Simulating Galaxy Evolution

The forwards approach to galaxy formation and evolution is extremely powerful but leaves several questions unanswered. Foremost among these is the origin of disks. A backwards approach is able to provide a more realistic treatment of star formation and feedback and provides a practical guide to eventually complement galaxy formation ab initio.Comment: 11 pages with 2 figures, to appear in "After the Dark Ages: When Galaxies were Young", proceedings of the 9th annual October Astrophysics Conference, ed. S. Holt and E. Smith, simulated images available at http://astro.berkeley.edu/~bouwens/simulation.htm

Inside-Out Infall Formation of Disk Galaxies: Do Predictions Differ from Models without Size Evolution?

We develop an idealized inside-out formation model for disk galaxies to include a realistic mix of galaxy types and luminosities that provides a fair match to the traditional observables. The predictions of our infall models are compared against identical models with no-size evolution by generating fully realistic simulations of the HDF, from which we recover the angular size distributions. We find that our infall models produce nearly identical angular size distributions to those of our no-size evolution models in the case of a Omega = 0 geometry but produce slightly smaller sizes in the case of a Omega = 1 geometry, a difference we associate with the fact that there is a different amount of cosmic time in our two models for evolving to relatively low redshifts (z \approx 1-2). Our infall models also predict a slightly smaller (11% - 29%) number of large (disk scale lengths > 4 h_{50} ^{-1} kpc) galaxies at z \approx 0.7 for the CFRS as well as different increases in the central surface brightness of the disks for early-type spirals, the infall model predicting an increase by 1.2 magnitudes out to z \approx 2 (Omega = 0), 1 (Omega = 1), while our no-size evolution models predict an increase of only 0.5 magnitude. This result suggests that infall models could be important for explaining the 1.2-1.6 magnitude increase in surface brightness reported by Schade et al. (1995, 1996a, 1996b).Comment: 12 pages, LaTeX (aaspp4.sty), accepted by ApJ Letter

Cloning Dropouts: Implications for Galaxy Evolution at High Redshift

The evolution of high redshift galaxies in the two Hubble Deep Fields, HDF-N and HDF-S, is investigated using a cloning technique that replicates z~ 2-3 U dropouts to higher redshifts, allowing a comparison with the observed B and V dropouts at higher redshifts (z ~ 4-5). We treat each galaxy selected for replication as a set of pixels that are k-corrected to higher redshift, accounting for resampling, shot-noise, surface-brightness dimming, and the cosmological model. We find evidence for size evolution (a 1.7x increase) from z ~ 5 to z ~ 2.7 for flat geometries (Omega_M+Omega_LAMBDA=1.0). Simple scaling laws for this cosmology predict that size evolution goes as (1+z)^{-1}, consistent with our result. The UV luminosity density shows a similar increase (1.85x) from z ~ 5 to z ~ 2.7, with minimal evolution in the distribution of intrinsic colors for the dropout population. In general, these results indicate less evolution than was previously reported, and therefore a higher luminosity density at z ~ 4-5 (~ 50% higher) than other estimates. We argue the present technique is the preferred way to understand evolution across samples with differing selection functions, the most relevant differences here being the color cuts and surface brightness thresholds (e.g., due to the (1+z)^4 cosmic surface brightness dimming effect).Comment: 56 pages, 22 figures, accepted for publication in Ap

AGN Feedback Causes Downsizing

We study the impact of outflows driven by active galactic nuclei (AGN) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires highly energetic outflows, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the anti-hierarchical quenching of star-formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGN, galaxy formation is hierarchical and follows the growth of dark-matter halos. The transition between the two regimes lies at the z ~ 2 peak of AGN activity.Comment: 6 pages, 2 figures, ApJL in pres

The Rest Frame UV to Optical Colors and SEDs of z~4-7 Galaxies

We use the ultra-deep HUDF09 and the deep ERS data from the HST WFC3/IR camera, along with the wide area Spitzer/IRAC data from GOODS-S to derive SEDs of star-forming galaxies from the rest-frame UV to the optical over a wide luminosity range (M_1500 ~ -21 to M_1500 ~ -18) from z ~ 7 to z ~ 4. The sample contains ~ 400 z ~ 4, ~ 120 z ~ 5, ~ 60 z ~ 6, and 36 prior z ~ 7 galaxies. Median stacking enables the first comprehensive study of very faint high-z galaxies at multiple redshifts (e.g., [3.6] = 27.4 +/- 0.1 AB mag for the M_1500 ~ -18 sources at z ~ 4). At z ~ 4 our faint median-stacked SEDs reach to ~ 0.06 L*(z=4) and are combined with recently published results at high luminosity L > L* that extend to M_1500 ~ -23. We use the observed SEDs and template fits to derive rest frame UV-to-optical colors (U - V) at all redshifts and luminosities. We find that this color does not vary significantly with redshift at a fixed luminosity. The UV-to-optical color does show a weak trend with luminosity, becoming redder at higher luminosities. This is most likely due to dust. At z >~ 5 we find blue colors [3.6]-[4.5] ~ -0.3 mag that are most likely due to rest-frame optical emission lines contributing to the flux in the IRAC filter bandpasses. The scatter across our derived SEDs remains substantial, but the results are most consistent with a lack of any evolution in the SEDs with redshift at a given luminosity. The similarity of the SEDs suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5 Gyr that encompasses very substantial growth in the stellar mass density in the universe (from ~ 4x10^6 to ~ 2x10^7 Msun Mpc^-3).Comment: 15 pages, 11 figures, 3 tables, submitted to Ap