Amanda A. Kepley

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

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Publications (34)78.33 Total impact

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    ABSTRACT: A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.
    04/2015; 808(1). DOI:10.1088/2041-8205/808/1/L1
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    ABSTRACT: We mapped 3 mm continuum and line emission from the starburst galaxy M82 using the Combined Array for Research in Millimeter-wave Astronomy. We targeted the HCN, HCO$^{+}$, HNC, CS and HC$_{3}$N lines, but here we focus on the HCN and HCO$^{+}$ emission. The map covers a field of 1.2' with a ~5" resolution. The HCN and HCO$^{+}$ observations are combined with single dish images. The molecular gas in M82 had been previously found to be distributed in a molecular disk, coincident with the central starburst, and a galactic scale outflow which originates in the central starburst. With the new short spacings-corrected maps we derive some of the properties of the dense molecular gas in the base of the outflow. From the HCN and HCO$^{+}$ J=(1-0) line emission, and under the assumptions of the gas being optically thin and in local thermodynamic equilibrium, we place lower limits to the amount of dense molecular gas in the base of the outflow. The lower limits are $7\times10^{6}$ $M_{\odot}$ and $21\times10^{6}$ $M_{\odot}$, or $\gtrsim2\%$ of the total molecular mass in the outflow. The kinematics and spatial distribution of the dense gas outside the central starburst suggests that it is being expelled through chimneys. Assuming a constant outflow velocity, the derived outflow rate of dense molecular gas is $\geq0.3$ $M_{\odot}$ yr$^{-1}$, which would lower the starburst lifetime by $\geq5\%$. The energy required to expel this mass of dense gas is $(1-10)\times10^{52}$ erg.
    The Astrophysical Journal 10/2014; 797(2). DOI:10.1088/0004-637X/797/2/134 · 6.28 Impact Factor
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    ABSTRACT: Compact groups of galaxies provide a unique environment to study the evolution of galaxies amid continuous gravitational encounters. These groups provide a nearby environment with conditions similar to those in the earlier universe when galaxies were assembled and give us the opportunity to witness hierarchical formation in progress. In order to understand how the compact group environment affects galaxy evolution, it is important to study the gas and dust processes in these groups. Single-dish neutral hydrogen (HI) observations of compact groups allow us to measure the HI mass of each group in our survey, as well as to determine whether there is a significant amount of HI in the intragroup medium. We compare the HI to stellar mass ratio with mid-IR indicators of star formation and optical [g-r] color to search for correlations between gas content and star formation activity. We find that quiescent galaxies tend to live in HI-poor groups, and galaxies with active star formation are more commonly found in HI-rich groups regardless of the location and distribution of the HI gas, though we do see "rogue" galaxies whose activity does not correlate with group HI content. Ultimately, we will use these results to understand gas processing and the ISM in environments similar to that in the earlier Universe.
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    ABSTRACT: Luminous compact blue galaxies (LCBGs) are a heterogeneous class of galaxies that are currently experiencing a burst of star formation. While they were common at 1, they are an order of magnitude rarer in the local universe. This discrepancy implies that this type of galaxy is a transient phase; these galaxies must evolve relatively rapidly. While it is known that LCBGs must stop forming stars in the near future and subsequently evolve, it is not known when their star formation ends, what processes cause the quenching, and what these galaxies become. To probe the global properties of the galaxies' starbursts, we have observed 40 local (D < 80 Mpc) LCBGs with the Caltech Continuum Backend and MUSTANG bolometer array on the Green Bank Telescope at frequencies where tracers of very recent star formation are strong. When this data is combined with archival observations at radio and infrared wavelengths, we can form a picture of the recent star formation histories of these galaxies. In modeling LCBGs' SEDs, we can constrain their timescales of recent star formation and correlate their star formation properties with other known properties of LCBGs. From analysis of these properties and timescales, we can determine the likelihood of their star formation continuing at its current pace, and thus constrain their evolutionary paths.
  • Susan Schmitz · A. A. Kepley · R. Beck · C. C. Lang · E. M. Wilcots
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    ABSTRACT: The magnetic fields of irregular galaxies may be important in their interstellar medium; however, not much is currently known about them. We have obtained continuum polarization observations from the VLA and Effelsberg telescopes of the irregular galaxy NGC 1156 to increase the sample of irregular galaxies with observed magnetic field structures. Significant polarization emission was observed at all three observed frequencies in the galaxy which shows the presence of the magnetic field and allows for study of its properties. From these data we obtain the synchrotron fraction, magnetic field strength, and magnetic field orientation. The rotation measure between the 6cm and 3cm observations provides information about the magnetic field strength and direction along the line of sight. Finally we compare the magnetic field strengths with the other forces in the galaxy including the hot, warm, and cold gas pressures to determine the dominant force in the galaxy and role of the magnetic field.
  • Amanda A. Kepley · A. K. Leroy · D. T. Frayer · A. Usero · J. Marvil · F. Walter
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    ABSTRACT: In both the Milky Way and nearby galaxies, the presence of dense molecular gas is correlated with recent star formation, suggesting that the formation of this gas may represent a key regulating step in the star formation process. Testing this idea requires wide-area, high-resolution maps of dense molecular gas in galaxies to explore how local physical conditions drive dense gas formation. Until now, these observations have been limited by the faintness of dense gas tracers like HCN and HCO+, but new instruments like the 4mm receiver on Robert C. Byrd Green Bank Telescope (GBT) -- the largest single-dish millimeter telescope -- are poised to change this picture. We present GBT maps of the dense gas tracers HCN and HCO+ in the prototypical nearby starburst galaxy M82. The HCN and HCO+ in the disk of M82 correlates both with recent star formation and the diffuse molecular gas and shows kinematics consistent with a rotating torus. HCO+ emission is also associated with the outflow of molecular gas previously identified in CO. These observations mark the first time that dense molecular gas like HCO+ has been associated with an outflow in a nearby galaxy and suggests that the outflow of dense molecular gas from the center of galaxies like M82 may regulate the star formation globally. Finally, the CO-to-HCN and CO-to-HCO+ line ratios reveal that there is more dense gas at the center of M82, pointing to the starburst as a key driver of this relationship. These results establish that the GBT can efficiently map the dense molecular gas at 90 GHz in nearby galaxies; this capability will increase further with the 16-element feed array currently being built for the GBT.
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    ABSTRACT: We present new KU band radio continuum imaging of three nearby starburst dwarf galaxies, NGC4449, NGC2366 and Holmberg II, obtained with the Karl G. Jansky Very Large Array. The wide bandwidth (2 GHz) allows a sensitive examination of the nature of radio continuum emission between 13 and 15 GHz. We compare these continuum images with tracers of very recent star formation (H alpha emission, probing the most recent few Myr) and with tracers of intermediate timescale star formation (GALEX near UV emission, probing timescales of ~200 Myr). When summed over the entire 2 GHz bandpass, the radio continuum emission morphology is qualitatively similar to that of the regions of highest H alpha surface brightness; however, in all three galaxies, there are regions of H alpha emission that are not detected at this sensitivity level in the 14 GHz continuum. Imaging individual 128 MHz spectral windows allows us to examine the variation of flux density with frequency on a spatially resolved basis. Combining with lower-frequency radio continuum imaging allows a separation of thermal and non-thermal emission components. A.N. and J.M.C. acknowledge support from Macalester College that made this work possible.
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    Amanda A. Kepley · Amy E. Reines · Kelsey E. Johnson · Lisa May Walker
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    ABSTRACT: The extent to which star formation varies in galaxies with low masses, low metallicities, and high star formation rate surface densities is not well-constrained. To gain insight into star formation under these physical conditions, this paper estimates the ionizing photon fluxes, masses, and ages for young massive clusters in the central region of II Zw 40 -- the prototypical low-metallicity dwarf starburst galaxy -- from radio continuum and optical observations. Discrete, cluster-sized sources only account for half the total radio continuum emission; the remainder is diffuse. The young (<5 Myr) central burst has a star formation rate surface density that significantly exceeds that of the Milky Way. Three of the 13 sources have ionizing photon fluxes (and thus masses) greater than R136 in 30 Doradus. Although isolating the effects of galaxy mass and metallicity is difficult, the HII region luminosity function and the internal extinction in the center of II Zw 40 appear to be primarily driven by a merger-related starburst. The relatively flat HII region luminosity function may be the result of an increase in ISM pressure during the merger and the internal extinction is similar to that generated by the clumpy and porous dust in other starburst galaxies.
    The Astronomical Journal 12/2013; 147(2). DOI:10.1088/0004-6256/147/2/43 · 4.05 Impact Factor
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    ABSTRACT: Observations of the Milky Way and nearby galaxies show that dense molecular gas correlates with recent star formation, suggesting that the formation of this gas phase may help regulate star formation. A key test of this idea requires wide-area, high-resolution maps of dense molecular gas in galaxies to explore how local physical conditions drive dense gas formation, but these observations have been limited because of the faintness of dense gas tracers like HCN and HCO+. Here we demonstrate the power of the Robert C. Byrd Green Bank Telescope -- the largest single-dish millimeter radio telescope -- for mapping dense gas in galaxies by presenting the most sensitive maps yet of HCN and HCO+ in the starburst galaxy M82. The HCN and HCO+ in the disk of this galaxy correlates with both recent star formation and more diffuse molecular gas and shows kinematics consistent with a rotating torus. The HCO+ emission extending to the north and south of the disk is coincident with the outflow previously identified in CO and traces the eastern edge of the hot outflowing gas. The central starburst region has a higher ratio of star formation to dense gas than the outer regions, pointing to the starburst as a key driver of this relationship. These results establish that the GBT can efficiently map the dense molecular gas at 90 GHz in nearby galaxies, a capability that will increase further with the 16 element feed array under construction.
    The Astrophysical Journal 12/2013; 780(1). DOI:10.1088/2041-8205/780/1/L13 · 6.28 Impact Factor
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    ABSTRACT: We observed radio continuum emission in 27 local (D < 70 Mpc) star-forming galaxies with the Robert C. Byrd Green Bank Telescope between 26 GHz and 40 GHz using the Caltech Continuum Backend. We obtained detections for 22 of these galaxies at all four sub-bands and four more marginal detections by taking the average flux across the entire bandwidth. This is the first detection (full or marginal) at these frequencies for 22 of these galaxies. We fit spectral energy distributions (SEDs) for all of the four-sub-band detections. For 14 of the galaxies, SEDs were best fit by a combination of thermal free-free and nonthermal synchrotron components. Eight galaxies with four-sub-band detections had steep spectra that were only fit by a single nonthermal component. Using these fits, we calculated supernova rates, total number of equivalent O stars, and star formation rates within each ~23 arcsecond beam. For unresolved galaxies, these physical properties characterize the galaxies' recent star formation on a global scale. We confirm that the radio-far-infrared correlation holds for the unresolved galaxies' total 33 GHz flux regardless of their thermal fractions, though the scatter on this correlation is larger than that at 1.4 GHz. In addition, we found that for the unresolved galaxies, there is an inverse relationship between the ratio of 33 GHz flux to total far-infrared flux and the steepness of the galaxy's spectral index between 1.4 GHz and 33 GHz. This relationship could be an indicator of the timescale of the observed episode of star formation.
    The Astrophysical Journal 11/2013; 780(1). DOI:10.1088/0004-637X/780/1/19 · 6.28 Impact Factor
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    ABSTRACT: We present the various science cases for building Band 1 receivers as part of ALMA's ongoing Development Program. We describe the new frequency range for Band 1 of 35-52 GHz, a range chosen to maximize the receiver suite's scientific impact. We first describe two key science drivers: 1) the evolution of grains in protoplanetary disks and debris disks, and 2) molecular gas in galaxies during the era of re-ionization. Studies of these topics with Band 1 receivers will significantly expand ALMA's Level 1 Science Goals. In addition, we describe a host of other exciting continuum and line science cases that require ALMA's high sensitivity and angular resolution. For example, ALMA Band 1 continuum data will probe the Sunyaev-Zel'dovich Effect in galaxy clusters, Very Small Grains and spinning dust, ionized jets from young stars, spatial and flaring studies of Sgr A*, the acceleration sites of solar flares, pulsar wind nebulae, radio supernovae, and X-ray binaries. Furthermore, ALMA Band 1 line data will probe chemical differentiation in cloud cores, complex carbon chain molecules, extragalactic radio recombination lines, masers, magnetic fields through Zeeman effect measurements, molecular outflows from young stars, the co-evolution of star formation and active galactic nuclei, and the molecular content of galaxies at z ~ 3. ALMA provides similar to better sensitivities than the JVLA over 35-50 GHz, with differences increasing with frequency. ALMA's smaller antennas and shorter baselines, greater number of baselines, and single-dish capabilities, however, give it a significant edge for observing extended emission, making wide-field maps (mosaics), or attaining high image fidelity, as required by the described science cases.
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    ABSTRACT: We present ALMA observations of 30 Doradus -- the highest resolution view of molecular gas in an extragalactic star formation region to date (~0.4pc x 0.6pc). The 30Dor-10 cloud north of R136 was mapped in 12CO 2-1, 13CO 2-1, C18O 2-1, 1.3mm continuum, the H30alpha recombination line, and two H2CO 3-2 transitions. Most 12CO emission is associated with small filaments and clumps (<1pc, ~1000 Msun at the current resolution). Some clumps are associated with protostars, including "pillars of creation" photoablated by intense radiation from R136. Emission from molecular clouds is often analyzed by decomposition into approximately beam-sized clumps. Such clumps in 30 Doradus follow similar trends in size, linewidth, and surface density to Milky Way clumps. The 30 Doradus clumps have somewhat larger linewidths for a given size than predicted by Larson's scaling relation, consistent with pressure confinement. They extend to higher surface density at a given size and linewidth compared to clouds studied at 10pc resolution. These trends are also true of clumps in Galactic infrared-dark clouds; higher resolution observations of both environments are required. Consistency of clump masses calculated from dust continuum, CO, and the virial theorem reveals that the CO abundance in 30 Doradus clumps is not significantly different from the LMC mean, but the dust abundance may be reduced by ~2. There are no strong trends in clump properties with distance from R136; dense clumps are not strongly affected by the external radiation field, but there is a modest trend towards lower dense clump filling fraction deeper in the cloud.
    The Astrophysical Journal 07/2013; 774(1). DOI:10.1088/0004-637X/774/1/73 · 6.28 Impact Factor
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    ABSTRACT: Using archival data from ATCA, WHISP, and the Very Large Array, we have analyzed the H I emission of 22 tidal tail regions of the Mullan et al. sample of pairwise interacting galaxies. We have measured the column densities, line-of-sight velocity dispersions, and kinetic energy densities on ~kpc scales. We also constructed a tracer of the line-of-sight velocity gradient over ~10 kpc scales. We compared the distributions of these properties between regions that do and do not contain massive star cluster candidates (MV < –8.5; ~104-106M ☉ as observed in Hubble Space Telescope WFPC2 VI data). In agreement with Maybhate et al., we find that a local, ~kpc-scale column density of log 20.6 cm–2 is frequently required for detecting clustered star formation. This H I gas also tends to be turbulent, with line-of-sight velocity dispersions σlos 10-75 km s–1, implying high kinetic energy densities (log ΣKE > 46 erg pc–2). Thus, high H I densities and pressures, partly determined by the tail dynamical age and other interaction characteristics, are connected to large-scale cluster formation in tidal tails overall. Last, we find that the high mechanical energy densities of the gas are likely not generally due to feedback from star formation. Rather, these properties are more likely to be a cause of star formation than a result.
    The Astrophysical Journal 04/2013; 768(2):194. DOI:10.1088/0004-637X/768/2/194 · 6.28 Impact Factor
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    ABSTRACT: Galactic centers represent a unique environment for star formation. Their high gas surface densities and short crossing times may lead to more efficient star formation than found in the disks of galaxies. Measuring the properties of the young massive clusters in galactic centers is critical to understanding star formation in this environment. One way to do this is to measure the properties of the gas ionized by the young massive clusters. Unfortunately, this gas is still obscured by the natal clouds of dust and gas surrounding the young massive clusters. Therefore, measuring the properties of this gas requires the use of an extinction-free ionized gas tracer like radio recombination lines. This poster presents radio recombination line observations of the center of the nearby face-on spiral galaxy IC 342. These observations constrain the density, filling factor, and kinematics of the obscured ionized gas in the center of this galaxy. The properties of the ionized gas are then used to constrain the properties of the young massive clusters and star formation efficiency in the center of IC 342.
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    ABSTRACT: We present preliminary EVLA/WIDAR snapshot L-band radio continuum imaging of four nearby dwarf galaxies: NGC 2366, NGC 783, WLM, and Pegasus. We exploit the 1 GHz bandwidth of these observations to create images of the sources in multiple 120 Mhz wide spectral windows. The radio continua of NGC 2366 and NGC 784 are compared with multiwavelength images (including GALEX near UV, continuum subtracted H alpha, and Spitzer FIR) to probe the nature of thermal and nonthermal emission on a spatially resolved basis. These data are the first results from a larger and more comprehensive study of the radio continua of nearby star-forming dwarf galaxies with the EVLA.
    American Astronomical Society Meeting Abstracts #219; 01/2012
  • Amanda A. Kepley · D. Balser · L. Chomiuk · W. Goss · K. Johnson · D. Meier · D. Pisano · A. Pope
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    ABSTRACT: I will present results showing the state of the art observations of the ionized gas in young star-forming regions in galaxies with extremely high star formation rates. Measuring the density, mass, and kinematics of the ionized gas in these regions tells us about the state of the interstellar medium in young star-forming regions and about the massive stars themselves. Unfortunately, the state of the ionized gas is difficult to determine in the optical due to the high extinction from the remnants of the surrounding natal cocoon of dust and gas. The ionized gas in these obscured regions can be more accurately traced using radio recombination lines (RRLs). RRLs have been difficult to detect in galaxies other than the brightest dozen nearby starburst galaxies because of the limited sensitivity and bandwidth of the previous generation of radio telescopes. RRL observations of fainter and more distant galaxies are now possible with the recent enormous improvements in radio telescope instrumentation. Our RRL observations showcase the power of these revitalized tracers of the ionized gas in young star-forming galaxies.
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    ABSTRACT: We present large field H I-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM. We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations. The ratio of H I at 1019 atoms cm–2 to optical extents of these galaxies is instead 2-3, which is normal for this type of galaxy. There is no evidence for a truncation in the H I distribution ≥1019 atoms cm–2.
    The Astronomical Journal 10/2011; 142(5):173. DOI:10.1088/0004-6256/142/5/173 · 4.05 Impact Factor
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    ABSTRACT: Radio recombination lines (RRLs) are powerful, extinction-free diagnostics of the ionized gas in young, star-forming regions. Unfortunately, these lines are difficult to detect in external galaxies. We present the results of Expanded Very Large Array (EVLA) observations of the RRL and radio continuum emission at 33 GHz from NGC 253, a nearby nuclear starburst galaxy. We detect the previously unobserved H58alpha and H59alpha RRLs and make simultaneous sensitive measurements of the continuum. We measure integrated line fluxes of 44.3 ± 0.7 W m-2 and 39.9 ± 0.8 W m-2 for the H58alpha and H59alpha lines, respectively. The thermal gas in NGC 253 is kinematically complex with multiple velocity components. We constrain the density of the thermal gas to (1.4-4) × 104 cm-3 and estimate an ionizing photon flux of 1 × 1053 s-1. We use the RRL kinematics and the derived ionizing photon flux to show that the nuclear region of NGC 253 is not gravitationally bound, which is consistent with the outflow of gas inferred from the X-ray and Halpha measurements. The line profiles, fluxes, and kinematics of the H58alpha and H59alpha lines agree with those of RRLs at different frequencies confirming the accuracy of the previous, more difficult, high-frequency observations. We find that the EVLA is an order of magnitude more efficient for extragalactic RRL observations than the Very Large Array. These observations demonstrate both the power of the EVLA and the future potential of extragalactic RRL studies with the EVLA.
    The Astrophysical Journal Letters 09/2011; 739(1). DOI:10.1088/2041-8205/739/1/L24 · 5.60 Impact Factor
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    ABSTRACT: We present large field HI-line emission maps obtained with the single-dish Green Bank Telescope centered on the dwarf irregular galaxies Sextans A, NGC 2366, and WLM. We do not detect the extended skirts of emission associated with the galaxies that were reported from Effelsberg observations (Huchtmeier et al. 1981). The ratio of HI at 10^19 atoms cm^-2 to optical extents of these galaxies are instead 2--3, which is normal for this type of galaxy. There is no evidence for a truncation in the HI distribution >/=10^19 atoms cm^-2.
  • A. Kepley · A. Reines · K. Johnson · A. Leroy · L. M. Walker
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    ABSTRACT: Star formation in the low mass and low metallicity environments of dwarf galaxies provides important clues about how star formation proceeded in the early universe. To measure the properties of youngest star forming regions - those closest to stellar birth conditions - one needs to use an extinction-free tracer of thermal emission, such as free-free continuum emission in the radio, to penetrate the dust surrounding these regions. We present high sensitivity and high resolution observations of the radio continuum emission in the low metallicity dwarf galaxy II Zw 40 and combine it with HST ACS data and single dish CO(1-0) to gain a more complete picture of star formation in this galaxy.
    EAS Publications Series 07/2011; DOI:10.1051/eas/1148034

Publication Stats

177 Citations
78.33 Total Impact Points

Institutions

  • 2013–2014
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2011–2013
    • University of Virginia
      • Department of Astronomy
      Charlottesville, Virginia, United States
  • 2007–2011
    • University of Wisconsin, Madison
      • Department of Astronomy
      Mississippi, United States
  • 2009
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      State College, PA, United States