Amanda A. Kepley

University of Virginia, Charlottesville, Virginia, United States

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Publications (28)68.37 Total impact

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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.
    01/2014;
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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.
    01/2014;
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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.
    01/2014;
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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.
    01/2014;
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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.
    01/2014;
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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.
    12/2013; 147(2).
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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). · 6.73 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). · 6.73 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. · 6.73 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.
    01/2013;
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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
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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.
    01/2012;
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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. · 4.97 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.
    09/2011;
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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 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 H58a and H59a RRLs and make simultaneous sensitive measurements of the continuum. We measure integrated line fluxes of $44.3 \pm 0.7$ W m$^{-2}$ and $39.9 \pm 0.8$ W m$^{-2}$ for the H58a and H59a lines, respectively. The thermal gas in NGC 253 is kinematically complex with multiple velocity components. We constrain the density of the thermal gas to $1.4 - 4 \times 10^4$ cm$^{-3}$ and estimate an ionizing photon flux of $1 \times 10^{53}$ 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 H58a and H59a 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 VLA. These observations demonstrate both the power of the EVLA and the future potential of extragalactic RRL studies with the EVLA.
    06/2011;
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    ABSTRACT: We examine the magnetic field in NGC 4214, a nearby irregular galaxy, using multi-wavelength radio continuum polarization data from the Very Large Array. We find that the global radio continuum spectrum shows signs that free-free absorption and/or synchrotron losses may be important. The 3cm radio continuum morphology is similar to that of the Halpha, while the 20cm emission is more diffuse. We estimate that 50% of the radio continuum emission in the center of the galaxy is thermal. Our estimate of the magnetic field strength is $30\pm 9.5$ \uG\ in the center and $10\pm3$ \uG\ at the edges. We find that the hot gas, magnetic, and the gravitational pressures are all the same order of magnitude. Inside the central star forming regions, we find that the thermal and turbulent pressures of the HII regions dominate the pressure balance. We do not detect any significant polarization on size scales greater than 200 pc. We place an upper limit of 8 \uG\ on the uniform field strength in this galaxy. We suggest that the diffuse synchrotron region, seen to the north of the main body of emission at 20cm, is elongated due to a uniform magnetic field with a maximum field strength of 7.6 \uG. We find that, while the shear in NGC 4214 is comparable to that of the Milky Way, the supernova rate is half that of the Milky Way and suggest that the star formation episode in NGC 4214 needs additional time to build up enough turbulence to drive an $\alpha-\omega$ dynamo.
    The Astrophysical Journal 05/2011; 736. · 6.73 Impact Factor
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    ABSTRACT: The radio continuum is an excellent probe of recent star formation in other galaxies. Free-free thermal emission is characterized by a flat spectrum and will be the strongest contributor to the radio continuum at frequencies between 30-100 GHz in star-forming galaxies. The thermal free-free emission constrains the number of ionizing photons, and thus the number of massive stars in star-forming regions. Observations at these frequencies are necessary in order to disentangle the relative contributions of thermal free-free emission and nonthermal synchrotron emission and are complementary to observations of star formation tracers at other wavelengths. We observed 26 local star-forming galaxies with the Caltech Continuum Backend on the Green Bank Telescope between 26 and 40 GHz. We have found that while some sources in our survey have relatively flat spectral indices, most have steep spectral indices over this range of frequencies. By separating the relative contributions from thermal and non-thermal flux and modeling the SEDs of these galaxies, we will infer their physical properties (temperature, density, pressure, size, etc.), and thus the dispersion timescale of star-forming regions, further constraining the star formation timescale. We have also mapped three luminous compact blue galaxies at 26-40 GHz with CCB and at 90 GHz with the MUSTANG bolometer array on the GBT. While we had expected these three galaxies to have the flat spectra characteristic of star-forming regions based on previous multi-wavelength observations, instead they had steep spectra indicative of a large non-thermal synchrotron component. Overall, our observations present a complex picture of star formation in nearby starbursting galaxies. Support for this work was provided by the NSF through award GSSP 10-0002 from the NRAO.
    05/2011;
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    ABSTRACT: We have searched for compact stellar structures within 17 tidal tails in 13 different interacting galaxies using F606W- and F814W-band images from the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The sample of tidal tails includes a diverse population of optical properties, merging galaxy mass ratios, H I content, and ages. Combining our tail sample with Knierman et al., we find evidence of star clusters formed in situ with M V < –8.5 and V – I < 2.0 in 10 of 23 tidal tails; we are able to identify cluster candidates to M V = –6.5 in the closest tails. Three tails offer clear examples of "beads on a string" star formation morphology in V – I color maps. Two tails present both tidal dwarf galaxy candidates and cluster candidates. Statistical diagnostics indicate that clusters in tidal tails may be drawn from the same power-law luminosity functions (with logarithmic slopes –2 to –2.5) found in quiescent spiral galaxies and interiors of interacting systems. We find that the tail regions with the largest number of observable clusters are relatively young (250 Myr old) and bright (V 24 mag arcsec–2), probably attributed to the strong bursts of star formation in interacting systems soon after periapse. Otherwise, we find no statistical difference between cluster-rich and cluster-poor tails in terms of many observable characteristics, though this analysis suffers from complex, unresolved gas dynamics and projection effects.
    The Astrophysical Journal 03/2011; 731(2):93. · 6.73 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 01/2011; 739(1). · 6.35 Impact Factor
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    ABSTRACT: How stars form out of dense gas and dust is one of the great remaining mysteries in astronomy. While there are viable theories of Galactic star formation, theories of star formation in environments different from those in the Milky Way, which must encompass everything from dwarf galaxies to luminous and ultra-luminous infrared galaxies, are relatively unconstrained. There is a great need for quantitative information about the physical conditions of the interstellar medium in extragalactic star-forming regions and how star formation might vary with the wide range of environments (metallicity, stellar density, turbulence, magnetic fields, etc.). Studies of young, embedded star-forming regions are particularly important because they give us crucial information on the condition of the interstellar medium in star-forming regions before the massive stars have moved off the main sequence, and thus tell us about the properties of the initial stellar population. However, because these regions are obscured by their natal dust and gas, they are poorly understood. Radio recombination lines provide an important extinction-free probe of obscured star-forming regions, yielding crucial constraints on the density, filling factor, and mass of the thermal gas in star-forming regions and number of ionizing photons present in these regions. Unfortunately, until recently these observations were tremendously difficult due to the limited bandwidth and sensitivity of previous radio interferometers. The Expanded Very Large Array (EVLA) has opened new frontiers of discovery for these powerful, but faint, diagnostic lines. We present the first observations of radio recombination lines in nearby star-forming galaxies with the recently updated Expanded Very Large Array (EVLA). These observations provide important constraints on the the physical properties of the thermal gas in these regions, and thus constraint extragalactic star formation theories.
    01/2011;

Publication Stats

125 Citations
68.37 Total Impact Points

Institutions

  • 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