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
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:
- Smercina et al. 2019, The Saga of M81: Global View of a Massive Stellar Halo in Formation, submitted to ApJ, arXiv:1910.14672
- D’Souza & Bell 2018b,The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor, Nature Astronomy, 2, 737
- D’Souza & Bell 2018a, The masses and metallicities of stellar haloes reflect galactic merger histories, MNRAS, 474, 5300
- Bell et al. 2017, Galaxies Grow Their Bulges and Black Holes in Diverse Ways, ApJL, 837, 8
- Harmsen et al. 2017, Diverse stellar haloes in nearby Milky Way mass disc galaxies, MNRAS, 466, 1491
- Monachesi et al. 2016, The GHOSTS survey – II. The diversity of halo colour and metallicity profiles of massive disc galaxies, MNRAS, 457, 1419
- Slater et al. 2016, The Stellar Density Profile of the Distant Galactic Halo, ApJ, 832, 206
- Slater et al. 2014, The Complex Structure of Stars in the Outer Galactic Disk as Revealed by Pan-STARRS1, ApJ, 791, 9
- Bell et al. 2008, The Accretion Origin of the Milky Way’s Stellar Halo, ApJ, 680, 295
Understanding the physical drivers of galaxy evolution
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:
- Terrazas et al. 2019, The relationship between black hole mass and galaxy properties: Examining the black hole feedback model in IllustrisTNG, submitted to MNRAS, arXiv:1906.02747
- Terrazas et al. 2017, Supermassive Black Holes as the Regulators of Star Formation in Central Galaxies, ApJ, 844, 170
- Bell et al. 2017, Galaxies Grow Their Bulges and Black Holes in Diverse Ways, ApJL, 837, 8
- Terrazas et al. 2016, Quiescence Correlates Strongly with Directly Measured Black Hole Mass in Central Galaxies, ApJL, 830, 12
- Terrazas et al. 2016, The diversity of growth histories of Milky Way-mass galaxies, MNRAS, 459, 1929
- Papovich et al. 2015, ZFOURGE/CANDELS: On the Evolution of M* Galaxy Progenitors from z = 3 to 0.5, ApJ, 803, 26
- McIntosh et al. 2014, A new population of recently quenched elliptical galaxies in the SDSS, MNRAS, 442, 533
- Bell et al. 2012, What Turns Galaxies Off? The Different Morphologies of Star-forming and Quiescent Galaxies since z ~ 2 from CANDELS, ApJ, 753, 167
- Robaina et al. 2010, The Merger-driven Evolution of Massive Galaxies, ApJ, 719, 844
Measuring the effects of dust on measurements of galaxy properties
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:
- Devour & Bell 2019, Circumventing the Effects of Projection and Dust Using Inclination-independent Infrared Galaxy Structure Measurements: Method, Error Analysis, and a New Public Catalog of Near-infrared Galaxy Structures, ApJS, 244, 3
- Devour & Bell 2017, Revealing strong bias in common measures of galaxy properties using new inclination-independent structures, MNRAS Letters, 468, 31
- Devour & Bell 2016, Global dust attenuation in disc galaxies: strong variation with specific star formation and stellar mass, and the importance of sample selection, MNRAS, 459, 2054
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.