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.

Current Research Projects

1) The missing satellites of the Milky Way
2) The structure of the dark matter halos of dwarf galaxies
3) Spitzer Survey of the SMC (S3MC)
4) A two-dimensional metallicity map of M33
5) Stellar kinematics of dwarf spheroidal galaxies
6) Are there dark galaxies?
7) High-resolution spectra of nearby supernovae
8) The fundamental line of disk galaxies
9) The triaxiality of dark matter halos
10) Arecibo HI mapping of the Leo Triplet

PhD Thesis

Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales


The Density Profile of the Dark Matter Halo of NGC 4605
The Absence of Stars in Compact High-Velocity Clouds
HI Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud
High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile
High-Resolution Measurements of the Halos of Four Dark Matter-Dominated Galaxies: Deviations from a Universal Density Profile*
The Molecular ISM of Dwarf Galaxies on Kiloparcsec Scales: A New Survey for CO in Northern, IRAS-Detected Dwarf Galaxies
Spitzer Space Telescope Detection of the Young Supernova Remnant 1E 0102.2-7219
The Cosmological Significance of High-Velocity Cloud Complex H
Is There a Fundamental Line for Disk Galaxies?
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands
A High-Resolution Spectrum of the Extremely Metal-Rich Bulge G-Dwarf OGLE-2006-BLG-265
Self-Consistent Massive Disks in Triaxial Dark Matter Halos
Detection of Circumstellar Material in a Normal Type Ia Supernova*
Upper Limit for Circumstellar Gas Around the Type Ia SN 2000cx
The Spitzer Survey of the Small Magellanic Cloud: Discovery of Embedded Protostars in the HII region NGC 346
The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem*
Constraints on Circumstellar Material Around the Type Ia Supernova 2007af
The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33
The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs
An Extremely Luminous X-ray Outburst at the Birth of a Supernova*
A Common Mass Scale for Milky Way Satellite Galaxies
Uncovering the Extremely Metal-Poor Stars in the Ultra-Faint Dwarf Spheoridal Galaxies
GALEX Spectroscopy of SN2005ay Suggests a UV Spectral Uniformity Among Type II-P Supernovae
The Chemical Abundances of Tycho G in Supernova Remnant 1572
The Least Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1
High-Resolution Specotroscopy of Extremely Metal-Poor Stars in the Least Evolved Galaxies: Ursa Major II and Coma Berenices
Variable Sodium Absorption in a Low-Extinction Type Ia Supernova
Spectroscopic Confirmation of the Pisces Overdensity
Accurate Masses for Dispersion-Supported Galaxies
Linking Dwarf Galaxies to Halo Building Blocks With the Most Metal-Poor Star in Sculptor*
High-Resolution Specotroscopy of Extremely Metal-Poor Stars in the Least Evolved Galaxies: Leo IV

and several more in progress . . .

A complete list of my "publications" (including AAS abstracts) is available on ADS. I don't know what this Joan Simon's deal is, but he's really messing up my list. Bad Joan! (Tim Robishaw has pointed out that this problem can be solved like this: better publications list. Thanks for making that exact name search thing work so nicely, ADS.)

* See press coverage below

Media/Popular Press Coverage

Dark Galaxies:
    New Scientist (October 2003)

Dark Matter Density Profiles:
    Science News (April 2005)
    Sky and Telescope (March 2005)

Type Ia Supernova Progenitors:
    ESO Press Release (July 2007)
    Caltech Press Release
    Physics World
    and many more . . .

Missing Satellites:
    Keck Observatory Cosmic Matters e-magazine, including a brief video interview (August 15, 2008)  
    Scientific American podcast interview with George Musser (January 23, 2008)       Also available on Scientific American's website or with iTunes.
    Keck Press Release (September 2007)
    Caltech Press Release
    Discover --- part of the #7 science story of 2007!
    Sky and Telescope
    USA Today
    George Musser's AAS blog entry (January 2008)
    Honolulu Star-bulletin
    Science Centric
    KPUA - Hilo, Hawaii

SN 2008D Caught in the Act:
    Caltech press release (May 2008)

The Most Metal-Poor Star Outside the Milky Way
    Carnegie press release (March 2010)
    CfA press release

The Darkest Galaxy
    Science News
    Keck press release (July 2011)
    Universe Today
    Daily Mail
    Discovery News
    Astronomy magazine

More Projenitors of Type Ia Supernovae
    Carnegie press release (August 2011)
    ABC Science

Conference Proceedings and Non-refereed Publications

Dark Matter in Dwarf Galaxies: The First Dark Galaxy?*
Dark Matter in Dwarf Galaxies: Latest Density Profile Results
Dark Matter in Dwarf Galaxies: High-Resolution Observations
Molecular Gas in Nearby Dwarf Galaxies: Single Dish and Interferometric Results
Infrared Sources in the Small Magellanic Cloud: First Results
Supernova 2007on Is Probably a Young Type Ia Event
A Metallicity Map of M33
ROTSE3 J125642.7+273041 Is Probably a Type Ia Supernova of Intergalactic Origin

Undergraduate Honors Thesis

Probing Bulk Flows With Spiral Galaxy Rotation Curves and the Tully-Fisher Relation
I worked for Jeff Willick on a project called Shellflow, in which we were trying to determine whether the mass causing the Local Group to move with respect to the rest frame of the universe is within about 85 megaparsecs of us or not. If you have a Postscript viewer, you can take a look (be warned: it's a big file).

Observing Experience

0.6-m Coude Auxiliary Telescope, Lick Observatory: 1 night of high-resolution spectroscopy (Hamilton Spectrograph)
1.0-m Nickel Telescope, Lick Observatory: 13 nights of imaging
1.8-m Perkins Telescope, Lowell Observatory: 6 nights of imaging
2.5-m du Pont Telescope, Las Campanas Observatory: 4 nights of multi-object spectroscopy (WFCCD)
3.0-m Shane Telescope, Lick Observatory: 7 nights of imaging (Prime Focus Camera)
3.0-m Shane Telescope, Lick Observatory: 1 night of low-resolution spectroscopy (Kast Spectrograph)
3.0-m Shane Telescope, Lick Observatory: 2 nights of high-resolution spectroscopy (Hamilton Spectrograph)
3.5-m WIYN Telescope, Kitt Peak: 13 nights of integral field spectroscopy (Densepak)
5.0-m Hale Telescope, Palomar Observatory: 3 nights of imaging (LFC)
6.5-m Magellan Telescope, Las Campanas Observatory: 4 nights of multi-object spectroscopy (IMACS)
10-m Keck Telescope, Hawaii: many nights of spectroscopy (LRIS, DEIMOS, HIRES)
12-m UASO telescope, Kitt Peak: 8 days of spectral line observing (CO)
305-m Arecibo telescope: ~200 hours of wide-field mapping (HI)
BIMA millimeter-wave interferometer, Hat Creek: 3 weeks as observer
                                                  6 nights of multiobject spectroscopy (DEIMOS)
                                                  4 nights of high-resolution spectroscopy (HIRES)

I've also worked with data from:
9.4-m Hobby-Eberly Telescope, McDonald Observatory (low-resolution and high-resolution spectroscopy)
4.2-m William Herschel Telescope (imaging)
0.85-m Spitzer Space Telescope (IRAC and MIPS imaging and IRS spectroscopy)

Current Research Project Descriptions

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).