Tesis Doctorado
Stellar mass buildup in galaxies in the first 1.5 gyr of the universe
Autor
González-Corvalán, Valentino Gastón
Institución
Resumen
The buildup of galaxies is one of the most fundamental questions in modern cosmology. The study of this process in the first few Gyr of the Universe, starting from the first stars, isa challenging endeavor. In this thesis we have made extensive use of the deepest optical and infrared images currently available from the Hubble Space Telescope (HST) and the Spitzer Space Telescope to study the properties of the stellar populations and the stellar mass buildupin galaxies in the first 1.5 Gyr after the Big Bang.We have studied the spectral energy distributions (SED8) of z - 4- 8 Lyman break galaxies (LBGs) in the rest-frame UV and optical and compared them to synthetic stellar population models to learn about the properties of these galaxies. We have found that the typicalbest fit ages for these systems are in the range 300 - 600 M?r. In a more general context this is not a very old population but at z > 4 this represents a large fraction of the cosmic time,indicating that these galaxies likely started forming stars much earlier, at z > 10.The star formation Rates (SFRs) estimated for LBGs at z > 4 are generally in therange 1 - 100 Mo ?-1. The stellar mass estimates are most robust for sources with good Spitzer/IRAC detections, corresponding to galaxies with stellar masses > 108.5 Mo at z - 4 (> 109.5 Mo at z - 7). For sources with lower rest-frame optical luminosities, that, as a result,are individually undetected in IRAC, their average stellar masses have been studied in a stacking analysis of a large number of sources. This enables us to reach stellar masses - 107.8 Mo atz - 4. The stellar masses show a fairly tight correlation with UV luminosity or SFR, and the zeropoint of the relation does not seem to evolve strongly with redshift. This relation is a direct refiection of a correlation between the UV and optical colors and it favors a typical star formation history (SFH) at high redshift in which the SFR of a galaxy increases as a function oftime. This is consistent with the observed brightening of the UV luminosity function (UV LF) ancl with cxpcctatiom; from numcrical simulations.We have taken advantage of the UV luminosity vs. stellar mass relation observed in LBGs at z > 4 - 7 to derive the stellar mass function (SMF) of galaxies at these redshifts. Themethod uses a combination of the UV LF and the mean UV vs. stellar mass relation (including the scatter, estimated to be - 0.5 dex at bright luminositics at z - 4). This method allows an analytic estimate of the low mass slope of the SMF. This slope (the power-law exponent of theSMF at low masses), is estimated to be in the -1.44--1.55, range which is fiatter than the UV LF faint enrl slope at these redshifts (< -l.74). This means that low mass systems contribute less to the total stellar mass density (SMD) of the Universe than would have been estimatedassuming a constant mass-to-UV-light ratio. We show that this is also much fiatter than the theoretical predictions from simulations, which generally over-predict the number density of lowmass systems at these redshifts.The UV luminosity vs. stellar mass relation indicates only a small variation of the mass-to-light ratio as a function of UV luminosity. This is confirmed in a stacking analysis of a large number of sources from the HUDF and the Early Release Science fields (- 400 z - 4, - 120 z - 5, - 60 z - 6.36 at z - 7). Interestingly, the stacked SEDs at z > 5 in the rest-frame optical shows a color [3.6]- [4.5] - 0.3 mag. This color is hard to reproduce by synthetic stellarpopulation models that only include stellar continua, and it probably indicates the presence of moderately strong emission lines (H? EWrest - 300 Á). The contribution from such emissionlines in the IRAC fluxes idicates that the stellar masses and ages could both be over-estimated by a factor - 2.One of the most interesting results presented in this thesis is the apparent plateau of the specific SFR (sSFR = SFR / stellar mass). In early results, the similarity in the SEDs of galaxies at a given UV luninosity in the z - 4-7 redshift range resulted in very similar estimates of the SFR and stellar masses of these galaxies. Furthermore, we find that the reported sSFR estima tes at z - 2 are also very similar to the ones in the z - 4 - 7 redshift range (- 2 Gyr-1for - 5 x 109 Mo galaxies). A puzzle arises frorn the fact that the dark matter accretion rate onto halos is predicted to decrease monotonically and rather fast as a function of cosmictime (approximately ?(1 + z) 2.5). If gas and star formation follow the inflow of dark matter, the sSFR at a constant mass should also decrease monotonically with time, which is contrary to the inclication from these observations. When we include the possible effects of emission lines, the stellar masses decrease by a factor - 2x at >, 5. The revised stellar masses may favor a slowly rising sSFR at z > 2, but the rise as a function of redshift is still much slower(sSFR(z) ? (1 + z) 0.7) than that of specific dark matter accretion rate. This suggests that the stellar mass buildup is somehow decoupled from the dark matter buildup at early times.A detailed understanding of the connection between the buildup of galaxy mass and dark matter is key for models of galaxy formation in the early Universe. It will be crucial to expand on analyses like the one presented here, including larger samples and broader stellarmass ranges, to explore the buildup of galaxies with improved statistics. Wide-area surveys with newly acquired HST and Spitzer data, and the upcoming generation of instruments, will likely provide the opportunity to make such a connection. PFCHA-Becas Doctor en Filosofía , Astronomía y Astrofísica 181p. PFCHA-Becas TERMINADA