| My research focuses on using observations of nearby galaxies to study problems of astrophysical and cosmological interest. The work I'm doing now is primarily related to dark matter, chemical evolution, star formation, and the process of galaxy evolution, but that's subject to change without prior notice. I've become involved with a lot of projects; some of them are described below. Proceed farther down the page for links to my PhD thesis and other publications. |
The missing satellites of the Milky Way
| Numerical simulations of galaxy formation in the currently-favored Cold Dark Matter (CDM) model predict that Milky Way-sized galaxies should be surrounded by hundreds of low-mass dwarf galaxies. However, until very recently there were only 11 known dwarfs orbiting the Milky Way, in stark contrast to the theoretical predictions. In just the last 2 years, the Sloan Digital Sky Survey has revealed a large population of new dwarf galaxies, which are up to 100 times fainter than any previously known galaxies. In collaboration with Marla Geha, I am using Keck to obtain very accurate velocity measurements of stars in these new dwarf galaxies so that we can constrain the dark matter content of these objects and determine whether they can resolve the missing satellite problem. We have found that these ultra-faint dwarfs are the most dark matter-dominated galaxies discovered to date, and after adding them to the previously known population of brighter dwarfs, we are able to resolve the missing satellite problem (Simon & Geha 2007). |
The structure of the dark matter halos of dwarf galaxies
| In the CDM model, dark matter halos are supposed to have density profiles with central cusps (i.e., the density increases towards the center as ρ ∝ r-1). Most observations have suggested instead that galaxies have constant-density cores at their centers. Along with Alberto Bolatto, Adam Leroy, and Leo Blitz, I am carrying out a survey of 2D Hα and CO velocity fields of nearby dwarfs, which we can use to obtain very accurate constraints on the central density profiles of their dark matter halos. In contrast to much of the previous work in this field, we find halos with relatively cuspy profiles (ρ ∝ r-0.7 on average), substantially lessening the disagreement between observations and numerical simulations (see Bolatto et al. (2002), Simon et al. (2003), and Simon et al. (2005) for more information). |
Spitzer
Survey of the SMC (S3MC)
| With a team led by Alberto Bolatto, we have acquired Spitzer maps of the Small Magellanic Cloud (SMC) in all 7 IRAC and MIPS bands. The primary goal of the project is to study dust and star formation in the low-metallicity environment provided by the SMC, but we are also finding a lot of interesting things about the young stellar content of the galaxy (e.g., this conference proceeding article). The first paper describing the survey is now available. See the S3MC project website linked above for more information. |
A two-dimensional metallicity map of M33
| In collaboration with Erik Rosolowsky, I am using Keck to obtain deep spectroscopy of a large sample of HII regions in M33, covering the entire disk of the galaxy. This data set will be the first to reveal the detailed structure of the distribution of heavy elements in another galaxy. |
Stellar kinematics of dwarf spheroidal galaxies
| Along with Steve Majewski, Tony Sohn, Mike Siegel, and others, I am working on Keck spectroscopy of large samples of stars in two Local Group dwarf spheroidal galaxies. We are using these data as an independent way to study the dark matter density profiles (see first project in this list) of galaxies that are even more dark matter-dominated. It may also be possible to put significant constraints on the properties of the dark matter particle with the measurements we make. |
Are there dark galaxies?
|
As described above, a potential problem with the CDM model is that it
predicts that there should be 10-100 times as many dwarf galaxies in
the Local Group as we actually observe. Because this prediction
appears quite robust in the generic CDM paradigm, it is plausible that
the Local Group contains large numbers of low-mass dark halos that
host very small numbers of stars (or perhaps none at all) --- dark
galaxies. If such objects exist, can they be detected? Despite claims to the contrary, the first (and perhaps best?) dark galaxy candidate was identified in 2002 by Tim Robishaw, Leo Blitz, and me (see this paper and this conference proceeding). We used the Arecibo telescope to make a high-resolution (for a single-dish HI telescope, at least!) HI map of HVC 127-41-330. We showed that this high-velocity cloud (HVC) appears to be rotating, with a total mass that makes it dark matter-dominated for any reasonable distance. This object thus has a dark matter halo, a substantial amount of cool gas, and no stars that we have been able to detect, suggesting that it may be a dark galaxy. We have also acquired maps of 12 other HVCs, several of which show similar kinematics, and we are currently trying to determine whether these clouds may be dark galaxies as well. |
High-resolution spectra of nearby supernovae
| In collaboration with Avishay Gal-Yam and Doug Leonard, I am studying supernovae (SNe) at high spectral resolution. One of my main interests in this project is using the SNe as bright background sources to probe HVCs along the line of sight. We are also searching for clues to the progenitor systems of Type Ia SNe and measuring the extinction to each SN. |
The fundamental line of disk galaxies
| Along with Francisco Prada and several others, I recently finished a paper about the fundamental line of disk galaxies. Prada & Burkert (2002) showed that the global mass-to-light ratio (M/L), mean metallicity, and central surface brightness of Local Group dwarf galaxies are strongly correlated. We used data from the literature to demonstrate that similar correlations can be found within the disk of a single galaxy (M33). We have also been trying to extend this study to other galaxies for several years to see if the fundamental line of disk galaxies is universal, but the observations have been weathered out three times in a row. |
The triaxiality of dark matter halos
| I am working on several efforts to understand the effects that the triaxiality of dark matter halos has on the observed kinematics of galaxies (closely related to several projects above). The noncircular motions that we frequently see in our velocity fields suggest that the dark matter density profiles inferred from observations may be systematically biased. Jeremy Bailin and Rob Crain are separately producing simulations of triaxial galaxies with which we can investigate this possibility. |
Arecibo HI mapping of the Leo Triplet
| Alberto Bolatto and I mapped the neutral hydrogen in this nearby group of interacting galaxies (and no, it's not named after that Leo). |