests a scenario where this object formed in a magnetic field that was compressed by the expanding Orion-Eridanus superbubble, suggesting that the object is related to this superbubble and implying a distance of ∼400 pc.
... Radio observations are not susceptible to dust obscuration, and are one of the best methods for studying the nonthermal synchrotron processes that arise due to the interaction between relativistic electrons and magnetic fields, as well as the thermal radio emission originating from ionized gas surrounding regions of star formation. One of the most notable discoveries that arose from radio observations of the GC was that of nonthermal radio filaments (Yusef-Zadeh et al. 1984). These are a population of highly linear, polarized, synchrotronemitting features that are apparently unique to the GC region. ...
... Several radio features in the region are highlighted, including the Mouse (Yusef-Zadeh & Bally 1987), the Snake (Gray et al. 1991) and the Pelican , the Harp and the Christmas Tree (Thomas et al. 2020), and numerous supernova remnants as cataloged by Green (2019). The Radio Arc (Yusef-Zadeh et al. 1984;Paré et al. 2019) is highlighted, now known to be coincident with part of the eastern boundary of the 430 pc bipolar radio bubbles that span the GC (Heywood et al. 2019). These are also annotated, although their full extent is not covered by the image above. ...
... Radio observations are not susceptible to dust obscuration, and are one of the best methods for studying the nonthermal synchrotron processes that arise due to the interaction between relativistic electrons and magnetic fields, as well as the thermal radio emission originating from ionized gas surrounding regions of star formation. One of the most notable discoveries that arose from radio observations of the GC was that of nonthermal radio filaments (Yusef-Zadeh et al. 1984). These are a population of highly linear, polarized, synchrotronemitting features that are apparently unique to the GC region. ...
... Several radio features in the region are highlighted, including the Mouse (Yusef-Zadeh & Bally 1987), the Snake (Gray et al. 1991) and the Pelican , the Harp and the Christmas Tree (Thomas et al. 2020), and numerous supernova remnants as cataloged by Green (2019). The Radio Arc (Yusef-Zadeh et al. 1984;Paré et al. 2019) is highlighted, now known to be coincident with part of the eastern boundary of the 430 pc bipolar radio bubbles that span the GC (Heywood et al. 2019). These are also annotated, although their full extent is not covered by the image above. ...
The inner ∼200 pc region of the Galaxy contains a 4 million M ⊙ supermassive black hole (SMBH), significant quantities of molecular gas, and star formation and cosmic-ray energy densities that are roughly two orders of magnitude higher than the corresponding levels in the Galactic disk. At a distance of only 8.2 kpc, the region presents astronomers with a unique opportunity to study a diverse range of energetic astrophysical phenomena, from stellar objects in extreme environments, to the SMBH and star-formation-driven feedback processes that are known to influence the evolution of galaxies as a whole. We present a new survey of the Galactic center conducted with the South African MeerKAT radio telescope. Radio imaging offers a view that is unaffected by the large quantities of dust that obscure the region at other wavelengths, and a scene of striking complexity is revealed. We produce total-intensity and spectral-index mosaics of the region from 20 pointings (144 hr on-target in total), covering 6.5 square degrees with an angular resolution of 4″ at a central frequency of 1.28 GHz. Many new features are revealed for the first time due to a combination of MeerKAT’s high sensitivity, exceptional u , v -plane coverage, and geographical vantage point. We highlight some initial survey results, including new supernova remnant candidates, many new nonthermal filament complexes, and enhanced views of the Radio Arc bubble, Sagittarius A, and Sagittarius B regions. This project is a South African Radio Astronomy Observatory public legacy survey, and the image products are made available with this article.
... Non-thermal filaments (NTF) and radio continuum threads (hereafter, threads) in the Galactic Center (GC) are unique for their straight and narrow morphology perpendicular to the galactic plane (Yusef-Zadeh et al. 1984Morris & Yusef-Zadeh 1985;Tsuboi et al. 1986;Anantharamaiah et al. 1991;Lang et al. 1999aLang et al. , 1999bLaRosa et al. 2004). The wide-field image of the GC at 1.3 GHz with the MeerKAT radio telescope (Heywood et al. 2019) has revealed a large-scale poloidal magnetic structure composed of coherently aligned vertical threads, which penetrate through the central molecular zone (CMZ: Oka et al. 1998Oka et al. , 2012Tsuboi et al. 2015). ...
... The waves are confined near the origin, and exhibit a variety of filamentary structures. The bunched NTF in the radio Arc (Yusef-Zadeh et al. 1984) could be due to a past temporal enhancement of activity associated with multiple disturbances at a shorter time interval. Figure 8 shows an enlarged portion of the simulated waves from t = 3 to 4 at an interval of δt = 0.1, where the tangential projection of the fronts resembles the observed NTF in the Arc. ...
Propagation of fast-mode magnetohydrodynamic (MHD) compression waves is traced in the Galactic Center with a poloidal magnetic cylinder. MHD waves ejected from the nucleus are reflected and guided along the magnetic field, exhibiting vertically stretched fronts. The radio threads and non-thermal filaments are explained as due to tangential views of the waves driven by sporadic activity in Sgr A$^*$, or by multiple supernovae. In the latter case, the threads could be extremely deformed relics of old supernova remnants exploded in the nucleus.
... The length of the Arc is about 15' or 40 pc. High resolution observations have resolved it into a number of linear filaments (Yusef-Zadeh et al., 1984). Linear polarization from the Galactic Center Arc has been reported from a series of observations in the frequency range between 4.5 GHz and 15 GHz (Sofue et al., 1987 and references therein; Yusef-Zadeh and Morris, 1987). ...
The Galactic Center Arc has been observed with the Effelsberg 100-m telescope at 32 GHz. The percentage polarization is of the order of 50%. The magnetic field structure is uniform in a direction parallel to the Arc.
... On the other hand, the vertical branch of the 'radio arc' is located just along the extension of the eastern ridge of the GCL, and shows a significant filamentary structure suggesting some effects of a strong magnetic field (Yusef-Zadeh et al. 1984). This suggestion, especially the presence of strong poloidal magnetic field, was confirmed later by observation of radio polarization (Inoue et al. 1984;Tsuboi et al. 1985). ...
I critically discuss three possible MHD mechanisms for the formation of Galactic center lobes (GCL) found by Sofue and Handa (1984) from the theoretical point of view. The three mechanisms I shall discuss are: (1) sweeping-magnetic-twist mechanism, (2) explosion in a disk with a vertical magnetic field, and (3) nonlinear Parker instability. I review the characteristics of these mechanisms, which are mainly obtained from nonlinear 2D MHD numerical simulations, and discuss their merits and demerits as possible mechanisms for the formation of GCL and related magnetic structures.
... The first NTF detected within the GC is also the most prominent and is known as the Radio Arc (Yusef-Zadeh et al. 1984, 1986Yusef-Zadeh & Morris 1987a). The Radio Arc consists of 10 individual filaments in close proximity to one another that are all oriented perpendicular to the Galactic plane. ...
The nonthermal filament (NTF) radio structures clustered within a few hundred parsecs of the Galactic center (GC) are apparently unique to this region of the Galaxy. Recent radio images of the GC using MeerKAT at 1 GHz have revealed a multitude of faint, previously unknown NTF bundles (NTFBs), some of which are comprised of as many as 10 or more individual filaments. In this work we present Very Large Array observations at the C - and X -bands (4–12 GHz) at arcsecond-scale resolutions of three of these newly discovered NTFBs, all located at southern Galactic latitudes. These observations allow us to compare their total-intensity properties with those of the larger NTF population. We find that these targets generally possess properties similar to what is observed in the larger NTF population. However, the larger NTF population generally has steeper spectral indices than what we observe for our chosen targets. The results presented here based on the total-intensity properties of these structures indicate that the NTFs are likely a result of synchrotron emission from relativistic electrons that have been generated either by a nearby compact source or by extended magnetic field structures in which the magnetic field line reconnection has accelerated the electrons. In either scenario, once the relativistic electrons are produced and injected locally into the field they diffuse along the magnetic field lines, producing the filaments.
... Closer to home, in the vastly different environment of the Galactic center (GC), radio observations have uncovered a population of magnetized radio filaments with linearly polarized synchrotron emission tracing cosmic-ray activity throughout the inner few hundred parsecs of the galaxy (Yusef-Zadeh et al. 1984Liszt 1985;Bally & Yusef-Zadeh 1989;Gray et al. 1991;Haynes et al. 1992;Staguhn et al. 1998Staguhn et al. , 2019Lang et al. 1999;LaRosa et al. 2001LaRosa et al. , 2004Nord et al. 2004;Law et al. 2008;Pound & Yusef-Zadeh 2018;Arendt et al. 2019). MeerKAT observations have provided a remarkable mosaic of the inner few degrees of the Galactic center revealing hundreds of radio filaments, housed within a ∼400 pc bipolar radio bubble filled with thermal X-ray gas that surrounds the Galactic center (Heywood et al. 2019(Heywood et al. , 2022Yusef-Zadeh et al. 2022a, 2022b. ...
Magnetized radio filaments are found in abundance in the inner few hundred parsecs of our galaxy. Progress in understanding this population of filaments has been slow over the last few decades, in part due to a lack of detection elsewhere in the galaxy or in external galaxies. Recent highly sensitive radio continuum observations of radio galaxies in galaxy clusters have revealed remarkable isolated filamentary structures in the intracluster medium (ICM) that are linked to radio jets, tails, and lobes. The origin of this class of filaments is not understood either. Here, we argue that the underlying physical mechanisms responsible for the creation of the two populations are the same because of their similarities in morphology, spacing between the filaments, aspect ratio, and magnetic energy densities to the thermal pressure of the medium and that both populations have undergone synchrotron aging. These similarities provide an opportunity to investigate the physical processes in the interstellar medium (ISM) and ICM for the first time. We consider that the origin of the filaments in both the Galactic center and ICM is a result of the interaction of a large-scale wind with clouds, or the filaments arise through the stretching and collection of field lines by turbulence in a weakly magnetized medium. We examine these ideas in four radio galaxy filaments associated with four radio galaxies—IC 40B, IC 4496, J1333–3141, ESO 137–006—and argue that much can be understood in the future by comparing these two populations of filaments.
... Radio observations of the Galactic center region show many isolated, elongated filaments (Yusef-Zadeh et al., 1984;Lang et al., 1999;LaRosa et al., 2001;Nord et al., 2004;Yusef-Zadeh et al., 2004). Recent high-resolution observations with the MeerKAT radio telescope found that the filaments trace bipolar bubbles that are rising from the CMZ (Heywood et al., 2019). ...
Cosmic rays (CRs) are a ubiquitous and an important component of astrophysical environments such as the interstellar medium (ISM) and intracluster medium (ICM). Their plasma physical interactions with electromagnetic fields strongly influence their transport properties. Effective models which incorporate the microphysics of CR transport are needed to study the effects of CRs on their surrounding macrophysical media. Developing such models is challenging because of the conceptional, length-scale, and time-scale separation between the microscales of plasma physics and the macroscales of the environment. Hydrodynamical theories of CR transport achieve this by capturing the evolution of CR population in terms of statistical moments. In the well-established one-moment hydrodynamical model for CR transport, the dynamics of the entire CR population are described by a single statistical quantity such as the commonly used CR energy density. In this work, I develop a new hydrodynamical two-moment theory for CR transport that expands the well-established hydrodynamical model by including the CR energy flux as a second independent hydrodynamical quantity. I detail how this model accounts for the interaction between CRs and gyroresonant Alfvén waves. The small-scale magnetic fields associated with these Alfvén waves scatter CRs which fundamentally alters CR transport along large-scale magnetic field lines. This leads to the effects of CR streaming and diffusion which are both captured within the presented hydrodynamical theory. I use an Eddington-like approximation to close the hydrodynamical equations and investigate the accuracy of this closure-relation by comparing it to high-order approximations of CR transport. In addition, I develop a finite-volume scheme for the new hydrodynamical model and adapt it to the moving-mesh code Arepo. This scheme is applied using a simulation of a CR-driven galactic wind. I investigate how CRs launch the wind and perform a statistical analysis of CR transport properties inside the simulated circumgalactic medium (CGM). I show that the new hydrodynamical model can be used to explain the morphological appearance of a particular type of radio filamentary structures found inside the central molecular zone (CMZ). I argue that these harp-like features are synchrotron-radiating CRs which are injected into braided magnetic field lines by a point-like source such as a stellar wind of a massive star or a pulsar. Lastly, I present the finite-volume code Blinc that uses adaptive mesh refinement (AMR) techniques to perform simulations of radiation and magnetohydrodynamics (MHD). The mesh of Blinc is block-structured and represented in computer memory using a graph-based approach. I describe the implementation of the mesh graph and how a diffusion process is employed to achieve load balancing in parallel computing environments. Various test problems are used to verify the accuracy and robustness of the employed numerical algorithms.
... They predominately "radiate" away from the most active portion of the CMZ. Some of these filaments were detected previously (e.g., [37,38]) and were shown to be strongly polarized. Therefore, they are primarily synchrotron in origin and are sometimes called nonthermal radio filaments (NTFs). ...
Nuclear regions of galaxies apparently play a disproportionately large role in regulating their formation and evolution. How this regulation works, however, remains very uncertain. Here we review a few recent X-ray studies of our Galactic center and the inner bulge region of our major neighboring galaxy, M31, and focusing on addressing such questions as: Why are the majority of supermassive black holes (e.g., Sgr A*) so faint? What regulates the Galactic nuclear environment? Furthermore, what impact does a recent active galactic nucleus have on the ionization state of surrounding gas? These studies have provided new insight into how various relevant high-energy phenomena and processes interplay with extreme galactic nuclear environments and affect global galactic ecosystems.
... More than 35 yr have passed since the discovery of the magnetized radio filaments associated with the Galactic center Radio Arc near l ∼ 0°.2 (Yusef-Zadeh et al. 1984). These observations showed linear, magnetized features running mainly perpendicular to the Galactic plane. ...
We present high-pass-filtered continuum images of the inner 3.°5 × 2.°5 of the Galactic center at 20 cm with 6.″4 resolution. These mosaic images are taken with MeerKAT and reveal a large number of narrow filaments, roughly an order of magnitude increase in their numbers compared to past measurements. For the first time, we carry out population studies of the spectral index and magnetic field of the entire region. The mean spectral indices of the filaments are steeper than supernova remnants (SNRs) (−0.62) with a value of α ∼ −0.83. The variation in α is much larger than for the SNRs, suggesting that these characteristics have a different origin. A large-scale cosmic-ray-driven wind has recently been proposed to explain the origin of filaments and the large-scale 430 pc bipolar radio and X-ray structure. This favors the possibility that the large-scale bipolar radio/X-ray structure is produced by past activity of Sgr A* rather than a coordinated burst of supernovae. A trend of steeper indices is also noted with increasing distance from the Galactic plane. This could be explained either by synchrotron cooling or weak shocks accelerating cosmic-ray particles in the context of the cosmic-ray-driven wind. The mean magnetic field strengths along the filaments range from ∼100 to 400 μ G depending on the assumed ratio of cosmic-ray protons to electrons. Given that there is a high cosmic-ray pressure in the Galactic center, the large equipartition magnetic field implies that the magnetic filed is weak in most of the interstellar volume of the Galactic center.
... The Radio Arc, which is one of the most prominent radio continua features in the Galactic center (Yusef-Zadeh, Morris & Chance 1984 ), is observed at frequency 1420.406 MHz with the Very Large Array (VLA) telescope (Lang et al. 2010 ). ...
The central molecular zone (CMZ) plays an essential role in regulating the nuclear ecosystem of our Galaxy. To get an insight into magnetic fields of the CMZ, we employ the Gradient Technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel $70\, {\mu }{\rm m}$ image, as well as ionized [Ne II] and Paschen-alpha emissions. The results are compared with the observations of Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWAC+) $53\, {\mu }{\rm m}$ polarized dust emissions. We map the magnetic fields orientation at multiple wavelength across the central molecular zone, including close-ups of the Radio Arc and Sagittarius A West regions, on multi scales from ∼ 0.1 pc to 10 pc. The magnetic fields towards the central molecular zone traced by GT are globally compatible with the polarization measurements, accounting for the contribution from the galactic foreground and background. This correspondence suggests that the magnetic field and turbulence are dynamically crucial in the galactic center. We find that the magnetic fields associated with the Arched filaments and the thermal components of the Radio Arc are in good agree with the HAWC+ polarization. Our measurement towards the non-thermal Radio Arc reveals the poloidal magnetic field components in the galactic center. For Sagittarius A West region, we find a great agreement between the GT measurement using [Ne II] emission and HWAC+ $53\, {\mu }{\rm m}$ observation. We use GT to predict the magnetic fields associated with ionized Paschen-alpha gas down to scales of 0.1 pc.
... Across all wavelengths, filamentary structures are increasingly evident as the resolution and sensitivity (both brightness and spatial scale) of observations improve. Radio data have revealed a plethora of nonthermal filaments in the Galactic center (Yusef-Zadeh et al. 1984;MeerKAT Collaboration 2018), which Sofue (2020) proposes may be relics of old SNRs, and isolated filaments from LOFAR measurements (Zaroubi et al. 2015;Jelić et al. 2015). Fine filamentary structure can be seen in the NPS in reprocessed data from the NRAO VLA Sky Survey (Rudnick & Brown 2009). ...
We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show that this model is consistent with the large number of observational studies on these regions and is able to resolve an apparent contradiction in the literature that suggests that the high-latitude portion of the NPS is nearby, while lower-latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar nonthermal, filamentary structures that are increasingly being observed with modern radio telescopes.
... The coherent structures visible in the different colors of Figures 4 and 5 arise from spatial variations in the relative strengths of the various emission mechanisms. The radio spectrum of supernova remnants (SNRs) originates primarily from synchrotron emission (Weiler & Sramek 1988), and thus objects like the SNR candidate G357.7-0.1 ("the Tornado") (Milne 1970) and SNR0.9+0.1 (Helfand & Becker 1987) appear reddish yellow in Figure 4. Similarly, prominent radio sources, including Sgr A*, the GCRA, and Sgr B1 (see, e.g., Yusef-Zadeh et al. 1984;Pedlar et al. 1989;Bally et al. 1991) are strikingly highlighted in this color, consistent with their strong synchrotron emission spectrum. Pulsar wind nebulae (PWN), like the Crab Nebula, also emit highly polarized synchrotron emission with a flat spectral index (Gaensler & Slane 2006), in contrast to SNRs, which generally emit synchrotron with a slightly lower polarization fraction and a steeper spectrum. ...
We present arcminute-resolution intensity and polarization maps of the Galactic center made with the Atacama Cosmology Telescope (ACT). The maps cover a 32 deg2 field at 98, 150, and 224 GHz with |l|≤4∘, |b|≤2∘. We combine these data with Planck observations at similar frequencies to create coadded maps with increased sensitivity at large angular scales. With the coadded maps, we are able to resolve many known features of the Central Molecular Zone (CMZ) in both total intensity and polarization. We map the orientation of the plane-of-sky component of the Galactic magnetic field inferred from the polarization angle in the CMZ, finding significant changes in morphology in the three frequency bands as the underlying dominant emission mechanism changes from synchrotron to dust emission. Selected Galactic center sources, including Sgr A*, the Brick molecular cloud (G0.253+0.016), the Mouse pulsar wind nebula (G359.23-0.82), and the Tornado supernova remnant candidate (G357.7-0.1), are examined in detail. These data illustrate the potential for leveraging ground-based Cosmic Microwave Background polarization experiments for Galactic science.
... Across all wavelengths, filamentary structures are increasingly evident as the resolution and sensitivity (both brightness and spatial scale) of observations improve. Radio data have revealed a plethora of non-thermal filaments in the Galactic centre (Yusef-Zadeh et al. 1984;MeerKAT Collaboration 2018), which Sofue (2020) proposes may be relics of old SNRs, and isolated filaments from LOFAR measurements (Zaroubi et al. 2015;Jelić et al. 2015). Fine filamentary structure can be seen in the NPS in reprocessed data from the NVSS survey (Rudnick & Brown 2009). ...
We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show this model is consistent with the large number of observational studies on these regions, and is able to resolve an apparent contradiction in the literature that suggests the high latitude portion of the NPS is nearby, while lower latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar non-thermal, filamentary structures that are increasingly being observed with modern radio telescopes.
... The coherent structures visible in the different colors of Figures 4 and 5 arise from spatial variations in the relative strengths of the various emission mechanisms. The radio spectrum of supernova remnants (SNR) originates primarily from synchrotron emission (Weiler & Sramek 1988), and thus objects like the SNR candidate G357.7-0.1 ("the Tornado") (Milne 1970) and SNR0.9+0.1 (Helfand & Becker 1987) appear reddish yellow in Figure 4. Similarly, Sgr A* and the GCRA (see, e.g., Yusef-Zadeh et al. 1984;Pedlar et al. 1989) are strikingly highlighted in this color, consistent with their strong synchrotron emission spectrum. Pulsar wind nebulae (PWN), like the Crab Nebula, also emit highly polarized synchrotron emission with a flat spectral index (Gaensler & Slane 2006), in contrast to SNRs which generally emit synchrotron with a slightly lower polarization fraction and a steeper spectrum. ...
We present arcminute-resolution intensity and polarization maps of the Galactic center made with the Atacama Cosmology Telescope (ACT). The maps cover a 32 deg$^2$ field at 98, 150, and 224 GHz with $\vert l\vert\le4^\circ$, $\vert b\vert\le2^\circ$. We combine these data with Planck observations at similar frequencies to create coadded maps with increased sensitivity at large angular scales. With the coadded maps, we are able to resolve many known features of the Central Molecular Zone (CMZ) in both total intensity and polarization. We map the orientation of the plane-of-sky component of the Galactic magnetic field inferred from the polarization angle in the CMZ, finding significant changes in morphology in the three frequency bands as the underlying dominant emission mechanism changes from synchrotron to dust emission. Selected Galactic center sources, including Sgr A*, the Brick molecular cloud (G0.253+0.016), the Mouse pulsar wind nebula (G359.23-0.82), and the Tornado supernova remnant candidate (G357.7-0.1), are examined in detail. These data illustrate the potential for leveraging ground-based Cosmic Microwave Background polarization experiments for Galactic science.
... They are seen not only in the CMZ, where such features were mostly known (e.g. Yusef-Zadeh, Morris & Chance 1984;LaRosa, Lazio & Kassim 2001), but also abundant in central regions relatively far away from the Galactic plane. Some of the radio filaments, observed to be strongly polarized, are clearly synchrotron in origin and are sometimes called non-thermal radio filaments (NTFs). ...
Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2° × 4° field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic Centre (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground H ii region, and that the southern plume is well wrapped by a corresponding radio lobe recently discovered by MeerKAT. In particular, we find that a distinct X-ray thread, G0.17−0.41, is embedded well within a non-thermal radio filament, which is locally inflated. This thread with a width of ∼1.6 arcsec (FWHM) is ∼2.6 arcmin or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically-thin thermal plasma with temperature ≳ 3 keV. The X-ray-emitting material is likely confined within a strand of magnetic field with its strength ≳ 1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source. Such re-connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic ray acceleration and turbulence.
... In addition, the VLA is also part of the National Radio Astronomy Observatory (NRAO) and has produced the largest radio survey to date, the NRAO VLA Sky Survey (NVSS), which covers the entire sky north of −40 • declination. The VLA has been crucial to numerous important science discoveries, some of which include the imaging of magnetic filaments at the galactic centre (Yusef-Zadeh et al. 1984), the discovery of the first Einstein ring (Hewitt et al. 1988), and the revelation of internal structures within radio jets (Swain et al. 1998). To further enhance its scientific capabilities, the VLA has been upgraded and renamed in 2012 as the Jansky Very Large Array (JVLA). ...
Magnetic fields are ubiquitous, permeating across all scales from interstellar space to voids. Their origins and evolution still remain as open questions. On galactic scales and beyond, Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose magnetic fields, and its spatial correlation gives the characteristic length scales of the field variation. This work shows how the RM is derived from the polarised radiative transfer equations under restrictive conditions and assesses the merit of RM fluctuations (RMF) for large-scale magnetic field diagnostics. The interpretation of RMF analyses is ambiguous for an ill-defined characteristic density, such as lognormal-distributed and fractal-like structures. The RMF approach also falls short under radiative absorption, emission, Faraday mixing and the contribution of non-thermal electrons. Notably, correlations along the line-of-sight and across the sky plane are generally dissimilar, therefore the context of RMF must be clarified when inferring from observations. Magnetic fields can also imprint observational signatures in the radio synchrotron emission, whose total intensity reveals the field strength and polarisation traces the field orientation. A point-by-point comparison between the X-ray and radio emissions of a simulated galaxy cluster follows a linear best-fit slope of almost unity, indicating that the magnetic field scales with density locally. On smaller scales, magnetic fields may have an important role during the formation and evolution of molecular clouds. The effects of magnetic fields on the ionisation and heating rates of cosmic rays in IC 5146 are quantified, assuming that the fields are traceable via optical and near-infrared starlight polarisations. While the ionisation rate is fairly constant across the cloud, cosmic-ray heating is capable of raising temperatures by order of 1 K in a Galactic environment, or even higher in actively star-forming regions. This may lead to an increase in the Jeans mass and consequently affect the onset of star formation.
... More than 30 yr have elapsed since the non-thermal radio filaments (NRFs) associated with the Galactic centre radio Arc near l ∼ 0.2 • were first reported (Yusef-Zadeh, Morris & Chance 1984). These observations showed linear, magnetized features running perpendicular to the Galactic plane and since then more than 100 NRFs with similar characteristics have been discovered (Liszt 1985;Yusef-Zadeh 1986;Morris & Yusef-Zadeh 1989;Gray et al. 1991;Haynes et al. 1992;Lang, Morris & Echevarria 1999;LaRosa et al. 2004;Yusef-Zadeh, Hewitt & Cotton 2004;Law, Yusef-Zadeh & Cotton 2008;Heywood et al. 2019). ...
The recent detection of an X-ray filament associated with the radio filament G0.173 − 0.42 adds to four other nonthermal radio filaments with X-ray counterparts, amongst the more than 100 elongated radio structures that have been identified as synchrotron-emitting radio filaments in the inner couple of degrees of the Galactic center. The synchrotron mechanism has also been proposed to explain the emission from X-ray filaments. However, the origin of radio filaments and the acceleration sites of energetic particles to produce synchrotron emission in radio and X-rays remain mysterious. Using MeerKAT, VLA, Chandra, WISE and Spitzer, we present structural details of G0.173-0.42 which consists of multiple radio filaments, one of which has anX-ray counterpart. A faint oblique radio filament crosses the radio and X-ray filaments. Based on the morphology, brightening of radio and X-ray intensities, and radio spectral index variation, we argue that a physical interaction is taking place between two magnetized filaments. We consider that the reconnection of the magnetic field lines at the interaction site leads to the acceleration of particles to GeV energies. We also argue against the synchrotron mechanism for the X-ray emission due to the short ∼30 year lifetime of TeV relativistic particles. Instead, we propose that the inverse Compton scattering mechanism is more likely to explain the X-ray emission by upscattering of seed photons emitted from a 106 L⊙ star located at the northern tip of the X-ray filament.
... It is argued by Sofue (1985) and in following studies that the east and west ridges are connected to the radio arc near the Galactic plane and to the star-forming region Sgr C, respectively, both of which are known to be physically situated in the Galactic center region (Lasenby et al. 1989). The emission of the radio arc is synchrotron radiation from highenergy electrons in a strong magnetic field of up to 1 mG (Yusef-Zadeh et al. 1984). On the basis of this, Reich et al. (1987) argued that the radiation of the entire GCL would be synchrotron radiation. ...
An observational result of a radio continuum and H92α radio recombination line of the Galactic center lobe (GCL), using the Yamaguchi 32 m radio telescope, is reported. The obtained spatial intensity distribution of the radio recombination line shows two distinctive ridge-like structures extending from the Galactic plane vertically to the north at the eastern and western sides of the Galactic center, which are connected to each other at a latitude of ${1{^{\circ}_{.}}2}$ to form a loop-like structure as a whole. This suggests that most of the radio continuum emission of the GCL is free–free emission, and that the GCL is filled with thermal plasma. The east ridge of the GCL observed with the radio recombination line separates 30 pc from the radio arc, which has been considered a part of the GCL, but coincides with a ridge of the radio continuum at a Galactic longitude of 0°. The radial velocity of the radio recombination line is found to be between −4 and +10 km s−1 across the GCL. This velocity is much smaller than expected from the Galactic rotation, and hence indicates that the GCL exists apart from the Galactic center. These characteristics of the GCL suggest that the long-standing hypothesis that the GCL was created by explosive activity in the Galactic center is unlikely, but favor that the GCL is a giant H ii region.
... Radio observations have identified a system of magnetized filamentary structures and large-scale radio bubble within the inner two degrees of the Galactic Centre and vertical lobes at the edges of the bubble (e.g. Sofue & Handa 1984;Yusef-Zadeh, Morris & Chance 1984;Yusef-Zadeh et al. 1986;Yusef-Zadeh et al. 2004;Heywood et al. 2019). Over the years dozens of long and narrow linear filaments have been found with aspect ratio of 10-100 (Liszt 1985;Yusef-Zadeh et al. 1986Gray et al. 1991;Haynes et al. 1992;Lang, Morris & Echevarria 1999;LaRosa et al. 2004;Law et al. 2008). ...
Radio, X-ray, and infrared observations of the inner few hundred parsecs of the Galactic Centre have highlighted two characteristics of the interstellar medium. The cosmic-ray ionization rate derived from molecular ions such as H$^+_3$ is at least two to three orders of magnitude higher than in the Galactic disc. The other is bipolar X-ray and radio emission away from the Galactic plane. These features are consistent with a scenario in which high cosmic-ray pressure drives large-scale winds away from the Galactic plane. The interaction of such a wind with stellar wind bubbles may explain the energetic non-thermal radio filaments found throughout the Galactic Centre. Some of the implications of this scenario is the removal of gas driven by outflowing winds, acting as a feedback to reduce the star formation rate in the central molecular zone (CMZ), and the distortion of azimuthal magnetic field lines in the CMZ to vertical direction away from the plane. The combined effects of the wind and the vertical magnetic field can explain why most magnetized filaments run perpendicular to the galactic plane. This proposed picture suggests our Milky Way nucleus has recently experienced starburst or black hole activity, as recent radio and X-ray observations indicate.
... The low DLP may be originated by Faraday depolarization of the foreground ionized gas, or the WPGCL may be the H II region in the near-side region of the GC. Figure 1c shows the continuum image of the Galactic center region at 333 MHz by the Karl G. Jansky Very Large Array (VLA) (LaRosa et al. 2000). The vertical filaments (VFs) of the Galactic Center Arc (Yusef-Zadeh et al. 1984;Yusef-Zadeh & Morris 1987), threads (e.g., Morris & Yusef-Zadeh 1985), and the linear non-thermal filaments of Sgr C (Liszt 1985) are clearly seen in the figure. The VFs have also been observed to interact with the Galactic center molecular clouds (GCMCs) (e.g., Yusef-Zadeh & Morris 1987;Tsuboi et al. 1997). ...
The Galactic Center Lobe (GCL) is a peculiar object widely protruding from the Galactic plane toward the positive Galactic latitude, which had been found toward the Galactic Center (GC) in the early days of the radio observation. The peculiar shape has suggested a relation with historical events, star burst, large explosion, and so on in the GC. However, the issue of whether the GCL is a single large structure located in the GC region is not yet settled conclusively. In the previous observations, the silhouette against the low-frequency emission was found in the western part of the GCL (WPGCL); this suggests that the part is located in front of the GC region. On the other hand, the Local Standard of Rest (LSR) velocity of the radio recombination line toward it was found to be as low as 0 km s−1. However, these observations cannot determine the exact position on the line-of-sight. There is still another possibility that it is in the near-side area of the GC region. In this analysis, we compare these results with the visual extinction map toward the GC. We found that the distribution of the visual extinction larger than 4 mag clearly corresponds to the silhouette of the WPGCL. The WPGCL must be located at most within a few kpc from us and not in the GC region. This would be a giant H ii region in the Galactic disk.
... The GC contains a number of radio-bright filaments, which are extended ( tens of pc long) and narrow ( 0.5 pc wide) tendrils of gas, the observations and analyses of which (e.g., Ekers et al. 1983;Yusef-Zadeh et al. 1984;Morris & Yusef-Zadeh 1985;Yusef-Zadeh et al. 1986;Yusef-Zadeh & Morris 1987;Bally & Yusef-Zadeh 1989;Gray et al. 1991;Uchida et al. 1992;Gray et al. 1995;Morris & Serabyn 1996;Lang et al. 1999;LaRosa et al. 2000;Bicknell & Li 2001a,b;LaRosa et al. 2004;Nord et al. 2004;Yusef-Zadeh et al. 2004;Morris et al. 2014;Yusef-Zadeh et al. 2016;Morris et al. 2017;Heywood et al. 2019) find that they possess the following properties: ...
Synchrotron-emitting, nonthermal filaments (NTFs) have been observed near the Galactic center for nearly four decades, yet their physical origin remains unclear. Here we investigate the possibility that NTFs are produced by the destruction of molecular clouds by the gravitational potential of the Galactic center. We show that this model predicts the formation of a filamentary structure with length on the order of tens to hundreds of pc, a highly ordered magnetic field along the axis of the filament, and conditions conducive to magnetic reconnection that result in particle acceleration. This model therefore yields the observed magnetic properties of NTFs and a population of relativistic electrons, without the need to appeal to a dipolar, $\sim$ mG, Galactic magnetic field. As the clouds can be both completely or partially disrupted, this model provides a means of establishing the connection between filamentary structures and molecular clouds that is observed in some, but not all, cases.
... Most noticeable are large-scale diffuse radio lobes, as well as numerous narrow filaments or their bundles, predominately "radiating" away from the most active portion of the CMZ. They are seen not only in the CMZ, where such features were mostly known (e.g., Yusef-Zadeh et al. 1984;LaRosa et al. 2001), but also abundant in central regions relatively far away from the Galactic plane. Some of the radio filaments, observed to be strongly polarized, are clearly synchrotron in origin and are sometimes called nonthermal radio filaments (NTFs). ...
Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2 times 4 deg^2 field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic center (GC). In particular, we find that a distinct X-ray thread, G0.17-0.41, is embedded well within a nonthermal radio filament, which is locally inflated. This thread with a width of ~1.6'' (FWHM) is ~2.6' or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically-thin thermal plasma with temperature ~8 keV. The X-ray-emitting material is likely confined within a rope of magnetic field with its strength ~1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source, providing probably the best evidence so far for this process to occur in the interstellar space. Such re-connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic-ray acceleration and turbulence.
The energy storage and dynamics at the center of galaxies is explained using a new construct, the gravitationally bound current loop (GBCL), produced when the galaxy formed under gravitational collapse. Thin toroidal plasma around the slender intense relativistic current loop is bound to it by the Maxwell “frozen-field” condition, and also binds gravitationally to the central object (presumably a black hole). The Strong Magnetic Field model (SMF) explains directly the Milky Way (MW) galactic center radio observations of a vertical magnetic field perpendicular to the galactic disk and the extended radio arcs, as well as the production of successive radio blobs ejected from the compact cores of active galactic nuclei (AGN) or quasars.
Active Galactic Nuclei and quasars are characterized at radio wavelengths by a high surface brightness, a flat spectrum, variations in intensity on time scales of a few days to a few years, and by large internal motions which probably reflect Doppler beaming from a synchrotron source which is moving with relativistic velocity away from the central energy source and toward the observer. The observations do not support interpretations based on simple ballistic models where the observed motion and core strength depends only on the geometric orientation of a relativistic beam, but appear to require significant dispersion in intrinsic properties as well as complex dynamics.
Radio continuum observations of the galactic center region have revealed a number of vertical structures running across the galactic plane, most of which are reasonably attributed either to vertical magnetic fields or to energy release out of the galactic plane. We review the observed radio structures and discuss their properties and origins with a particular attention to the unusual manifestation of energy release in the galactic center. The relation of the continuum structures to the expanding and/or contracting molecular gas rings is also discussed.
We have made 55″ resolution maps of the 158 μm [CII] emission line in the region of the curved, thermal filaments and the +20 / +50 kms ⁻¹ molecular clouds in Sgr A. The [CII] emission is spatially well correlated with the radio continuum in the filaments. The large intensity of the [CII] radiation excludes shocks as the origin of the ionization and we conclude that the curved filaments are most likely photo-ionized HII regions at the surface of dense molecular clouds. Our [CII] maps of the +20 / +50 kms ⁻¹ clouds indicate that the +50 kms ⁻¹ cloud is close to (<10pc) Sgr A west while the more massive +20 kms ⁻¹ cloud is at a greater distance from the center (>30pc).
A review is given of large-scale magnetic fields in disks and halos of spiral galaxies. A particular attention is given to vertical field structures, and we discuss their origin and implication on their interaction with halo gas. We point out that the disk-halo magnetic interface plays an important role in circulation of interstellar gas in galaxies, in particular a large-scale circulartion from the galactic center to outer disk regions.
We critically review the status of our current understanding of galactic dynamo theory. In brief, a definitive model for the production of magnetic fields on galactic scales remains to be constructed.
We present new 30″ resolution J=2–1 ¹² CO observations of the molecular gas located close to a group of non-thermal radio emitting filaments recently detected near the Galactic center (Bally and Yusef-Zadeh, 1989). The new data provide some support for the association of the non-thermal filaments G359.54+0.18 with the projected edge of a molecular cloud.
Symmetrical structures do exist, in the Galactic Centre region, In this article we attempt to summarize their properties and draw the attention of the scientific community to the advantages of taking them into account when working with models of the Centre of our Galaxy, Our work is corroborated by two new maps of the region at 10.7 GHz.
Preliminary results of a systematic survey of H78 α , H91 α and H98 β emission from the inner 40′ of the Galactic center region are presented. This region consists of two prominent continuum features, the Sgr A complex and the radio continuum Arc. In spite of much nonthermal emission arising from these two features, we detected strong line emission with large line widths in more than half of the observed 130 positions. Many of the detections are new, in particular −50 km s ⁻¹ ionized gas linking the Sgr A complex and the Arc, β line emission from GO.1+0.08 (the arched filaments), and α line emission from the loop-like structures which surround the non-thermal filaments near G0.2−0.05. We find that much of the detected lines are probably associated with the −50 km s ⁻¹ and the 20 km s ⁻¹ molecular clouds, known to lie near the Galactic center. We present line profiles of a number of Galactic center sources including Sgr B1, Sgr C and Sgr D.
Molecular clouds moving at Keplerian velocities can induce strong electric fields in ionized regions near the galactic center. The induced field can drive currents over equivalent circuits ~ 100 pc. along the highly ordered magnetic fields, B ~ 10 ⁻³ G. Such current paths drive low-level ion acoustic turbulence, providing a resistance in the circuit. Small magnetic pinches form which are generally kink unstable, but can organize into larger, long-lived structures. Ohmic losses are energetically important in the molecular clouds, where high density regions should be most luminous. Electrons can accelerate in the induced fields to relativistic energies, yielding the radio luminosity. Electrodynamic flares may occur on year timescales. Such electrodynamic deceleration of clouds can powerfully increase accretion toward galactic centers and enhance their luminosities.
Recent studies of the Galactic center environment have revealed a wealth of new thermal and nonthermal features with unusual characteristics. A system of nonthermal filamentary structures tracing magnetic field lines are found to extend over 200pc in the direction perpendicular to the Galactic plane. Ionized structures, like nonthermal features, appear filamentary and show forbidden velocity fields in the sense of Galactic rotation and large line widths. Faraday rotation characteristics and the flat spectral index distributions of the nonthermal filaments suggest a mixture of thermal and nonthermal gas. Furthermore, the relative spatial distributions of the magnetic structures with respect to those of the ionized and molecular gas suggest a physical interaction between these two systems. In spite of numerous questions concerning the origin of the large-scale organized magnetic structures, the mechanism by which particles are accelerated to relativistic energies, and the source or sources of heating the dust and gas, recent studies have been able to distinguish the inner 200pc of the nucleus from the disk of the Galaxy in at least two more respects: (1) the recognition that the magnetic field has a large-scale structure and is strong, uniform and dynamically important; and (2) the physics of interstellar matter may be dominated by the poloidal component of the magnetic field.
The Galactic Center of the Milky Way, thanks to its proximity, allows to perform astronomical observations that investigate physical phenomena at the edge of astrophysics and fundamental physics. As such, our Galactic Center offers a unique laboratory to test gravity. In this review we provide a general overview of the history of observations of the GC, focusing in particular on the smallest-observable scales, and on the impact that such observations have on our understanding of the underlying theory of gravity in the surrounding of a massive compact object.
The mechanism to produce the numerous Galactic-Centre filaments (GCF) that vertically penetrate the Galactic plane without clear evidence of connection to the disc remains a mystery. Here we show that the GCFs are explained by relics of supernova remnants (rSNR) driven by hundreds of supernovae (SNe) exploded in the star-forming ring of the central molecular zone (CMZ) at an SN rate of ∼2 × 10−4 y−1 in the past ∼0.5 My. The evolution of rSNRs is simulated by the propagation of fast-mode magneto-hydrodynamic (MHD) waves, which are shown to converge around the Galactic rotation axis by the focusing effect. Tangential projection of the cylindrical wave fronts on the sky constitutes the vertical filaments. The SNR model explains not only the morphology, but also the nonthermal radio spectrum , smoothed brightness over the distribution area consistent with the Σ − D relation of SNR and the heating mechanism of hot plasma in the GC.
We carry out population study of magnetized radio filaments in the Galactic center using MeerKAT data by focusing on the spacing between the filaments that are grouped. The morphology of a sample of 43 groupings containing 174 magnetized radio filaments are presented. Many grouped filaments show harp-like, fragmented cometary tail-like, or loop-like structures in contrast to many straight filaments running mainly perpendicular to the Galactic plane. There are many striking examples of a single filament splitting into two prongs at a junction, suggestive of a flow of plasma along the filaments. Spatial variations in spectral index, brightness, bending and sharpening along the filaments indicate that they are evolving on a 105 − 6-year time scale. The mean spacings between parallel filaments in a given grouping peaks at ∼16″. We argue by modeling that the filaments are approximately cylindrical rather than sheet-like structures and that the groupings are isotropically oriented in 3D space. One candidate for the origin of filamentation is interaction with an obstacle, which could be a compact radio source, before a filament splits and bends into multiple filaments. In this picture, the obstacle or sets the length scale of the separation between the filaments. Another possibility is synchrotron cooling instability occurring in cometary tails formed as a result of the interaction of cosmic-ray driven Galactic center outflow with obstacles such as stellar winds. In this picture, the mean spacing and the mean width of the filaments are expected to be a fraction of a parsec, consistent with observed spacing.
The inner $\sim$200 pc region of the Galaxy contains a 4 million M$_{\odot}$ supermassive black hole (SMBH), significant quantities of molecular gas, and star formation and cosmic ray energy densities that are roughly two orders of magnitude higher than the corresponding levels in the Galactic disk. At a distance of only 8.2 kpc, the region presents astronomers with a unique opportunity to study a diverse range of energetic astrophysical phenomena, from stellar objects in extreme environments, to the SMBH and star-formation driven feedback processes that are known to influence the evolution of galaxies as a whole. We present a new survey of the Galactic center conducted with the South African MeerKAT radio telescope. Radio imaging offers a view that is unaffected by the large quantities of dust that obscure the region at other wavelengths, and a scene of striking complexity is revealed. We produce total intensity and spectral index mosaics of the region from 20 pointings (144 hours on-target in total), covering 6.5 square degrees with an angular resolution of 4$"$,at a central frequency of 1.28 GHz. Many new features are revealed for the first time due to a combination of MeerKAT's high sensitivity, exceptional $u,v$-plane coverage, and geographical vantage point. We highlight some initial survey results, including new supernova remnant candidates, many new non-thermal filament complexes, and enhanced views of the Radio Arc Bubble, Sgr A and Sgr B regions. This project is a SARAO public legacy survey, and the image products are made available with this article.
The Radio Arc is a system of organized nonthermal filaments (NTFs) located within the Galactic center (GC) region of the Milky Way. Recent observations of the Radio Arc NTFs revealed a magnetic field that alternates between being parallel and rotated with respect to the orientation of the filaments. This pattern is in stark contrast to the predominantly parallel magnetic field orientations observed in other GC NTFs. To help elucidate the origin of this pattern, we analyze spectro-polarimetric data of the Radio Arc NTFs using an Australian Telescope Compact Array data set covering the continuous frequency range from ∼4 to 11 GHz at a spectral resolution of 2 MHz. We fit depolarization models to the spectral polarization data to characterize Faraday effects along the line of sight. We assess whether structures local to the Radio Arc NTFs may contribute to the unusual magnetic field orientation. External Faraday effects are identified as the most likely origin of the rotation observed for the Radio Arc NTFs; however, internal Faraday effects are also found to be likely in regions of parallel magnetic field. The increased likelihood of internal Faraday effects in parallel magnetic field regions may be attributed to the effects of structures local to the GC. One such structure could be the Radio Shell local to the Radio Arc NTFs. Future studies are needed to determine whether this alternating magnetic field pattern is present in other multi-stranded NTFs, or is a unique property resulting from the complex interstellar region local to the Radio Arc NTFs.
Using recent observational data, we here test the theory, first proposed by Carlqvist and Gahm, that astrophysical filaments are formed by the Bennett pinch process. In this process, current flowing along the axis of the filament creates a toroidal magnetic field, which in turn creates an inward electromagnetic pressure that draws plasma toward the axis of the filament. In such filaments, the inward pinching force and outwards kinetic forces are approximately equal–the Bennett pinch condition. We find that the Bennett pinch condition does in fact hold, as predicted by the theory, for a sample of interstellar filaments, including molecular clouds, such as Taurus, IC5146, the pipe nebula, the infrared dark cloud G035.39–00.33, as well as
$H\text{I}$
fibers at high galactic latitudes, supernova remnant SN1006. The currents are estimated to be about 10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup>
–10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup>
A. These values are similar to that estimated by Carlqvist and Gahm for Lynds204. The Bennett pinch relation is also applied to intergalactic filaments, such as the ridge between Abell 0399 and 0401. This filament is almost in equilibrium. These results reinforce the validity of the theory.
We present Breakthrough Listen’s Exotica Catalog as the centerpiece of our efforts to expand the diversity of targets surveyed in the Search for Extraterrestrial Intelligence (SETI). As motivation, we introduce the concept of survey breadth, the diversity of objects observed during a program. Several reasons for pursuing a broad program are given, including increasing the chance of a positive result in SETI, commensal astrophysics, and characterizing systematics. The Exotica Catalog is a 963 entry collection of 816 distinct targets intended to include “one of everything” in astronomy. It contains four samples: the Prototype sample, with an archetype of every known major type of nontransient celestial object; the Superlative sample of objects, with the most extreme properties; the Anomaly sample of enigmatic targets that are in some way unexplained; and the Control sample, with sources not expected to produce positive results. As far as we are aware, this is the first object list in recent times with the purpose of spanning the breadth of astrophysics. We share it with the community in hopes that it can guide treasury surveys and as a general reference work. Accompanying the catalog is an extensive discussion of the classification of objects and a new classification system for anomalies. Extensive notes on the objects in the catalog are available online. We discuss how we intend to proceed with observations in the catalog, contrast it with our extant Exotica efforts, and suggest how similar tactics may be applied to other programs.
The Galactic Center (GC) region hosts a variety of powerful astronomical sources and rare astrophysical processes that emit a large flux of nonthermal radiation. The inner 375 pc × 600 pc region, called the Central Molecular Zone, is home to the supermassive black hole Sagittarius A*, massive cloud complexes, and particle accelerators such as supernova remnants (SNRs). We present the results of our improved analysis of the very-high-energy gamma-ray emission above 2 TeV from the GC using 125 hr of data taken with the Very Energetic Radiation Imaging Telescope Array System imaging-atmospheric Cerenkov telescope between 2010 and 2018. The central source VER J1745–290, consistent with the position of Sagittarius A*, is detected at a significance of 38 standard deviations above the background level (38 σ ), and we report its spectrum and light curve. Its differential spectrum is consistent with a power law with exponential cutoff, with a spectral index of , a flux normalization at 5.3 TeV of TeV ⁻¹ cm ⁻² s ⁻¹ , and cutoff energy of TeV. We also present results on the diffuse emission near the GC, obtained by combining data from multiple regions along the GC ridge, which yield a cumulative significance of 9.5 σ . The diffuse GC ridge spectrum is best fit by a power law with a hard index of 2.19 ± 0.20, showing no evidence of a cutoff up to 40 TeV. This strengthens the evidence for a potential accelerator of PeV cosmic rays being present in the GC. We also provide spectra of the other sources in our field of view with significant detections, composite SNR G0.9+0.1, and HESS J1746–285.
A pair of nonthermal radio bubbles recently discovered in the inner few hundred parsecs of the Galactic center bears a close spatial association with elongated, thermal X-ray features called the X-ray chimneys. While their morphology, position, and orientation vividly point to an outflow from the Galactic center, the physical processes responsible for the outflow remain to be understood. We use 3D magnetohydrodynamic simulations to test the hypothesis that the radio bubbles/X-ray chimneys are the manifestation of an energetic outflow driven by multiple core-collapsed supernovae (SNe) in the nuclear stellar disk, where numerous massive stars are known to be present. Our simulations are run with different combinations of two main parameters, the supernova birth rate and the strength of a global magnetic field being vertically oriented with respect to the disk. The simulation results show that a hot gas outflow can naturally form and acquire a vertically elongated shape due to collimation by the magnetic pressure. In particular, the simulation with an initial magnetic field strength of 80 μ G and a supernova rate of 1 kyr ⁻¹ can well reproduce the observed morphology, internal energy, and X-ray luminosity of the bubbles after an evolutionary time of 330 kyr. On the other hand, a magnetic field strength of 200 μ G gives rise to an overly elongated outflow that is inconsistent with the observed bubbles. The simulations also reveal that, inside the bubbles, mutual collisions between the shock waves of individual SNe produce dense filaments of locally amplified magnetic field. Such filaments may account for a fraction of the synchrotron-emitting radio filaments known to exist in the Galactic center.
Synchrotron-emitting, nonthermal filaments (NTFs) have been observed near the Galactic center for nearly four decades, yet their physical origin remains unclear. Here we investigate the possibility that NTFs are produced by the destruction of molecular clouds by the gravitational potential of the Galactic center. We show that this model predicts the formation of a filamentary structure with length on the order of tens to hundreds of pc, a highly ordered magnetic field along the axis of the filament, and conditions conducive to magnetic reconnection that result in particle acceleration. This model therefore yields the observed magnetic properties of NTFs and a population of relativistic electrons, without the need to appeal to a dipolar, ∼ mG, Galactic magnetic field. As the clouds can be both completely or partially disrupted, this model provides a means of establishing the connection between filamentary structures and molecular clouds that is observed in some, but not all, cases.
We suggest that narrow, long radio filaments near the Galactic Center arise as kinetic jets - streams of high energy particles escaping from ram-pressure confined pulsar wind nebulae (PWNe). The reconnection between the PWN and interstellar magnetic field allows pulsar wind particles to escape, creating long narrow features. They are the low frequency analogues of kinetic jets seen around some fast-moving pulsars, such as The Guitar and The Lighthouse PWNe. The radio filaments trace a population of pulsars also responsible for the Fermi GeV excess produced by the Inverse Compton scattering by the pulsar wind particles. The magnetic flux tubes are stretched radially by the large scale Galactic winds. In addition to PWNe accelerated particles can be injected at supernovae remnants. The model predicts variations of the structure of the largest filaments on scales of ∼ dozens of years - smaller variations can occur on shorter time scales. We also encourage targeted observations of the brightest sections of the filaments and of the related unresolved point sources in search of the powering PWNe and pulsars.
An HI absorption study of the central ~ 100 pc of the galactic centre was performed using the compact array of the VLA ² . The results of these observations lead us to believe that the body of gas at 40–50 km s ⁻¹ , known as the ‘40 km s ⁻¹ molecular cloud’, is placed behind the Arc. In addition, the distribution of gas around +20km s ⁻¹ across the Arc and its correlation with 3cm polarized emission suggests that it may be this 20km s ⁻¹ gas which is associated with the Arc and is depolarizing much of its synchrotron radiation. The kinematic structure of the negative velocity HI gas across the arched filaments implies an association between the neutral atomic gas and the ionized and molecular material.
We have used the 20-pixel IR camera to observe thermal IR emission from dust associated with the radio continuum Arc near the Galactic center and the cluster of HII regions in the immediate vicinity of Sgr A East. We detected strong 10 μ m emission from the eastern and western arched filaments (G0.1+0.08), from an unusual pistol-shaped structure known as G0.15–0.05 and from the brightest member of the Sgr A East HII region. Spatial maps of these features at 10 μ m with a resolution of 4.1″ × 4.2″ are presented and are compared with 5-GHz radio images. We find a general spatial correlation between the ionized gas and the dust distributions. The ratio of IR to radio flux densities is significantly different in the eastern and western arched filaments, which suggests that the source of heating has a softer spectrum along the eastern arched filaments. In addition, the ratio of IR to radio flux densities, which is typically ~10 in normal Galactic HII regions excited by O stars, is at least a factor of two higher than this value in almost all the sources we have observed. This suggests that additional mechanisms other than trapped Lyman α radiation should be present in heating the dust, e.g. stochastic heating of small dust grains by energetic particles associated with the nonthermal filaments.
We present Breakthrough Listen's "Exotica" Catalog as the centerpiece of our efforts to expand the diversity of targets surveyed in the Search for Extraterrestrial Intelligence (SETI). As motivation, we introduce the concept of survey breadth, the diversity of objects observed during a program. Several reasons for pursuing a broad program are given, including increasing the chance of a positive result in SETI, commensal astrophysics, and characterizing systematics. The Exotica Catalog is an 865 entry collection of 737 distinct targets intended to include "one of everything" in astronomy. It contains four samples: the Prototype sample, with an archetype of every known major type of non-transient celestial object; the Superlative sample of objects with the most extreme properties; the Anomaly sample of enigmatic targets that are in some way unexplained; and the Control sample with sources not expected to produce positive results. As far as we are aware, this is the first object list in recent times with the purpose of spanning the breadth of astrophysics. We share it with the community in hopes that it can guide treasury surveys and as a general reference work. Accompanying the catalog is extensive discussion of classification of objects and a new classification system for anomalies. We discuss how we intend to proceed with observations in the catalog, contrast it with our extant Exotica efforts, and suggest similar tactics may be applied to other programs.
The dynamics of the thermal plasma instability in a nonuniform magnetic
field, involved in the formation of solar filaments or prominences in
sheared coronal magnetic fields, are examined. An expression is derived
for a perturbation of equilibrium conditions corresponding to the
physical configuration to observed filaments, which allows the dynamic
response of plasma pressure, density and temperature to optically thin
radiation and field-collimated thermal conduction to be determined.
Numerical solutions to the equations demonstrate the formation of
characteristic knife blade condensations and can predict the initial
temporal and spatial scales of coronal filament appearance.
Observations with 10 arcmin resolution of a 2 sq deg portion of the galactic plane around the galactic center at 150, 200, and 300 microns are presented, obtained as part of a galactic plane survey. In the galactic center region the mean dust temperature is about 30 K. Both the temperature and the dust-to-gas mass ratio in the galactic center region are typical of a large portion of the galactic plane. Three main emission peaks, Sgr A, B, and C, are detected; these peaks appear as dust column density enhancements rather than temperature enhancements. The dust responsible for the FIR emission may only be a fraction of the dust responsible for the visual extinction toward the galactic center.
A 45′×30′ region around the galatic center was mapped with 1′ resolution at 55 μm and 125 μm using the Kuiper Airborne Observatory. Peaks in temperature of the dust are correlated with centimeter wavelength thermal continuum sources. The distribution of the column density of dust shows minima at the galactic center (Sgr A) and at the position of an HII region complex (G.07+04) 10′ to the North.
Recent research in radio astronomy has shown the desirability of making observations at shorter and shorter wavelengths. Although designed originally for operation at a wavelength of 21 cm, the Australian 210-ft telescope has given satisfactory performances at 11 cm, and operation at still shorter wavelengths appeared possible (Bowen and Minnett 1963).
Ionized gas extends from the center of the Galaxy to a radius of 150 pc;
molecular clouds extend to 300 pc. With the possible exception of the
Arc feature, the giant H II regions indicate recent large-scale star
formation. The nucleus of the Galaxy consists of the compact H II region
Sgr A West, which contains a cluster of O stars, surrounded by more
extended H II of lower density. The He(+) abundance in nuclear giant H
II regions is extremely low.
An attempt to determine the source structure of the galactic center
compact radio source through VLBI observations at 6.0, 3.6, 2.8, and
1.35 cm is reported. It is found that: (1) at 6.0 cm and 3.6 cm, the
sizes were about 0.05 and 0.015 arcsec, respectively; (2) the 0.001
arcsec core component reported by Kellerman et al. (1976) at 3.8 cm was
not detected at 3.6 cm; and (3) the observations do not allow a
distinction between suggestions that radio source apparent size is
controlled by interstellar electron scattering or by free-free
self-absorption. While the source structure is therefore not yet known
despite these measurements, an absolute upper limit of 10 to the 15th cm
can be set to the linear size, posing a constraint on possible physical
models of the underlying energy source. The plausibility of pulsar,
binary stellar radio source and massive collapsed object models is
discussed.
Although it is well known that HII regions are present in the innermost regions of the Galaxy their kinematics are still not fully understood. In one study Pauls et al . (1976) surveyed with a beamwidth of 3′ arc the 10 GHz recombination line emission in directions within 15′ arc of the nuclear radio source Sgr A. They found that the emission velocities varied from position to position within the range -50 to + 50 km s -1 but appeared to lack any overall pattern. In contrast, we have recently observed the recombination line emission from the galactic centre region with a beamwidth of 4′.5 arc, and find strong evidence of ordered motions near the galactic nucleus.
Observations of the 2.6-mm CO line have been obtained along the galactic equator covering longitudes |ℓ| ≤ 3° and velocities |V| ≤ 300 km s^(-1). The strongest emission is concentrated toward the center (|ℓ| < 1°.5) and at low velocities (<125 km s^-1)) The similarities in distribution and brightness temperature between the CO and 100-µ emissions suggest that the grains and the gas are colocated and nearly in thermal equilibrium. Kinematic models for the gas in the nucleus are briefly discussed. The mass of this gas (mostly H_2) is estimated as 10^7 - 10^8 M_⊙ within 600 pc of the center. This large mass of interstellar molecules indicates that the galactic nucleus may not be deficient in Population I material.
Observations of 2.6-mm CO emission in the inner Galaxy are presented,
together with an interpretation of the expanding molecular features and
of the more general distribution and kinematics of CO emission in
approximately the inner 3 kpc of the Galaxy. The observations are
interpreted in terms of the tilted-disk model of the inner Galaxy
described by Burton and Liszt (1978). The results show that: (1) the
same model that accounts for H I emission reproduces the molecular ring
and other features of the CO observations as artifacts of the
transformation of the emission from smoothly distributed molecular gas
in the tilted disk into the observed coordinates and intensities; (2)
the kinematics and distribution of H I and CO in the inner Galaxy are
essentially identical; (3) the kinematics of the molecular gas in the
inner Galaxy is extremely regular on large and small angular scales and
exhibits the predictions of the tilted-disk model; and (4) molecular gas
is overabundant in the inner Galaxy, relative to either the H I there or
the molecular density in the rest of the Galaxy.
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THE radio source Sgr A, which is located at or near the galactic centre, has now been observed with high resolution (<1 arc min) at several frequencies between 327 and 15 GHz1–4. Downes and Martin3 have suggested that two main components are present between 1,400 and 5,000 MHz: Sgr A-West, identified with the galactic nucleus, coincides with the position of the 2.2 µm source of Becklin and Neugebauer5. Its size is about 45 arc s and it may contain smaller scale structure3,4. Sgr A-East, about 1.5 arc min to the east, is larger (~2.5 arc min) and has a steeper spectrum3 so that it dominates at low frequencies.
INTERFEROMETRIC1,2 and lunar occultation3–5 observations show that the radio source Sgr A, which is located near the galactic centre, consists of several components varying in size from a few arc s to a few arc min. Lunar occultation observations made at Ootacamund on September 9, 1970, have provided detailed information on the structure of the source of 327 MHz and have revealed the presence of an extended component 10 × 4 arc min in size and centred approximately on Sgr A. The extended source has an inclination of 45° ± 15° to the galactic plane, similar to that of the 20 cm continuum ridges6.
RADIO observations have shown that the galactic centre region consists of a number of discrete sources. The brightest of these, Sagittarius A, is believed to represent the galactic nucleus. This communication describes a new series of observations of the region, made at frequencies of 8.25 and 15.50 Gc/s with a pencil-beam antenna. The angular resolutions were respectively 4.2 and 2.2 arc min, the latter being the highest pencil-beam resolution so far applied to the galactic centre region. The observations confirm that the microwave spectrum of Sagittarius A is non-thermal1, show that the angular diameter of the source is approximately 3.5 arc min, and demonstrate that, adjacent to Sagittarius A, there is an irregular emission region, which is apparently thermal in nature. The relation between radio data concerning the galactic centre and optical information about the centres of nearby normal galaxies is also examined.
A 1 arcsec resolution radio map shows a wealth of detail in the "spiral"
distribution of ionized gas within 1.5 pc of the galactic centre. Three
streams of ionized gas passing near the centre can be identified. These
streams could be due to molecular gas falling in towards the centre, in
which case one has the first direct observation of the paths of
accretion by a galactic nucleus.
Observations of infrared fine-structure line emission from compact clouds of ionized gas within Sgr A West are presented. These clouds have diameters of 0.1-0.5 pc, internal velocity dispersions of 100 km/s (FWHM), and line center velocities up to + or - 260 km/s. Their masses are not accurately determined but are probably between 0.1 and 10 solar masses. They are ionized by radiation like that of stars of effective temperature not greater than 35,000 K. The clouds are shown to have lifetimes of 10,000 yr and so must be generated and dissipated at a rate of a few per 1000 yr. From analysis of the distribution of the velocities of the clouds, a most probable mass distribution is derived which includes a central pointlike mass of several x 10 to the 6th solar masses in addition to several x 10 to the 6th solar masses of stars within 1 pc of the center.
Photometric maps of the central 1' of the galactic center are presented ; at 2.2 and 10 $mu$ with a spatial resolution of approx.2/sup double-prime/./sub /; 5. Most of the 2.2-$mu$ radiation within the central 2 pc comes from discrete ; sources with absolute 2.2-$mu$ magnitudes brighter than -8. At 10 $mu$, nine ; discrete sources plus an extended ridge of emission are seen. Five of the ; discrete sources are located within the extended ridge and have linear dimensions ; and 10-$mu$ luminosities similar to the planetary nebula NGC 7027.;
Maps of a region 10 arcmin in diameter around the galactic center made simultaneously in three wavelength bands at 30, 50, and 100 microns with about 1 arcmin resolution are presented, and the distribution of far-IR luminosity and color temperature across this region is derived. The position of highest far-IR surface brightness coincides with the peak of the late-type stellar distribution and with the H II region Sgr A West. The high spatial and temperature resolution of the data is used to identify features of the far-IR maps with known sources of near-IR, radio continuum, and molecular emission. The emission mechanism and energy sources for the far-IR radiation are analyzed qualitatively, and it is concluded that all of the observed far-IR radiation from the galactic-center region can be attributed to thermal emission from dust heated both by the late-type stars and by the ultraviolet sources which ionize the H II regions. A self-consistent model for the far-IR emission from the galactic-center region is presented. It is found that the visual extinction across the central 10 pc of the Galaxy is only about 3 mag and that the dust density is fairly uniform in this region. An upper limit of 10 million suns is set on the luminosity of any still unidentified source of 0.1- to 1-micron radiation at the galactic center.
Observations of the 55 micron and 125 micron emissions of the central 30 arcmin of the Galaxy were performed with 1 arcmin resolution with instrumentation on-board the Kuiper flying observatory. The flux distributions detected showed both wavelengths with peaks near the galactic center, some extension along the galactic plane, and the 125 micron feature located 30 arcsec south. An asymmetry was found around Sgr A, and the variation in dust temperature across the region was calculated from the ratio of the two wavelengths. The temperature peak was around Sgr A, with subsidiary peaks associated with the 55 micron emission. A common heating source was indicated by correlations between cm wavelength sources and the temperature maps. Additionally, the distributions of NH3 and formaldehyde were also correlated with the dust distribution at the center.
This paper presents and discusses far-infrared observations of nine H II regions within 1 deg of the galactic center, including Sgr A, Sgr B2, Sgr C, and G0.5-0.0. The far-infrared luminosity, color temperature, and optical depth of these regions and the ratio of infrared flux to radio-continuum flux lie in the range characteristic of spiral-arm H II regions. The far-infrared results are therefore consistent with the idea that the galactic-center H II regions are ionized by luminous early-type stars. Steep systematic gradients in far-infrared color temperature and optical depth are seen along the galactic plane between Sgr B2 and G0.5-0.0; the appearance of this area is similar to that of regions of star formation in the spiral arms.
Observations of infrared fine-structure line emission from compact clouds of ionized gas in the galactic center have been reported by Lacy et al (1979, 1980). These observations suggest the existence of a central black hole of nearly 3,000,000 solar masses and require mechanisms to generate, ionize, and dispose of the gas clouds. It is found that the best model to fulfill these requirements involves cloud generation through disruption of red giants by stellar collisions, ionization by a population of stars which is affected either by enhanced metal abundances or the death of the most massive stars, and gas disposal by star formation. Although the existence of a massive black hole cannot be ruled out, it would play no necessary role in this model and may cause the tidal disruption of stars at a rate such that their accretion into the black hole would produce more radiation than is observed.
Photometry from 1.25 to 12 micrometers and spectrophotometry from 8 to 13 micrometers of the compact sources found in the galactic-center region are reported. In addition, revised 10 and new 20 micrometers maps with 2''.3 resolution are given. The nature of the compact sources is discussed. Some are best identified as stars or star clusters; the brightest source at 2 micrometers is probably a supergiant, and the infrared source near the nonthermal radio source is probably a stellar cluster with density greater than 1 million solar masses/cu pc. Other sources emit most of their luminosity at wavelengths of 10 micrometers and greater; this emission is probably from heated dust. One of the sources is observationally similar to extremely red OH/infrared stars. Other sources have luminosities and linear sizes similar to those of compact H II regions; emission from optically thin silicate dust is seen in these.
Far-infrared observations of the central 4 arcmin of the Galaxy with 30-arcsec resolution made simultaneously at 30 microns, 50 microns, and 100 microns are presented. The 30-micron radiation peaks strongly at the position of the galactic center, as determined from the 2-micron surface brightness and the density of ionized gas. The 50- and 100-micron emission is much more extended along the plane and shows two emission lobes, one on either side of the 30-micron peak. At the position of the galactic center itself there is a local minimum in the 100-micron surface brightness. It is concluded that the dust density decreases inward over the central few parsecs of the Galaxy and that the dust density in the central parsec is so low that optical and ultraviolet radiation freely traverses this region. The total luminosity of the sources heating the dust which radiates the far-infrared emission from the central few parsecs is deduced to be between 1 x 10 to the 7th and 3 x 10 to the 7th solar luminosities.
A 15 by 15 arcmin region surrounding Sgr A has been mapped at a mean wavelength of 540 microns. The principal feature is a ridge about 10 arcmin long running parallel to the galactic equator and approximately centered on Sgr A but with no peak at that point. The ridge coincides with the 25- and 55-km/s clouds seen in molecular line observations. The mass of the clouds is estimated, and their positions with respect to the galactic center are discussed.
An infrared (K band) spectrum taken with a 3.8 arcsec-diameter aperture centered on the galactic center source IRS 16 exhibits weak CO band absorption and emission by hydrogen Brackett-gamma (Br gamma) and a singlet helium line near 4857/cm. The helium emission profile is much broader (FWHM about 1500 + or - 300 km/sec) than any feature seen previously at the galactic center. The broadening is probably due to Doppler motion of gas either flowing through an ionizing layer or orbiting about a massive object. The absence of hydrogen Br gamma emission with a similar profile suggests the galactic center H/He abundance ratio is reduced by a factor greater than about 500 from normal cosmic values.
A 2-cm wavelength map of the region in the vicinity of the galactic center is presented that represents an improvement in resolution by a factor of 2.7 over Hollinger's (1965) map and an improvement in sensitivity and coverage over the Downes, Maxwell, and Meeks (1965) map. The improved map reveals the great complexity of this region and the existence of a large number of densely clustered discrete sources and many new hitherto unresolved features.
Galactic Radio Astronomy
- A G Little
- A. G. Little