Robert A. Benjamin

University of Groningen, Groningen, Groningen, Netherlands

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Publications (121)351.72 Total impact

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    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; 799(1). DOI:10.1088/2041-8205/799/1/L7
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    ABSTRACT: The new generation of IR surveys are revealing and quantifying Galactic features, providing an improved 3-D interpretation of our own Galaxy. We present an analysis of the global distribution of dust clouds in the bulge using the near-IR photometry of 157 million stars from the VVV Survey. We investigate the color magnitude diagram of the Milky Way bulge which shows a red giant clump of core He burning stars that is split in two color components, with a mean color difference of (Z-Ks)=0.55 magnitudes equivalent to A_V=2.0 magnitudes. We conclude that there is an optically thick dust lane at intermediate latitudes above and below the plane, that runs across several square degrees from l=-10 deg to l=+10 deg. We call this feature the "Great Dark Lane". Although its exact distance is uncertain, it is located in front of the bulge. The evidence for a large-scale great dark lane within the Galactic bulge is important in order to constrain models of the barred Milky Way bulge and to compare our galaxy with external barred galaxies, where these kinds of features are prominent. We discuss two other potential implications of the presence of the Great Dark Lane for microlensing and bulge stellar populations studies.
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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). DOI:10.1088/0004-6256/148/6/127 · 4.05 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.
    The Astrophysical Journal 07/2014; 797(1). DOI:10.1088/0004-637X/797/1/53 · 6.28 Impact Factor
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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). DOI:10.1088/0004-637X/787/2/106 · 6.28 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). DOI:10.1088/0067-0049/212/1/13 · 14.14 Impact Factor
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    Alex S. Hill · L. M. Haffner · R. A. Benjamin · M. Gostisha · K. Barger
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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.
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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). DOI:10.1088/0004-637X/777/1/55 · 6.28 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). DOI:10.1088/0004-637X/777/1/19 · 6.28 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. DOI:10.1088/0004-6256/146/3/64 · 4.05 Impact Factor
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    ABSTRACT: Context. The ESO public survey VISTA variables in the Vía Láctea (VVV) started in 2010. VVV targets 562 sq. deg in the Galactic bulge and an adjacent plane region and is expected to run for about five years. Aims: We describe the progress of the survey observations in the first observing season, the observing strategy, and quality of the data obtained. Methods: The observations are carried out on the 4-m VISTA telescope in the ZYJHKs filters. In addition to the multi-band imaging the variability monitoring campaign in the Ks filter has started. Data reduction is carried out using the pipeline at the Cambridge Astronomical Survey Unit. The photometric and astrometric calibration is performed via the numerous 2MASS sources observed in each pointing. Results: The first data release contains the aperture photometry and astrometric catalogues for 348 individual pointings in the ZYJHKs filters taken in the 2010 observing season. The typical image quality is 0.9 arcsec {-1.0 arcsec}. The stringent photometric and image quality requirements of the survey are satisfied in 100% of the JHKs images in the disk area and 90% of the JHKs images in the bulge area. The completeness in the Z and Y images is 84% in the disk, and 40% in the bulge. The first season catalogues contain 1.28 × 108 stellar sources in the bulge and 1.68 × 108 in the disk area detected in at least one of the photometric bands. The combined, multi-band catalogues contain more than 1.63 × 108 stellar sources. About 10% of these are double detections because of overlapping adjacent pointings. These overlapping multiple detections are used to characterise the quality of the data. The images in the JHKs bands extend typically 4 mag deeper than 2MASS. The magnitude limit and photometric quality depend strongly on crowding in the inner Galactic regions. The astrometry for Ks = 15-18 mag has rms 35-175 mas. Conclusions: The VVV Survey data products offer a unique dataset to map the stellar populations in the Galactic bulge and the adjacent plane and provide an exciting new tool for the study of the structure, content, and star-formation history of our Galaxy, as well as for investigations of the newly discovered star clusters, star-forming regions in the disk, high proper motion stars, asteroids, planetary nebulae, and other interesting objects. Based on observations taken within the ESO VISTA Public Survey VVV, Programme ID 179.B-2002.
    Astronomy and Astrophysics 08/2013; 537:A107. DOI:10.1051/0004-6361/201118407 · 4.48 Impact Factor
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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.
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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.
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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.
  • 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.
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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.
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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.
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    The Astrophysical Journal 12/2012; 761(2):189. DOI:10.1088/0004-637X/761/2/189 · 6.28 Impact Factor
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    ABSTRACT: We apply a wind model, driven by combined cosmic-ray and thermal-gas pressure, to the Milky Way, and show that the observed Galactic diffuse soft X-ray emission can be better explained by a wind than by previous static gas models. We find that cosmic-ray pressure is essential to driving the observed wind. Having thus defined a “best-fit ” model for a Galactic wind, we explore variations in the base parameters and show how the wind’s properties vary with changes in gas pressure, cosmic-ray pressure and density. We demonstrate the importance of cosmic rays in launching winds, and the effect cosmic rays have on wind dynamics. In addition, this model adds support to the hypothesis of Breitschwerdt and collaborators that such a wind may help explain the relatively small gradient observed in γ-ray emission as a function of galactocentric radius. Subject headings: ISM:outflows – ISM:cosmic rays – ISM:magnetic fields – Galaxy:evolution – X-rays:diffuse background 1.
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    Richard J. Rand · Kenneth Wood · Robert A. Benjamin
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    ABSTRACT: We present infrared spectroscopy from the Spitzer Space Telescope at one disk position and two positions at a height of 1 kpc from the disk in the edge-on spiral NGC 891, with the primary goal of studying halo ionization. Our main result is that the [Ne III]/[Ne II] ratio, which provides a measure of the hardness of the ionizing spectrum free from the major problems plaguing optical line ratios, is enhanced in the extraplanar pointings relative to the disk pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for the effects of radiation field hardening, we find that this trend cannot be reproduced by any plausible photo-ionization model, and that a secondary source of ionization must therefore operate in gaseous halos. We also present the first spectroscopic detections of extraplanar PAH features in an external normal galaxy. If they are in an exponential layer, very rough emission scale-heights of 330 − 530 pc are implied for the various features. Extinction may be non-negligible in the midplane and reduce these scale-heights significantly. There is little significantMuch information regarding the source(s) of ionization have come from optical emission line ratios in the Milky Way and external galaxies (e.g., Rand 1997, 1998b; Greenawalt, Walterbos, & Braun – 2 – variation in the relative emission from the various features between disk and extraplanar environment. Only the 17.4 µm feature is significantly enhanced in the extraplanar gas compared to the other features, possibly indicating a preference for larger PAHs in the halo.

Publication Stats

3k Citations
351.72 Total Impact Points

Institutions

  • 2014
    • University of Groningen
      • Kapteyn Astronomical Institute
      Groningen, Groningen, Netherlands
  • 2005–2014
    • University of Wisconsin - Whitewater
      • Department of Physics
      Whitewater, Wisconsin, United States
  • 2012
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1998–2008
    • University of Wisconsin, Madison
      • Department of Physics
      Madison, MS, United States
  • 2006
    • University of New Mexico
      • Department of Physics & Astronomy
      Albuquerque, New Mexico, United States