For my thesis, I am investigating 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 am using the Very Large Array (VLA), the Green Bank Telescope (GBT), and the Australia Telescope Compact Array (ATCA) to observe the polarization of the radio continuum emission at three different frequencies from several irregular galaxies. These observations will 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. Check out my poster at the 2007 Winter AAS meeting for my latest results!
Advisor: Eric Wilcots
Collaborators: Tim Robishaw, Carl Heiles, Stefanie Müuhle, and Ellen Zweibel
This project was my introduction to radio astronomy. I used high resolution observations from the Very Large Array (VLA) to examine neutral hydrogen content of WLM. This galaxy is a dwarf irregular galaxy on the edge of our Local Group. We find that the neutral hydrogen content of this galaxy has significant substructure consisting of a broken ring of high column density neutral hydrogen. The origin of this ring is probably a burst of recent star formation propagating out from the center of the galaxy. The distribution of the Halpha emission and the young stars in WLM is closely correlated with the distribution of high column density neutral hydrogen. The gap in the central ring is likely the result of star formation in that region using up, blowing out, or ionizing all of the neutral hydrogen. We also find that WLM's neutral hydrogen only extends to 1.2 times its Holmberg diameter, which is a much smaller extent than found by Huchtmeier et al. (1981). Finally, we determine a dynamical mass for this galaxy of 2.2e9 Msun.
Reference: Kepley, A. A., Wilcots, E. M., Hunter, D. A., & Nordgren, T. ``A High-Resolution Study of the HI Content of Local Group Dwarf Irregular Galaxy WLM.'' 2007, AJ, 133, 2242. Version with high resolution figures.
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.
Reference: Kepley, A.A., Morrison, H.L., Helmi, A., Kinman, T.D., van Duyne, J., Martin, J.C., Harding, P., Norris, J.E., and Freeman, K.C. 2007, AJ, 134, 1579.
During the summer of 2000, I worked at the University of Chicago with Dr. Roger Hildebrand as a part of the Center for Astrophysical Research in Antarctica (CARA) Research Experience for Undergraduates (REU) program. I was involved in the initial stages of design and testing of the Hale Polarimeter, an instrument being proposed for use on the Stratospheric Observatory for Infrared Astronomy.
During summer 1999, I participated in the Summer Internships In Science and Technology (SIST) program at Fermi National Accelerator Laboratory. I worked with my advisor, Dr. Chandra Bhat, using models to examine the operating characteristics of a new particle accelerator being proposed for Fermilab.