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Belinda J. Wilkes,
Roy Kilgard,
Dong-Woo Kim,
Minsun Kim,
Mari Polletta,
Carol Lonsdale, Harding E. Smith,
Jason Surace,
Frazer N. Owen,
A. Franceschini,
Brian Siana,
and David Shupe
[show abstract]
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ABSTRACT: We report a moderate-depth (70 ks), contiguous 0.7 deg2 Chandra survey in the Lockman Hole Field of the Spitzer/SWIRE Legacy Survey coincident with a completed, ultra-deep VLA survey with deep optical and near-infrared imaging in-hand. The primary motivation is to distinguish starburst galaxies and active galactic nuclei (AGNs), including the significant, highly obscured (log N H > 23) subset. Chandra has detected 775 X-ray sources to a limiting broadband (0.3-8 keV) flux ~4 × 10–16 erg cm–2 s–1. We present the X-ray catalog, fluxes, hardness ratios, and multi-wavelength fluxes. The log N versus log S agrees with those of previous surveys covering similar flux ranges. The Chandra and Spitzer flux limits are well matched: 771 (99%) of the X-ray sources have infrared (IR) or optical counterparts, and 333 have MIPS 24 μm detections. There are four optical-only X-ray sources and four with no visible optical/IR counterpart. The very deep (~2.7 μJy rms) VLA data yield 251 (>4σ) radio counterparts, 44% of the X-ray sources in the field. We confirm that the tendency for lower X-ray flux sources to be harder is primarily due to absorption. As expected, there is no correlation between observed IR and X-ray fluxes. Optically bright, type 1, and red AGNs lie in distinct regions of the IR versus X-ray flux plots, demonstrating the wide range of spectral energy distributions in this sample and providing the potential for classification/source selection. Many optically bright sources, which lie outside the AGN region in the optical versus X-ray plots (fr /fx >10), lie inside the region predicted for red AGNs in IR versus X-ray plots, consistent with the presence of an active nucleus. More than 40% of the X-ray sources in the VLA field are radio-loud using the classical definition, RL . The majority of these are red and relatively faint in the optical so that the use of RL to select those AGNs with the strongest radio emission becomes questionable. Using the 24 μm to radio flux ratio (q 24) instead results in 13 of the 147 AGNs with sufficient data being classified as radio-loud, in good agreement with the ~10% expected for broad-lined AGNs based on optical surveys. We conclude that q 24 is a more reliable indicator of radio-loudness. Use of RL should be confined to the optically selected type 1 AGN.
The Astrophysical Journal Supplement Series 11/2009; 185(2):433. · 13.46 Impact Factor
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Carol J. Lonsdale,
Maria del Carmen Polletta,
Alain Omont,
Dave Shupe,
Stefano Berta,
Robert Zylka,
Brian Siana,
Dieter Lutz,
Duncan Farrah, Harding E. Smith, [......],
Frazer Owen,
Alexandre Beelen,
Dan Weedman,
Alberto Franceschini,
Dave Clements,
Linda Tacconi,
Alejandro Afonso-Luis,
Ismael Pérez-Fournon,
Pierre Cox,
and Frank Bertoldi
[show abstract]
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ABSTRACT: We report on-off pointed MAMBO observations at 1.2 mm of 61 Spitzer-selected star-forming galaxies from the Spitzer Wide Area Infrared Extragalactic Legacy survey (SWIRE). The sources are selected on the basis of bright 24 μm fluxes (f 24 μm > 0.4 mJy) and of stellar dominated near-infrared spectral energy distributions in order to favor z ~ 2 starburst galaxies. The average 1.2 mm flux for the whole sample is 1.5 ± 0.2 mJy. Our analysis focuses on 29 sources in the Lockman Hole field where the average 1.2 mm flux (1.9 ± 0.3 mJy) is higher than in other fields (1.1 ± 0.2 mJy). The analysis of the multiwavelength spectral energy distributions indicates that these sources are starburst galaxies with far-infrared luminosities from 1012 to 1013.3 L ☉, and stellar masses of ~0.2-6 × 1011 M ☉. Compared to submillimeter selected galaxies (SMGs), the SWIRE-MAMBO sources are among those with the largest 24 μm/1.2 mm flux ratios. The origin of such large ratios is investigated by comparing the average mid-infrared spectra and the stacked far-infrared spectral energy distributions of the SWIRE-MAMBO sources and of SMGs. The mid-infrared spectra, available for a handful of sources, exhibit strong polycyclic aromatic hydrocarbon (PAH) features, and a warm dust continuum. The warm dust continuum contributes ~34% of the mid-infrared emission, and is likely associated with an AGN component. This contribution is consistent with what is found in SMGs. The large 24 μm/1.2 mm flux ratios are thus not due to AGN emission, but rather to enhanced PAH emission compared to SMGs. The analysis of the stacked far-infrared fluxes yields warmer dust temperatures than typically observed in SMGs. Our selection favors warm ultraluminous infrared sources at high-z, a class of objects that is rarely found in SMG samples. Indeed SMGs are not common among bright 24 μm sources (e.g., only about 20% of SMGs have f 24 μm > 0.4 mJy). Our sample is the largest Spitzer-selected sample detected at millimeter wavelengths currently available.
The Astrophysical Journal 02/2009; 692(1):422. · 6.02 Impact Factor
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Maria del Carmen Polletta,
Belinda J. Wilkes,
Brian Siana,
Carol J. Lonsdale,
Roy Kilgard, Harding E. Smith,
Dong-Woo Kim,
Frazer Owen,
Andreas Efstathiou,
Tom Jarrett, [......],
Alberto Franceschini,
Michael Rowan-Robinson,
Tom S. R. Babbedge,
Stefano Berta,
Fan Fang,
Duncan Farrah,
Eduardo González-Solares,
Glenn Morrison,
Jason A. Surace,
and Dave L. Shupe
[show abstract]
[hide abstract]
ABSTRACT: Using the large multiwavelength data set in the Chandra/SWIRE Survey (0.6 deg2 in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGNs, estimate a lower limit to their surface density, and characterize their multiwavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain (1) five X-ray sources with hard X-ray spectra and column densities 1024 cm-2 and (2) 120 infrared sources with red and AGN-dominated infrared SEDs. We estimate a surface density of at least 25 Compton-thick AGNs deg-2 detected in the infrared in the Chandra/SWIRE field, of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remaining being dominated by the host galaxy emission. Only ~33% of all Compton-thick AGNs are detected in the X-rays at our depth [F(0.3-8 keV) > 10-15 ergs cm-2 s-1]. We report the discovery of two sources in our sample of Compton-thick AGNs, SWIRE J104409.95+585224.8 (z = 2.54) and SWIRE J104406.30+583954.1 (z = 2.43), which are the most luminous Compton-thick AGNs at high z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities, and black hole masses.
The Astrophysical Journal 12/2008; 642(2):673. · 6.02 Impact Factor
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ABSTRACT: We report 18 cm VLBI continuum imaging observations at ~5 mas resolution for UGC 5101, NGC 7469, and Mrk 231, all part of a sample of luminous infrared galaxies that were shown to have strong VLBI radio cores in a previous 18 cm VLBI survey. Mrk 231, generally considered to be a dust-enshrouded quasi-stellar object (QSO), shares many characteristics with quasars, including a broad Sy1 optical emission spectrum, optical and radio variability, and a broad absorption line spectrum. NGC 7469 is a classical Sy1 galaxy, and UGC 5101 is a LINER system. The radio morphology of these three systems on VLBI scales is like that of an active galactic nucleus (AGN), with well-defined ridgelines and high-brightness yet spatially resolved components. The structure and flux densities of these VLBI components are not consistent with starburst-generated radio supernovae of the type found in Arp 220. On scales of ~100 pc and perhaps beyond, the radio continuum in all three objects appears to be dominated by an AGN, not a starburst. Radio emission on larger scales may well originate, in part or in total, in a less compact circumnuclear star-forming region. This is in contrast to the situation thought to prevail in many luminous infrared galaxies (LIGs), which involves a very compact and dense nuclear star-forming region. By placing a lower limit on the AGN-related radio emission, it is possible to shed light on the relative luminosities of the AGN and starburst in these objects. Using the far-infrared-radio correlation for starburst-related radio emission, we show that, despite the unambiguous evidence for AGN activity in these systems, the AGN luminosity, although energetically important, may be exceeded by the extended starburst luminosity in all three cases, unless the bolometric luminosity of the entire system has been underestimated. The lower limits on AGN-related radio emission indicate that these AGNs are somewhat overluminous in the radio compared with radio-quiet Palomar-Green (PG) QSOs and thus that the AGN radio luminosity cannot be used to estimate the bolometric luminosity of the AGN. However, comparing near- and mid-IR luminosities of the LIGs to those of PG QSOs, we find evidence that a significant fraction of the bolometric luminosity of the AGN in Mrk 231 and to a lesser extent UGC 5101 may be radiated away from Earth and thus not detected at any wavelength. The VLBI structure observed in Mrk 231 allows additional interpretation. Confirming and extending VLBI imaging by Ulvestad, Wrobel, and Carilli, our continuum image shows a triple structure, with a core and two lobes, causing it to be classied as a compact symmetric object. It has been suggested that these sources are young, τ 106 yr, with the hot spots representing the working surface of a relativistic jet on the ambient medium. If the southern (primary) lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe advance speed, va ~ 10-4c, and an age for the jet/compact source, τ < 106 yr. We have also imaged the 1667 MHz OH maser emission in Mrk 231, which is extended on scales of 50-100 mas (40-80 pc) and probably coincides with the inner region of the disk, which is seen in CO emission and H I absorption. Among OH megamaser sources studied at high sensitivity with milliarcsecond resolution, Mrk 231 is unique in the stringent upper limits placed on the flux density of compact OH structures of the type found in Arp 220 and other LIGs. It is possible that the circumnuclear environment of Mrk 231 has been sufficiently disrupted by the emergent QSO that the cool dense clouds necessary for such compact masers no longer exist.
The Astrophysical Journal 12/2008; 592(2):804. · 6.02 Impact Factor
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Payam Davoodi,
Seb Oliver,
Maria del Carmen Polletta,
Michael Rowan-Robinson,
Richard S. Savage,
Ian Waddington,
Duncan Farrah,
Tom Babbedge,
Carol Lonsdale,
Tracey Evans,
Fan Fang,
Eduardo Gonzalez-Solares,
Tom Jarrett,
David L. Shupe,
Brian Siana, Harding E. Smith,
Jason Surace,
and C. Kevin Xu
[show abstract]
[hide abstract]
ABSTRACT: We fit a parametric model comprising a mixture of multidimensional Gaussian functions to the 3.6-8 μm color and optical photometric redshift distribution of galaxy populations in the ELAIS-N1 and Lockman Fields of the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy survey. For 16,698 sources in ELAIS-N1 we find our data are best modeled (in the sense of the Bayesian information criterion) by the sum of four Gaussian distributions or modes (Ca, Cb, Cc, and Cd). We compare the fit of our empirical model with predictions from existing semianalytic and phenomenological models. We infer that our empirical model provides a better description of the mid-infrared color distribution of the SWIRE survey than these existing models. This color distribution test is thus a powerful model discriminator and is entirely complementary to comparisons of number counts. We use our model to provide a galaxy classification scheme and explore the nature of the galaxies in the different modes of the model. Population Ca is found to consist of dusty star-forming systems such as ultraluminous infrared galaxies, over a broad redshift range. Low-redshift late-type spiral galaxies are found in population Cb, in which polycyclic aromatic hydrocarbon emission dominates at 8 μm, making these sources very red in longer wavelength Infrared Array Camera (IRAC) colors. Population Cc consists of dusty starburst systems with high levels of star formation activity at intermediate redshifts. Low-redshift early-type spiral and elliptical galaxies are found to dominate population Cd. We thus find a greater variety of galaxy types than one can with optical photometry alone. Finally, we develop a new technique to identify unusual objects and find a selection of outliers with very red IRAC colors. These objects are not detected in the optical but have very strong detections in the mid-infrared. These sources are modeled as dust-enshrouded, strongly obscured active galactic nuclei (AGNs), for which the high mid-infrared emission may be attributed to either dust heated by the AGNs or substantial star formation. These sources have zph ~ 2-4, making them incredibly infrared-luminous, with LIR ~ 1012.6-1014.1 L.
The Astronomical Journal 12/2007; 132(5):1818. · 4.03 Impact Factor
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Brian Siana,
Maria del Carmen Polletta, Harding E. Smith,
Carol J. Lonsdale,
Eduardo Gonzalez-Solares,
Duncan Farrah,
Tom S. R. Babbedge,
Michael Rowan-Robinson,
Jason Surace,
David Shupe,
Fan Fang,
Alberto Franceschini,
Seb Oliver
[show abstract]
[hide abstract]
ABSTRACT: We use a simple optical/infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman Break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z~3 (U-dropout) QSO candidates with 19<r'<22 over 11.7 deg^2 in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z~3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming they are all QSOs at 2.83<z<3.44. We find that our z~4$ (g'-dropout) sample suffers from both unreliability and incompleteness but present 7 previously unidentified QSOs at 3.50<z<3.89. Detailed simulations show our z~3 completeness to be ~80-90% from 3.0<z<3.5, significantly better than the ~30-80% completeness of the SDSS at these redshifts. The resulting luminosity function extends two magnitudes fainter than SDSS and has a faint end slope of beta=-1.42 +- 0.15, consistent with values measured at lower redshift. Therefore, we see no evidence for evolution of the faint end slope of the QSO luminosity function. Including the SDSS QSO sample, we have now directly measured the space density of QSOs responsible for ~70% of the QSO UV luminosity density at z~3. We derive a maximum rate of HI photoionization from QSOs at z~3.2, Gamma = 4.8x10^-13 s^-1, about half of the total rate inferred through studies of the Ly-alpha forest. Therefore, star-forming galaxies and QSOs must contribute comparably to the photoionization of HI in the intergalactic medium at z~3. Comment: Accepted for publication in ApJ. emulateapj format. 23 pages, 17 figures
The Astrophysical Journal 11/2007; · 6.02 Impact Factor
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ABSTRACT: We present parsec-resolution spectral-line VLBI data for two epochs separated by 15 months as a precise new probe of the innermost regions of the nearby Ultraluminous Infrared Galaxy (ULIRG) Arp 220. This galaxy hosts a powerful starburst, with an associated supernova (SN) rate of order 4/yr. An extensive population of compact continuum sources interpreted as radio supernovae (RSNe) and young supernova remnants (SNR) has been imaged. We show here that many of the supernova-related radio continuum point sources exhibit clear evidence of OH absorption or maser emission in the intervening gas, and as such provide us with a sampling of conditions along very narrow and specific lines of sight through the nuclear environment. The OH gas along these lines of sight exhibits velocity dispersions of up to several tens of km/sec, and that in some cases, multiple distinct concentrations of masing gas at different radial velocities can be discerned. There is evidence for variability in the OH properties on ~1yr timescales. Our results are discussed in the context of the overall OH megamaser properties of Arp 220.
Proceedings of the International Astronomical Union 02/2007; 3:432 - 436.
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ABSTRACT: We report the first detection at multiple radio wavelengths (13, 6 and 3.6
cm) of the compact sources within both nuclei of the Ultra Luminous Infra-Red
Galaxy Arp 220. We present the radio spectra of the 18 detected sources. In
just over half of the sources we find that these spectra and other properties
are consistent with the standard model of powerful Type IIn supernovae
interacting with their pre-explosion stellar wind. The rate of appearance of
new radio sources identified with these supernova events suggests that an
unusually large fraction of core collapse supernovae in Arp 220 are highly
luminous; possibly implying a radically different stellar initial mass function
or stellar evolution compared to galactic disks. Another possible explanation
invokes very short (~3 x 10^5 year) intense (~10^3 M_Sol year^-1) star
formation episodes with a duty cycle of ~10%. A second group of our detected
sources, consisting of the brightest and longest monitored sources at 18 cm do
not easily fit the radio supernova model. These sources show a range of
spectral indexes from -0.2 to -1.9. We propose that these are young supernova
remnants which have just begun interacting with a surrounding ISM with a
density between 10^4 and 10^5 cm^-3. One of these sources is probably resolved
at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate
that the ionized component of the ISM gives rise to foreground free-free
absorption with opacity at 18 cm of <0.6 along the majority of lines of sight.
Other sources may be affected by absorption with opacity in the range 1 to 2.
These values are consistent with previous models as fitted to the radio
recombination lines and the continuum spectrum.
12/2006;
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ABSTRACT: We have used a Very Long Baseline Interferometry (VLBI) array at 18cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, and with very high sensitivity. The resulting map has an rms of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison with high sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, though the western nucleus is more compact, and appears to be ~3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.
The Astrophysical Journal 05/2006; · 6.02 Impact Factor
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Payam Davoodi,
Seb Oliver,
Maria del Carmen Polletta,
Michael Rowan-Robinson,
Richard S. Savage,
Ian Waddington,
Duncan Farrah,
Tom Babbedge,
Carol Lonsdale,
Tracey Evans,
Fan Fang,
Eduardo Gonzalez-Solares,
Tom Jarrett,
David L. Shupe,
Brian Siana, Harding E. Smith,
Jason Surace,
C. Kevin Xu
[show abstract]
[hide abstract]
ABSTRACT: We fit a parametric model comprising a mixture of multi-dimensional Gaussian functions to the 3.6 to 8um colour and optical photo-z distribution of galaxy populations in the ELAIS-N1 and Lockman Fields of SWIRE. For 16,698 sources in ELAIS-N1 we find our data are best modelled (in the sense of the Bayesian Information Criterion) by the sum of four Gaussian distributions or modes (C_a, C_b, C_c and C_d). We compare the fit of our empirical model with predictions from existing semi-analytic and phenomological models. We infer that our empirical model provides a better description of the mid-infrared colour distribution of the SWIRE survey than these existing models. This colour distribution test is thus a powerful model discriminator and complementary to comparisons of number counts. We use our model to provide a galaxy classification scheme and explore the nature of the galaxies in the different modes of the model. C_a consists of dusty star-forming systems such as ULIRG's. Low redshift late-type spirals are found in C_b, where PAH emission dominates at 8um. C_c consists of dusty starburst systems at intermediate redshifts. Low redshift early-type spirals and ellipticals dominate C_d. We thus find a greater variety of galaxy types than one can with optical photometry alone. Finally we develop a new technique to identify unusual objects, and find a selection of outliers with very red IRAC colours. These objects are not detected in the optical, but have very strong detections in the mid-infrared. These sources are modelled as dust-enshrouded, strongly obscured AGN, where the high mid-infrared emission may either be attributed to dust heated by the AGN or substantial star-formation. These sources have z_ph ~ 2-4, making them incredibly infrared luminous, with a L_IR ~ 10^(12.6-14.1) L_sun.
05/2006;
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Carol Lonsdale,
Maria Polletta,
Jason Surace,
David Shupe,
Fan Fang,
C. Kevin Xu, Harding E. Smith,
Brian Siana,
Michael Rowan-Robinson,
Tom Babbedge, [......],
Nick Gautier,
Alberto Franceschini,
Stefano Berta,
Ismael Perez-Fournon,
Herve Dole,
Gordon Stacey,
Steve Sergeant,
Marguerite Pierre,
Matt Griffin,
Rick Puetter
[show abstract]
[hide abstract]
ABSTRACT: We characterize the SWIRE galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24$\mu$ Spitzer data and deep U,g',r',r' optical imaging within an area ~1/3 sq. deg for ~16,000 Spitzer-SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6$\mu$m and almost 350,000 at 24$\mu$m; ~70,000 of these will be 5-band 3.6-24$\mu$ detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations, however significant samples of unusually blue objects in the [3.6-4.5]$\mu$m color are found, as well as many objects very red in the 3.6-24$\mu$m mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts and AGN from z=0.37 to 2.8, with luminosities from L$_{IR}$=10$^{10.3}$ to 10$^{13.7}$ L$_{\odot}$ Comment: Accepted for publication in the ApJ Supplements Spitzer Special Issue 17 pages, 4 figures
06/2004;
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ABSTRACT: The largest of the SIRTF Legacy programs, SWIRE will survey 65 sq. deg. in seven high latitude fields selected to be the best
wide low-extinction windows into the extragalactic sky. SWIRE will detect millions of spheroids, disks and starburst galaxies
to z>3 and will map L* and brighter systems on scales up to 150 Mpc at z∼0.5–1. It will also detect ∼104 low extinction AGN and large numbers of obscured AGN. An extensive program of complementary observations is underway. The
data are non-proprietary and will be made available beginning in Spring 2004.
12/2003: pages 165-168;
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Carol J. Lonsdale, Harding E. Smith,
Michael Rowan-Robinson,
Jason Surace,
David Shupe,
Cong Xu,
Seb Oliver,
Deborah Padgett,
Fan Fang,
Alberto Franceschini,
Nick Gautier,
Matt Griffin,
Frank Masci,
Glenn Morrison,
JoAnn O'Linger,
Ismael Perez-Fournon,
Marguerite Pierre,
Richard Puetter,
Gordon Stacey
[show abstract]
[hide abstract]
ABSTRACT: The SIRTF Wide-area InfraRed Extragalactic survey (SWIRE), the largest SIRTF Legacy program, is a wide-area, imaging survey to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and AGN as a function of environment, from redshifts z~3 to the current epoch. SWIRE will survey 7 high-latitude fields, totaling 60 - 65 sq. deg. in all 7 SIRTF bands: IRAC 3.6, 4.5, 5.6, 8 microns and MIPS 24, 70, 160 microns. The Legacy Extragalactic Catalog may contain in excess of 2 million IR-selected galaxies, dominated by (1) ~150,000 luminous infrared galaxies (LIRGs: L{FIR}>10^11 L_sun), ~7000 of these with z>2; (2) 1 million early-type galaxies, ~10,000 with z>2; and (3) \~20,000 classical AGN, plus significantly more dust-obscured QSO/AGN among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGN. The key scientific goals of SWIRE are: (1) to determine the evolution of actively star-forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts and AGN in the key redshift range, 0.5<z<3, over which much of cosmic evolution has occurred; (3) to determine the evolutionary relationship between ``normal galaxies'' and AGN, and the contribution of AGN accretion energy vs stellar nucleosynthesis to the cosmic backgrounds. SWIRE's large area is important to create statistically significant population samples over enough volume cells that we can resolve the star formation history as a function of epoch and environment The large volume is also optimised for finding rare objects. Comment: 65 pages, 11 figures, accepted for publication in the P.A.S.P. Aug. 2003
05/2003;
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ABSTRACT: We report 18cm VLBI continuum imaging observations at 5 mas resolution for UGC 5101, NGC 7469, and Mrk 231, all part of a sample of Luminous Infrared Galaxies which have been shown to have strong VLBI radio cores. The radio morphology of these three systems on VLBI scales is AGN-like, with well-defined ridgelines and high-brightness yet spatially resolved components. The structure and flux densities of these VLBI components are not consistent with starburst generated radio supernovae of the type found in Arp 220. On scales of 100pc the radio continuum in all three objects appears to be dominated by an AGN, not a starburst. Radio emission on larger scales may well originate in a less compact circumnuclear star-forming region. Confirming and extending VLBI imaging of Mrk 231 by Ulvestad et al. (1999), our continuum image shows a triple structure, with a core and two lobes, classifying it as a Compact Symmetric Object (CS0). If the southern (primary) lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe advance speed, $v_a \sim 10^{-4}c$, and an age for the jet/compact source, $< 10^6 yr$. We have also imaged the 1667 MHz OH maser emission in Mrk 231, which is extended on scales of 50--100 milliarcsec (40--80 pc) and probably coincides with the inner region of the disk which is seen in CO emission and HI absorption. Among OH megamasers studied at high sensitivity with mas resolution, Mrk 231 is unique in the stringent upper limits placed upon the flux density of compact OH structures of the type found in Arp 220 and other LIGs. It is possible that the circumnuclear environment of Mrk 231 has been sufficiently disrupted by the emergent QSO that the cool, dense clouds necessary for such compact masers no longer exist. Comment: 43 pages, 7 figures
04/2003;
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ABSTRACT: We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp220. the correct spatial configuration of the various componnents of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern nuclear region in MERLIN scales; this gradient is three times smaller than seen in OH and implies that the OH gas lies inside the HI. A re-analysis of previously presented global VLBI data (Lonsdale et al. 1998) reveals a very high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the site of a heavily obscured AGN. Comment: 10 pages, 11 figures, accepted by MNRAS
Monthly Notices of the Royal Astronomical Society 02/2003; · 4.90 Impact Factor
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ABSTRACT: As described by Colin Lonsdale in this Workshop, VLBI observations reveal thepresence of both AGN and Starburst activity in Luminous Infrared Galaxies.We highlight new results on two classical LIGs which span the range of LIGradio activity: Arp 220, which reveals luminous radio supernovae (RSN)produced in an intense starburst, and Mrk231, which has a powerful compact,AGN radio core. Second epoch observations of the compact radio sources inArp 220 confirm their nature as luminous Radio Supernovae, but indicate alower luminous RSN frequency (LRSN 0.3 yr-1) andconsequent slower decay rate than previously suggested. We interpret thisas due to the dense starburst medium into which the supernovaedetonate. The compact radio morphology of Mrk231 places it among theCompact Symmetric Objects (CSOs) which are suggested to be young radio sources in which asymmetric lobes or hotspots reveal theworking surface of a relativistic jet upon the ambient medium. Assumingthat the lobes in Mrk 231 are confined by ram pressure, we estimate an agefor the jet/compact source, 106yr. We interpret Mrk 231as a newly formed QSO emerging from a starburst.
Astrophysics and Space Science 02/1999; 266(1):125-130. · 1.69 Impact Factor
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ABSTRACT: Mrk 231, a Luminous Infrared Galaxy with log Lfir =
12.35(L_sun) is generally considered to be a dust-enshrouded quasar. Mrk
231 shares many characteristics with quasars including a broad, Sy1
optical emission spectrum, optical and radio variability, and
broad-absorption-line (BAL) spectrum. In common with other Luminous IR
Galaxies, Mrk 231 is a molecular-gas-rich system showing evidence for a
merger or other dynamical disturbance in the host galaxy, which is
itself a luminous system with evidence for rapid star formation. We have
obtained VLBI imaging observations in the 18cm continuum and 1667MHz OH
maser line. The continuum shows a strong, unresolved nuclear core with
an extended, limb-brightened lobe extending about 30mas (26pc) to the
south, and a much fainter lobe 25mas (21pc) to the north. The compact
radio morphology places it among the Compact Symmetric Objects (CSOs) in
which radio emission originates from hot spots or lobes symmetrically
placed on sub-kpc scales about a central compact source. It has been
suggested that these sources are young, tau << 10(6) yr, with the
hot spots representing the working surface of a relativistic jet upon
the ambient medium. If the southern (primary) lobe/hot-spot in Mrk 231
is confined by ram pressure, we estimate a lobe advance speed, v_a
10(-4) c and an age for the jet/compact source, tau < 10(6) yr. The
OH 1667MHz emission is extended over a region of angular dimension
approximately 100 x 50 mas (85 x 43pc) and is probably the inner region
of the disk which is seen in CO emission (Bryant & Scoville 1996,
Ap.J., 457, 678) and HI absorption (Carilli, Wrobel & Ulvestad 1998,
A.J., 115, 928). The radio continuum emission associated with this
gaseous disk is likely to be produced by a circumnuclear starburst; if
this starburst has a normal FIR-to-radio ratio, q = 2.34, then up to 2/3
of the far-infrared continuum in Mrk 231 may come from star-formation.
We suggest that Mrk 231 is a nascent quasar (tau < 10(6) yr) emerging
from its dust-enshrouded starburst phase along the lines of the
evolutionary scenario suggested by Sanders et al.(1988, Ap.J., 325, 74).
04/1998; 30:833.
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ABSTRACT: Through analysis of available optical spectrophotometric data and radio flux density measurements in the literature, it is demonstrated that a good correlation exists between the radio power and bolometric luminosity of the optically-selected OSOs in the Bright Quasar Sample (BOS) of Schmidt and Green (1983). We have recently used VLBI measurements of a sample of ultraluminous infrared galaxies to infer the likely existence of radio-quiet Active Galactic Nuclei (AGNs) deeply enshrouded in dust within their nuclei (Lonsdale, Smith, and Lonsdale 1993). We employ the radio-bolometric luminosity correlation for the BQS quasars to test whether these hypothetical buried AGNs can be energetically responsible for the observed far-infrared luminosities of the ultraluminous infrared galaxies. The ultraluminous infrared galaxies are shown to follow the same relation between radio core power and bolometric luminosity as the radio-quiet QSOs, suggesting that buried AGNs can account for essentially all the observed infrared luminosity, and raising the possibility that any starburst which may be in progress may not be energetically dominant. The broader implications of the radio-optical correlation in quasars for AGNs and luminous infrared galaxy models and the use of radio astronomy as a probe of the central powerhouse in radio quiet AGNs and luminous infrared galaxies are briefly discussed.
02/1995;
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ABSTRACT: In a recent VLBI survey of a sample of 31 Compact (theta <=
0arcsecpoint 25 at 8.4GHz), Luminous (log LFIR >= 11.25
L_sun) Far Infrared Galaxies (Lonsdale, Smith and Lonsdale 1993, Ap. J.
(Letters), 405, L9), we demonstrated that high--T_b AGN-like radio cores
are common, perhaps universal in these galaxies, comprising generally a
few percent of the total 18cm radio flux density. The VLBI core
characteristics do not correlate strongly with other radio, optical or
infrared properties of the sample galaxies. However, we have compared
the radio--infrared properties of the Luminous FIR Galaxy Sample with
the properties of Radio-Quiet QSOs under the working hypotheses that: 1)
The FIR luminosity of the Infrared Galaxies represents optical/UV
radiation, absorbed and re-radiated by dust. 2) The observed radio
emission from Radio Quiet QSOs comes from compact, high-T_b cores like
those found for the Luminous FIR Galaxies. Under these assumptions the
Luminous FIR Galaxies and Radio-Quiet QSOs show a common, uniform
correlation between UV/Optical luminosity and Radio Core luminosity.
This correlation is consistent with a common energy source for the
UV/Optical/Radio core luminosity in both sets of objects and suggests
that the Luminous FIR Galaxies may be powered by AGN. Further
implications for the origin of the emission from Luminous FIR Galaxies
and possible evolutionary relationships with classical AGN will be
discussed.
04/1993; 25:891.
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ABSTRACT: Results are presented of Mk III 18-cm VLBI observations of a small sample of IR-bright galaxies: four starburst galaxies which have no evidence of an AGN from the optical spectrum, two Seyfert 2 galaxies, and one previously unclassified Markarian galaxies. In Mrk 297 the compact radio source lies in an obscure clump near the edge of the galaxy. This is either the first known case of a detached nucleus (presumably as a result of disruption during a merger) or a radio supernova of unprecedented luminosity. No VLBI-scale emission was detected in the nucleus of the starburst galaxy Mrk 201 or in Mrk 928a, although the Seyfert 2 nucleus Mrk 928b was detected. Mrk 620 was found to have a milliarcsecond core, but Mrk 520 was not detected. Optical spectroscopy was obtained for five galaxies, including Mrk 520. It is demonstrated that, in luminous starburst galaxies, an active nucleus can effectively be completely hidden from view in the optical by dust. The results also support a physical or causal connection between classical nuclear activity and the starburst phenomenon.
07/1992;
