Radio recombination lines (RRLs) are a powerful, extinction-free diagnostic of the conditions in the ionized gas in star-forming regions still embedded in their natal clouds of dust and gas. I am leading a project to use RRLs to determine the conditions in obscured star-forming regions in galaxies with a wide variety of stellar masses, metallicities, and star formation rates to provide constraints for theories of extragalactic star formation. I have received time to use the Green Bank Telescope (GBT) to expand the sample of galaxies with detected RRLs. I am following up these observations with EVLA Resident Shared Risk Observing (RSRO) and Open Shared Risk Observing (OSRO) programs.
I am also analyzing the high resolution radio continuum from the blue compact dwarf II Zw 40 (Kepley et al. 2012, submitted). At the frequencies we have observed, the radio continuum is almost entirely free-free emission from thermal electrons. We identified 11 thermal sources in this galaxies and speculate that it is undergoing a second (and possibly final) wave of star formation and has either used up or destroyed most of its molecular gas.
Main Collaborators: Kelsey Johnson, Dana Balser, Laura Chomiuk, Miller Goss, and D.J. Pisano.
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For my Ph.D. thesis, I investigated the magnetic fields of irregular galaxies. Magnetic fields are an important part of the interstellar medium in galaxies: they distribute energy from supernovae, channnel gas flows, and provide a heating mechanism for the ISM. Despite the importance of magnetic fields in the ISM, it is not well known what generates and sustains galactic magnetic fields and how magnetic fields, gas, and stars interact in galaxies. Magnetic fields are especially important in low mass galaxies like dwarf irregulars, where the magnetic field energy may be larger than the gravitational energy. However, only four dwarf irregular galaxies besides the LMC and SMC have well-studied magnetic field structures! The goal of my project is to significantly increase the number of irregular galaxies with observed magnetic field structure. To accomplish this, I used the Very Large Array (VLA) radio continuum polarization data from several irregular galaxies. These observations allow me to constrain the magnetic field strength and structure in each galaxy, investigate the conditions necessarily for the formation of a large scale field in irregular galaxies, and examine the interaction of the magnetic fields in these galaxies with their interstellar medium.
Advisor: Eric Wilcots
Main Collaborators: Ellen Zweibel, John Everett, Stefanie Müuhle, Uli Klein, Tim Robishaw, and Carl Heiles
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I have been involved with several projects exploring the relationship between neutral hydrogen and star formation in two extreme environments: dwarf galaxies and tidal tails. Dwarf galaxies have low masses, which means that they can easily be disrupted, and mostly solid body rotation, which means that they cannot drive star formation with spiral density waves. I have been part of two studies looking at dwarf galaxies with extended disks. In WLM, I found that the disk is probably not as extended as previously thought and that the star formation in the ring surrounding the center of the galaxy is possibly the result of star formation propagating out from the center of the galaxy. In the second project, we observed the extended neutral hydrogen disks of several galaxies using the very sensitive Green Bank Telescope (GBT) to determine whether the disks were really extended. Tidal tails are another extreme environment. These features result from the merger of two galaxies. I am part of a large collaboration lead by Jane Charlton at Penn State exploring the nature of star clusters in tidal tails. My primary responsibility for this project has been to explore the correlations between the neutral hydrogen distribution and star cluster population.
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The goal of this project, which was my senior research project with Dr. Heather Morrison, was to use a local (distances less than 2.5 kpc from the Sun) sample of halo stars to look for halo substructure. In 1994, Ibata, Gilmore, & Irwin discovered that the Galaxy is currently accreting the Sagittarius dwarf spheroidal galaxy. Stars from this system are not yet well-mixed with the rest of the Galactic halo either spatially or in velocity space. Could other halo accretion events be detected if the stars from the accreted object have become well-mixed with the stars in the halo spatially, but not in velocity or angular momentum space? Helmi et al. (1999), hereafter H99, found evidence in angular momentum space for streams in the solar neighborhood using a sample of 97 stars within 1 kpc of the Sun and, using the sample properties, generated a model of the streams. With our data set, we confirm the existence of the streams found by Helmi et al. (1999), hereafter H99. These streams have a double-peaked velocity distribution in the z direction (out of the Galactic plane). We use the results of H99 to test how one might use v_z velocity information and radial velocity information to detect kinematic substructure in the halo. We find that detecting the H99 streams with radial velocities alone would require a large sample (e.g., approximately 150 stars within 2 kpc of the Sun within 20 degrees of the Galactic poles). In addition, we use the velocity distribution of the H99 streams to estimate their age. From our model of the H99 progenitor, we determine that the H99 streams' progenitor was accreted about 6 Gyr old. The H99 streams have [alpha/Fe] abundances similar to other halo stars in the solar neighborhood, suggesting that the gas that formed these stars were enriched mostly by Type II SNe. We have also discovered in angular momentum space two other possible substructures, which we refer to as the retrograde and prograde outliers. The retrograde outliers are likely to be halo substructure, but the prograde outliers are most likely part of the smooth halo. The retrograde outliers have significant structure in the v_phi direction and show a range of [alpha/Fe], with two having low [alpha/Fe] for their [Fe/H]. The fraction of substructure stars in our sample is between 5% and 7%. The methods presented in this paper can be used to exploit the kinematic information present in future large databases like RAVE, SDSSII/SEGUE, and Gaia.
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