R. A. Benjamin

University of Wisconsin - Whitewater, Whitewater, Wisconsin, United States

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Publications (113)366.55 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Giant lobes of plasma extend 55 degrees above and below the Galactic Center, glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP haze) and polarized radio waves. We use ultraviolet absorption-line spectra from the Hubble Space Telescope to constrain the velocity structure of the outflowing gas within these regions, targeting the quasar PDS 456 (Galactic coordinates l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical structure seen in hard X-ray and gamma-ray emission near the base of the northern Fermi Bubble. We report two high-velocity metal absorption components, at v_LSR=-235 and +250 km/s, which cannot be explained by co-rotating gas in the Galactic disk or halo. Their velocities are suggestive of an origin on the front and back side of an expanding biconical outflow emanating from the Galactic Center. We develop simple kinematic biconical outflow models that can explain these observed profiles with an outflow velocity of ~900 km/s and a full opening angle of ~110 degrees (matching the X-ray bicone). This indicates Galactic Center activity over the last ~2.5-4.0 Myr, in line with age estimates of the Fermi Bubbles. The observations illustrate the use of UV absorption-line spectroscopy to probe the properties of swept-up gas venting into the Fermi Bubbles.
    12/2014;
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    ABSTRACT: We present a study of the vertical structure of the gaseous and stellar disks in a sample of edge-on galaxies (NGC 4157, 4565, and 5907) using BIMA/CARMA 12CO (J = 1 --> 0), VLA H I, and Spitzer 3.6 micron data. In order to take into account projection effects when we measure the disk thickness as a function of radius, we first obtain the inclination by modeling the radio data. Using the measurement of the disk thicknesses and the derived radial profiles of gas and stars, we estimate the corresponding volume densities and vertical velocity dispersions. Both stellar and gas disks have smoothly varying scale heights and velocity dispersions, contrary to assumptions of previous studies. Using the velocity dispersions, we find that the gravitational instability parameter Q follows a fairly uniform profile with radius and is greater than or equal to 1 across the star forming disk. The star formation law has a slope that is significantly different from those found in more face-on galaxy studies, both in deprojected and pixel-by-pixel plots. Midplane gas pressure based on the varying scale heights and velocity dispersions appears to roughly hold a power-law correlation with the midplane volume density ratio.
    The Astronomical Journal 08/2014; 148(6). · 4.97 Impact Factor
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    ABSTRACT: The very long and thin infrared dark cloud "Nessie" is even longer than had been previously claimed, and an analysis of its Galactic location suggests that it lies directly in the Milky Way's mid-plane, tracing out a highly elongated bone-like feature within the prominent Scutum-Centaurus spiral arm. Re-analysis of mid-infrared imagery from the Spitzer Space Telescope shows that this IRDC is at least 2, and possibly as many as 5 times longer than had originally been claimed by Nessie's discoverers (Jackson et al. 2010); its aspect ratio is therefore at least 300:1, and possibly as large as 800:1. A careful accounting for both the Sun's offset from the Galactic plane ($\sim 25$ pc) and the Galactic center's offset from the $(l^{II},b^{II})=(0,0)$ position shows that the latitude of the true Galactic mid-plane at the 3.1 kpc distance to the Scutum-Centaurus Arm is not $b=0$, but instead closer to $b=-0.4$, which is the latitude of Nessie to within a few pc. An analysis of the radial velocities of low-density (CO) and high-density (${\rm NH}_3$) gas associated with the Nessie dust feature suggests that Nessie runs along the Scutum-Centaurus Arm in position-position-velocity space, which means it likely forms a dense `spine' of the arm in real space as well. The Scutum-Centaurus arm is the closest major spiral arm to the Sun toward the inner Galaxy, and, at the longitude of Nessie, it is almost perpendicular to our line of sight, making Nessie the easiest feature to see as a shadow elongated along the Galactic Plane from our location. Future high-resolution dust mapping and molecular line observations of the harder-to-find Galactic "bones" should allow us to exploit the Sun's position above the plane to gain a (very foreshortened) view "from above" of the Milky Way's structure.
    07/2014;
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    ABSTRACT: We present Wisconsin H-Alpha Mapper [S II] {\lambda}6716 and H{\alpha} spectroscopic maps of the warm ionized medium (WIM) in the Scutum-Centaurus Arm at Galactic longitudes 310{\deg} < l < 345{\deg}. Using extinction-corrected H{\alpha} intensities (IH{\alpha}c), we measure an exponential scale height of electron density-squared in the arm of H_ne^2 = 0.30 kpc (assuming a distance of 3.5 kpc), intermediate between that observed in the inner Galaxy and in the Perseus Arm. The [S II]/H{\alpha} line ratio is enhanced at large |z| and in sightlines with faint IH{\alpha}c. We find that the [S II]/H{\alpha} line ratio has a power law relationship with IH{\alpha}c from a value of ~=1.0 at IH{\alpha}c < 0.2 R (Rayleighs) to a value of ~=0.08 at IH{\alpha}c >= 100 R. The line ratio is better correlated with H{\alpha} intensity than with height above the plane, indicating that the physical conditions within the WIM vary systematically with electron density. We argue that the variation of the line ratio with height is a consequence of the decrease of electron density with height. Our results reinforce the well-established picture in which the diffuse H{\alpha} emission is due primarily to emission from in situ photoionized gas, with scattered light only a minor contributor.
    The Astrophysical Journal 04/2014; 787(2). · 6.73 Impact Factor
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    ABSTRACT: We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multi-wavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3' of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding-wind binaries, X-ray binaries, and magnetars. There is also a fifth population that is still unidentified but, based on its X-ray and infrared properties, likely comprise partly of Galactic sources and partly of active galactic nuclei.
    The Astrophysical Journal Supplement Series 03/2014; 212(1). · 16.24 Impact Factor
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    ABSTRACT: We present early results from the Wisconsin H-Alpha Mapper (WHAM) Southern Sky Survey, the completion of the first kinematically-resolved, all sky survey of diffuse H-Alpha emission from the Galaxy. We show spectroscopic maps of H-Alpha and [S II] lambda 6716 emission from the warm ionized medium in the Scutum-Centaurus Arm in the inner Galaxy. We measure the scale height of the ionized gas in the arm, finding it to be less than that observed in the Perseus Arm. The [S II]/Halpha line ratio is enhanced in faint sightlines. The trends in the line ratio are most consistent with emission from in situ photoionized gas in which the physical conditions are primarily a function of density. WHAM is supported by the NSF through grant AST 1108911.
    01/2014;
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    ABSTRACT: We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures towards extragalactic radio sources behind the Smith Cloud, new HI observations from the Green Bank Telescope, and a spectroscopic map of H{\alpha} from the Wisconsin H-Alpha Mapper Northern Sky Survey. There are enhancements in rotation measure of approximately 100 rad m^(-2) which are generally well correlated with decelerated H{\alpha} emission. We estimate a lower limit on the line-of-sight component of the field of approximately 8 {\mu}G along a decelerated filament; this is a lower limit due to our assumptions about the geometry. No RM excess is evident in sightlines dominated by HI or H{\alpha} at the velocity of the Smith Cloud. The smooth H{\alpha} morphology of the emission at the Smith Cloud velocity suggests photoionization by the Galactic ionizing radiation field as the dominant ionization mechanism, while the filamentary morphology and high (approximately 1 Rayleigh) H{\alpha} intensity of the lower-velocity magnetized ionized gas suggests an ionization process associated with shocks due to interaction with the Galactic interstellar medium. The presence of the magnetic field may contribute to the survival of high velocity clouds like the Smith Cloud as they move from the Galactic halo to the disk. We expect these data to provide a test for magnetohydrodynamic simulations of infalling gas.
    The Astrophysical Journal 09/2013; 777(1). · 6.73 Impact Factor
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    ABSTRACT: We find evidence for the impact of infalling, low-metallicity gas on the Galactic disk. This is based on FUV absorption line spectra, 21-cm emission line spectra, and FIR mapping to estimate the abundance and physical properties of IV21 (IVC135+54-45), a galactic intermediate-velocity molecular cloud (IVMC) that lies ~300 pc above the disk. The metallicity of IV21 was estimated using observations toward the sdB star PG1144+615, located at a projected distance of 16 pc from the cloud's densest core, by measuring ion and HI column densities for comparison with known solar abundances. Despite the cloud's bright FIR emission and large column densities of molecular gas as traced by CO, we find that it has a sub-solar metallicity of log(Z/Z_Sun)=-0.43 +/- 0.12dex. IV21 is thus the first known sub-solar metallicity cloud in the solar neighborhood. In contrast, most intermediate-velocity clouds (IVC) have near-solar metallicities and are believed to originate in the Galactic Fountain. The cloud's low metallicity is also atypical for Galactic molecular clouds, especially in the light of the bright FIR emission which suggest a substantial dust content. The measured I_100mu/N(HI) ratio is a factor of three below the average found in high latitude \HI clouds within the solar neighborhood. We argue that IV21 represents the impact of an infalling, low-metallicity high-velocity cloud (HVC) that is mixing with disk gas in the lower Galactic halo.
    The Astrophysical Journal 08/2013; 777(1). · 6.73 Impact Factor
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    ABSTRACT: Open stellar clusters are extremely valuable probes of Galactic structure, star formation, kinematics, and chemical abundance patterns. Near-infrared (NIR) data have enabled the detection of hundreds of clusters hidden from optical surveys, and mid-infrared (MIR) data are poised to offer an even clearer view into the most heavily obscured parts of the Milky Way. We use new MIR images from the Spitzer GLIMPSE-360, Cyg-X, and SMOG surveys to visually identify a large number of open cluster candidates in the outer disk of the Milky Way (65° < l < 265°). Using NIR color-magnitude diagrams, stellar isochrones, and stellar reddening estimates, we derive cluster parameters (metallicity, distance, reddening) for those objects without previous identification and/or parameters in the literature. In total, we present coordinates and sizes of 20 previously unknown open cluster candidates; for 7 of these we also present metallicity, distance, and reddening values. In addition, we provide the first estimates of these values for nine clusters that had been previously cataloged. We compare our cluster sizes and other derived parameters to those in the open cluster catalog of Dias et al. and find strong similarities except for a higher mean reddening for our objects, which signifies our increased detection sensitivity in regions of high extinction. The results of this cluster search and analysis demonstrate the ability of MIR imaging and photometry to augment significantly the current census of open clusters in the Galaxy.
    The Astronomical Journal 08/2013; 146(3):64. · 4.97 Impact Factor
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    ABSTRACT: The GLIMPSE and additional surveys observed the full 360 degrees of longitude of the Galactic Plane with a width that ranged from 2-9 degrees in latitude using the Spitzer Space Telescope. Some regions of the survey include Spitzer MIPS 24 micron as well as the 4 IRAC bands (3.6, 4.5, 5.8, and 8 microns), while others were only observed in 3.6 and 4.5 microns during the “warm mission.” Here we tabulate and describe the different surveys and observing modes that were processed by the GLIMPSE team. We describe the data products (point source lists and cleaned mosaic images) and how to get the data.
    06/2013;
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    ABSTRACT: Based on the 3.6 and 4.5 micron images from the Spitzer GLIMPSE360 survey, combined with the WISE 12 micron image, we identified a sample of star formation regions in the outer Galaxy. We study their correlation with the CO clouds from "The Outer Galaxy Molecular Cloud Catalog" (Brunt et al. 2003; based on the FCRAO survey), as well as with known H II regions and masers. We investigate the CO and mid-IR properties as a function of distance and the association with various dynamical features in the Galaxy.
    06/2013;
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    ABSTRACT: Velocity-resolved surveys of the Galactic plane with the Wisconsin H-alpha Mapper indicate a thick distribution of ~1 kpc for the ionized gas layer of the Galaxy, but also show that the emission is enhanced in the vicinity of spiral arms. We characterize the vertical scale-heights of the Perseus Arm and Scutum-Centaurus Arm as a function of azimuth and compare the structure of these arms in ionized gas (from WHAM) and neutral gas (from the Leiden-Argentina-Bonn survey). We then explore the hypothesis that these arms are the sources of correlated outflow from the Galactic disk and compare the observed velocity structure of the arms with different predictions for outflow kinematics.
    06/2013;
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    ABSTRACT: We present an preliminary analysis of an on-going Wisconsin H-alpha Mapper (WHAM) survey of the fourth quadrant of the Galactic plane in diffuse emission from [S II] 6716 A, covering fourth galactic quadrant (Galactic longitude=270-360 degrees) and galactic latitude |b|<12 degrees. Because of the high atomic mass and narrow thermal line widths of sulfur (as compared to hydrogen or nitrogen), this emission line serves as the best tracer of the kinematics of the warm ionized medium in the mid plane of the Galaxy. We detect extensive emission at velocities as negative as -100 km/s indicating that we are seeing much further into the center of the Milky Way than was found for the first quadrant. We discuss constraints on the velocity and vertical density structure of this gas, and compare this distribution with what is observed in CO and HI surveys. This work was partially supported by the National Science Foundation's REU program through NSF Award AST-1004881. WHAM research and on-going operations are supported by NSF award AST-1108911.
    01/2013;
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    ABSTRACT: The Smith Cloud is a high velocity cloud (HVC) with a cometary morphology indicating an ongoing interaction with the Galactic interstellar medium. Although it is among the best-studied HVCs and the only large HVC mapped in both H I and Halpha, its origin remains unknown. Most formation scenarios have difficulty explaining its coherence after passage through the Galactic halo, but magnetic fields may help to stabilize HVCs against disruption. We present measurements of Faraday rotation of extragalactic radio sources behind the Smith Cloud derived from the NRAO VLA Sky Survey as well as new Karl G. Jansky Very Large Array observations of Faraday rotation. The data show evidence of Faraday rotation due to the cloud; the Faraday rotation is better correlated with the Halpha emission than with the H I emission.
    01/2013;
  • Ed Churchwell, Robert A. Benjamin
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    ABSTRACT: As infrared surveys have reached optical-quality angular resolution, they have revealed new information on the stellar, interstellar, and star-formation components of the Galaxy. The distance to the Galactic center appears to be known to within 5%: R o = 8. 0 ± 0. 4 kpc. Measurements of the stellar scalelength of the disk, R d = 2-4 kpc, continue to show a large range; the origin of this scatter needs to be understood. The exponential disk does not continue into the center of the Galaxy, with an inner radius of R h ˜ 3 kpc. Claims exist for a truncation, or change in scalelength, in the outer disk, but are not yet confirmed. The stellar disk is warped, with a similar nonsymmetric azimuthal dependence as the HI disk, but a lower amplitude and uncertain radial extent. There is extensive evidence for two non-axisymmetric structures in the inner galaxy: the Galactic bar (or triaxial bulge) and the Long Bar, which differ in angle by ˜ 20∘. The existence of an inner (nuclear) bar seems likely, but studies have not converged on its parameters. There is no compelling evidence for a ring in stellar mass, but a case can be made for a star-forming ring.
    Planets, Stars and Stellar Systems. Volume 5: Galactic Structure and Stellar Populations. 01/2013;
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    The Astrophysical Journal 12/2012; 761(2):189. · 6.73 Impact Factor
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    ABSTRACT: We present a self-consistent three-dimensional Monte-Carlo radiative transfer model of the stellar and dust emission in the Milky-Way, and have computed synthetic observations of the 3.6 to 100 microns emission in the Galactic mid-plane. In order to compare the model to observations, we use the GLIMPSE, MIPSGAL, and IRAS surveys to construct total emission spectra, as well as longitude and latitude profiles for the emission. The distribution of stars and dust is taken from the SKY model, and the dust emissivities includes an approximation of the emission from polycyclic aromatic hydrocarbons in addition to thermal emission. The model emission is in broad agreement with the observations, but a few modifications are needed to obtain a good fit. Firstly, by adjusting the model to include two major and two minor spiral arms rather than four equal spiral arms, the fit to the longitude profiles for |l|>30 degrees can be improved. Secondly, introducing a deficit in the dust distribution in the inner Galaxy results in a better fit to the shape of the IRAS longitude profiles at 60 and 100 microns. With these modifications, the model fits the observed profiles well, although it systematically under-estimates the 5.8 and 8.0 microns fluxes. One way to resolve this discrepancy is to increase the abundance of PAH molecules by 50% compared to the original model, although we note that changes to the dust distribution or radiation field may provide alternative solutions. Finally, we use the model to quantify which stellar populations contribute the most to the heating of different dust types, and which stellar populations and dust types contribute the most to the emission at different wavelengths.
    Astronomy and Astrophysics 08/2012; · 5.08 Impact Factor
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    ABSTRACT: We present the first results from the CHANG-ES survey, a new survey of 35 edge-on galaxies to search for both in-disk as well as extra-planar radio continuum emission. The motivation and science case for the survey are presented in a companion paper (Paper I). In this paper (Paper II), we outline the observations and data reduction steps required for wide-band calibration and mapping of EVLA data, including polarization, based on C-array test observations of NGC 4631. With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes at 6 GHz for the test data) we have achieved best rms noise levels of 22 and 3.5 $\mu$Jy beam$^{-1}$ at 1.5 GHz and 6 GHz, respectively. New disk-halo features have been detected, among them two at 1.5 GHz that appear as loops in projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be formed from a single wide-band observation in a single array configuration. This map represents tangent slopes to the intensities within the band centered at 1.5 GHz, rather than fits across widely separated frequencies as has been done in the past and is also the highest spatial resolution spectral index map yet presented for this galaxy. The average spectral index in the disk is $\bar\alpha_{1.5 GHz}\,=\,-0.84\,\pm\,0.05$ indicating that the emission is largely non-thermal, but a small global thermal contribution is sufficient to explain a positive curvature term in the spectral index over the band. Two specific star forming regions have spectral indices that are consistent with thermal emission. Polarization results (uncorrected for internal Faraday rotation) are consistent with previous observations and also reveal some new features. On broad scales, we find strong support for the notion that magnetic fields constrain the X-ray emitting hot gas.
    The Astronomical Journal 05/2012; 144(2). · 4.97 Impact Factor
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    ABSTRACT: We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.
    The Astronomical Journal 05/2012; 144(2). · 4.97 Impact Factor
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    ABSTRACT: We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near infrared (2MASS and UKIDSS) surveys. VLBI parallax measurements of masers around massive young stars provide the most direct and bias-free measurement of the distance to these dark clouds. We compare the extinction distance estimates to these maser parallax distances. We also compare these distances to kinematic distances, including recent re-calibrations of the Galactic rotation curve. The extinction distance methods agree with the maser parallax distances (within the errors) between 66% and 100% of the time (depending on method and input survey) and between 85% and 100% of the time outside of the crowded Galactic center. Although the sample size is small, extinction distance methods reproduce maser parallax distances better than kinematic distances; furthermore, extinction distance methods do not suffer from the kinematic distance ambiguity. This validation gives us confidence that these extinction methods may be extended to additional dark clouds where maser parallaxes are not available.
    The Astrophysical Journal 04/2012; 751(2). · 6.73 Impact Factor

Publication Stats

2k Citations
366.55 Total Impact Points

Institutions

  • 2005–2014
    • University of Wisconsin - Whitewater
      • Department of Physics
      Whitewater, Wisconsin, United States
  • 2013
    • University of Virginia
      • Department of Astronomy
      Charlottesville, Virginia, United States
  • 2012
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
    • American Museum of Natural History
      New York City, New York, United States
  • 1998–2008
    • University of Wisconsin, Madison
      • • Department of Astronomy
      • • Department of Physics
      Mississippi, United States
  • 2006
    • University of New Mexico
      • Department of Physics & Astronomy
      Albuquerque, New Mexico, United States