Research

I am interested in learning about how galaxies form, grow and change from the earliest times until the present day. Here is a link to my papers on ADS.

Learning about the growth of galaxies and their satellite systems

Map of the stellar density in the extended stellar envelope of the Milky Way as shown in an equal-area polar projection (1/4 of the sky). Intensity scale shows density of stars; color shows distance (blue = 30000 light years; red > 100000 light years). One can clearly discern overdensities corresponding to non-phase-mixed debris from dwarf galaxies, and phase-mixed older debris. From Bell et al. 2008, The Accretion Origin of the Milky Way’s Stellar Halo, ApJ, 680, 295

One of the central predictions of our understanding of how galaxies form in a dark matter-dominated universe is that galaxies and galaxy groups like our own will grow, in part, through the infall and merger of galaxies with the large central galaxies. This growth should evidence itself in the surviving satellite galaxies in a group, and in a diffuse halo of stars predominantly made of the debris from the tidal disruption of many dwarf galaxies. Such a stellar halo exists around our Milky Way and a number of other nearby spiral galaxies, containing ~1-20% of the stellar mass of such systems; in elliptical galaxies, such halos may be more massive.

Our research group argues that these stellar halos are the most powerful and informative probe of the merger and growth history of galaxies, where particularly strong constraints are placed on the properties of the most massive satellite to have previously merged with a galaxy. We use a variety of datasets – for example, the HST GHOSTS survey, the Subaru-GHOSTS survey with HyperSuprimeCam, or SMASH – in conjunction with simulations of stellar halo growth through the accretion of dwarf galaxies – for example, Illustris or Bullock and Johnston 2005 – to measure and quantitatively interpret the properties of stellar halos. This kind of work gives powerful insight into the merger and growth histories of galaxies, and is the foundation of the empirical test of how merging and growth histories affect galaxy evolution, but is observationally very demanding. We are also engaged in trying to understand the most fruitful observational and analysis techniques for future facilities such as LSST or WFIRST.

Selected papers on this topic include:

Understanding the physical drivers of galaxy evolution

A dry merger in GEMS (with a background spiral galaxy). Mergers between elliptical galaxies are thought to be an important driver of the present-day properties of more massive elliptical galaxies.

Galaxies have a huge range of properties, spanning a factor of nearly a billion in stellar mass, factors of more than a thousand in size. Some are essentially devoid of star formation; others form stars quickly enough to double their mass in less than a billion years (which is a short time for astronomers!)

What drives this diversity? I have long been interested in the physical drivers of the evolution of the galaxy population. For a number of years I focused on the role of merging in growing and assembling the early-type (largely non star-forming galaxies whose stellar motions are supported primarily by random motions). While I have broad interests, I have recently focused on two problems. On one hand, we are trying to understand the processes that shut off star formation on galactic scales.  In galaxies that are in the center of their dark matter halos, we maintain that energy injected into the galaxy by material very close to the central supermassive black hole (AGN feedback) is the driving force, as the degree to which galaxies fail to form stars correlates strongly with the mass of the supermassive black hole. On the other hand, we are trying to understand the diversity of growth histories of Milky Way-like galaxies from the look-back observations, with the ultimate goal of being able to connect the insight we get from the study of individual galaxies’ growth using galactic archaeology with the insight we get from look-back observations about the growth of the galaxy population.

Selected papers on this topic include:

Measuring the effects of dust on measurements of galaxy properties

These panels show galaxies with intrinsically very similar stellar masses, SFRs, inclination-independent sizes and concentrations at two different viewing angles – the top panels nearly face-on, the bottom panels close to edge-on. The right-hand panels shows nearly attenuation-independent K-band images, whereas the left shows the SDSS gri-band cutouts, which are clearly affected dramatically by viewing angle.

With the advent of large, homogenous, multi-passband digital sky survey datasets, many of the most important errors to affect our inferences are systematic rather than random. One of the most poorly-understood of these systematic sources of uncertainty is dust attenuation. Astrophysical dust both absorbs and scatters light, and in practice is distributed widely, mixed in with the stars in galaxies. Such complexity makes it difficult to predict the effects of dust a priori, and may make it dangerous rely on methods with some model-dependence (e.g., spectral energy distribution fitting). Accordingly, we been studying the properties of galaxies as a function of inclination, as a function of their properties. A crucial part of this effort is devising inclination-independent metrics of galaxy structure. We find that widely-used measures of galaxy luminosity and color, structures (e.g., sizes and Sersic indices or concentrations), and ‘dust-corrected’ SFR estimates are all a strong function of inclination for samples of galaxies that are identical save for their viewing angle.

Our papers on this topic include:

In addition, Brian Devour’s PhD thesis has one unpublished chapter that shows that reddening is a strong function of galaxy size – an effect that has not been incorporated in many previous analyses.