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• W320539476 abstract "Author(s): Gonzalez, Valentino | Advisor(s): Illingworth, Garth | Abstract: The buildup of galaxies is one of the most fundamental questions inmodern cosmology. The study of this process in the first few Gyr of theUniverse, starting from the first stars, is a challenging endeavor. Inthis thesis we have made extensive use of the deepest optical andinfrared images currently available from the Hubble SpaceTelescope (HST) and the Spitzer SpaceTelescope to study the properties of the stellar populationsand the stellar mass buildup in galaxies in the first 1.5 Gyr after theBig Bang.We have studied the spectral energy distributions (SEDs) ofz∼4-8 Lyman break galaxies (LBGs) in the rest-frameUV and optical and compared them to synthetic stellar population modelsto learn about the properties of these galaxies. We have found that thetypical best fit ages for these systems are in the range 300-600 Myr. Ina more general context this is not a very old population but atz≥4 this represents a large fraction of the cosmictime, indicating that these galaxies likely started forming stars muchearlier, at z≥10.The star formation Rates (SFRs) estimated for LBGs atz≥4 are generally in the range 1-100 Msunyr-1. The stellar mass estimates are most robust forsources with good Spitzer/IRAC detections,corresponding to galaxies with stellar masses≥108.5 Msun at z∼4(≥109.5 Msun at z∼7).For sources with lower rest-frame optical luminosities, that, as aresult, are individually undetected in IRAC, their average stellarmasses have been studied in a stacking analysis of a large number ofsources. This enables us to reach stellar masses∼107.8Msun at z∼4.The stellar masses show a fairly tight correlation with UV luminosity orSFR, and the zeropoint of the relation does not seem to evolve stronglywith redshift. This relation is a direct reflection of a correlationbetween the UV and optical colors and it favors a typical star formationhistory (SFH) at high redshift in which the SFR of a galaxy increases asa function of time. This is consistent with the observed brightening ofthe UV luminosity function (UV LF) and with expectations from numericalsimulations.We have taken advantage of the UV luminosity vs. stellar mass relationobserved in LBGs at z≥4-7 to derive the stellar massfunction (SMF) of galaxies at these redshifts. The method uses acombination of the UV LF and the mean UV vs. stellar mass relation(including the scatter, estimated to be ∼0.5 dex at brightluminosities at z∼4). This method allows ananalytic estimate of the low mass slope of the SMF. This slope (thepower-law exponent of the SMF at low masses), is estimated to be in the-1.44--1.55, range which is flatter than the UV LF faint end slope atthese redshifts (≤-1.74). This means that low mass systemscontribute less to the total stellar mass density (SMD) of the Universethan would have been estimated assuming a constant mass-to-UV-lightratio. We show that this is also much flatter than the theoreticalpredictions from simulations, which generally over-predict the numberdensity of low mass systems at these redshifts.The UV luminosity vs. stellar mass relation indicates only a smallvariation of the mass-to-light ratio as a function of UV luminosity.This is confirmed in a stacking analysis of a large number of sourcesfrom the HUDF and the Early Release Science fields (∼400z∼4, ∼120 z∼5, ∼60 z∼6, 36 at z∼7).Interestingly, the stacked SEDs at z≥5 in therest-frame optical shows a color [3.6]-[4.5]∼0.3 mag. This color ishard to reproduce by synthetic stellar population models that onlyinclude stellar continua, and it probably indicates the presence ofmoderately strong emission lines (Hα EWrest∼300A). The contribution from such emission lines in the IRAC fluxesindicates that the stellar masses and ages could both be over-estimatedby a factor ∼2.One of the most interesting results presented in this thesis is theapparent plateau of the specific SFR (sSFR = SFR / stellar mass). Inearly results, the similarity in the SEDs of galaxies at a given UVluminosity in the z∼4-7 redshift range resulted invery similar estimates of the SFR and stellar masses of these galaxies.Furthermore, we find that the reported sSFR estimates atz∼2 are also very similar to the ones in thez∼4-7 redshift range (∼2 Gyr-1for ∼5×109 Msun galaxies). A puzzlearises from the fact that the dark matter accretion rate onto halosis predicted to decrease monotonically and rather fast as a functionof cosmic time (approximately propotional to(1+z)2.5). If gas and star formationfollow the inflow of dark matter, the sSFR at a constant mass shouldalso decrease monotonically with time, which is contrary to theindication from these observations. When we include the possibleeffects of emission lines, the stellar masses decrease by a factor∼2× at z≥5. The revised stellar massesmay favor a slowly rising sSFR at z≥2, but therise as a function of redshift is still much slower(sSFR(z) propto(1+z)0.6) than that of specific darkmatter accretion rate. This suggests that the stellar mass buildupis somehow decoupled from the dark matter buildup at early times.A detailed understanding of the connection between the buildup of galaxymass and dark matter is key for models of galaxy formation in the earlyUniverse. It will be crucial to expand on analyses like the onepresented here, including larger samples and broader stellar massranges, to explore the buildup of galaxies with improved statistics.Wide-area surveys with newly acquired HST andSpitzer data, and the upcoming generation ofinstruments, will likely provide the opportunity to make such aconnection." @default.
• W320539476 created "2016-06-24" @default.
• W320539476 creator A5076956680 @default.
• W320539476 date "2012-01-01" @default.
• W320539476 modified "2023-09-23" @default.
• W320539476 title "Stellar Mass Buildup in Galaxies in the First 1.5 Gyr of the Universe" @default.
• W320539476 hasPublicationYear "2012" @default.
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