In this thesis I used a sample of radio galaxies with 1.4GHz radio luminosity, S1.4 > 10mJy, with the aim of finding high-redshift radio galaxies in the Lockman Hole, ELAIS-N1, ELAISN2, XMM-LSS and CDFS using near-infrared data from the Spitzer Space Telescope, and in the VIMOS4/SSA22 field using near-infrared data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS-DXS). I used these near-infrared data to filter out low redshift (z < 2) radio galaxies by only including sources with either a very faint identification or non-detection in the K−band or at 3.6μm. I then applied a radio selection criteria based on the compactness of the radio structure in the FIRST survey, to ensure that it was possible to correctly associate a near-infrared identification if detected. Using these filtering critera I reduced the sample from 498 to 220, and I obtained spectroscopic observations for 46 of these. I successfully measured redshifts for 22 of these. Four of these sources were found to have redshifts at z > 3, thus proving the efficiency of the selection. Using these spectroscopically identified sources I investigated the correlation between radio luminosity and Lyα emission line luminosity and showed that these radio sources lie on the same correlations as their more radio-luminous counterparts, suggesting that accretion rate on to the central supermassive black hole determines the power of the AGN, both in terms of photoionizing radiation and radio power. I also investigated the near infrared properties of the host galaxy for these sources. Studying the 3.6μm magnitude versus redshift, I found that many of the radio galaxies in our sample lie on a similar relation as other powerful radio sources studied by different methods, although in my selection there is a bias to the fainter end of the distribution, likely due to the selection bias that I impose. The selection criteria has led us to report on the discovery of the highest redshift radio galaxy ever found, however the robustness of the redshift should be confirmed by follow up spectroscopy as the source has KAB magnitude of ∼ 20.7 which is in an area of the K − z diagram which makes it consistent with both z∼ 1 (in the case of [Oii]) and z ∼ 5 (in the case of Lyα). Finally I determined the number density of high-redshift sources in my sample at different epochs and show that the SKADS simulation is consistent with our observed radio galaxy sample. Using this knowledge I go on to predict the number of sources for current and future near-infrared surveys, where the future appears very bright for this kind of work.

In this paper, we investigate the relationship between star formation and structure, using a mass-complete sample of 27,893 galaxies at 0.5. <. z. <. 2.5 selected from 3D-HST. We confirm that star-forming galaxies are larger than quiescent galaxies at fixed stellar mass (M*). However, in contrast with some simulations, there is only a weak relation between star formation rate (SFR) and size within the star-forming population: when dividing into quartiles based on residual offsets in SFR, we find that the sizes of star-forming galaxies in the lowest quartile are 0.27. +/-. 0.06 dex smaller than the highest quartile. We show that 50% of star formation in galaxies at fixed M. takes place within a narrow range of sizes (0.26 dex). Taken together, these results suggest that there is an abrupt cessation of star formation after galaxies attain particular structural properties. Confirming earlier results, we find that central stellar density within a 1 kpc fixed physical radius is the key parameter connecting galaxy morphology and star formation histories: galaxies with high central densities are red and have increasingly lower SFR/M., whereas galaxies with low central densities are blue and have a roughly constant (higher) SFR/M. at a given redshift. We find remarkably little scatter in the average trends and a strong evolution of > 0.5 dex in the central density threshold correlated with quiescence from z.similar to. 0.7-2.0. Neither a compact size nor high-n are sufficient to assess the likelihood of quiescence for the average galaxy; instead, the combination of these two parameters together with M* results in a unique quenching threshold in central density/velocity.

Although there has been much progress in understanding how galaxies evolve, we still do not understand how and when they stop forming stars and become quiescent. We address this by applying our galaxy spectral energy distribution models, which incorporate physically motivated star formation histories (SFHs) from cosmological simulations, to a sample of quiescent galaxies at 0.2 < z < 2.1. A total of 845 quiescent galaxies with multi-band photometry spanning rest-frame ultraviolet through near-infrared wavelengths are selected from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) data set. We compute median SFHs of these galaxies in bins of stellar mass and redshift. At all redshifts and stellar masses, the median SFHs rise, reach a peak, and then decline to reach quiescence. At high redshift, we find that the rise and decline are fast, as expected, because the universe is young. At low redshift, the duration of these phases depends strongly on stellar mass. Low-mass galaxies (log(M*/M-circle dot) similar to 9.5) grow on average slowly, take a long time to reach their peak of star formation (greater than or similar to 4 Gyr), and then the declining phase is fast (less than or similar to 2 Gyr). Conversely, high-mass galaxies (log(M*/M-circle dot) similar to 11) grow on average fast (less than or similar to 2 Gyr), and, after reaching their peak, decrease the star formation slowly (greater than or similar to 3). These findings are consistent with galaxy stellar mass being a driving factor in determining how evolved galaxies are, with high-mass galaxies being the most evolved at any time (i.e., downsizing). The different durations we observe in the declining phases also suggest that low- and high-mass galaxies experience different quenching mechanisms, which operate on different timescales.

We present results from a survey of the internal kinematics of 49 star-forming galaxies at z similar to 2 in the CANDELS fields with the Keck/MOSFIRE spectrograph, Survey in the near-Infrared of Galaxies with Multiple position Angles (SIGMA). Kinematics (rotation velocity V-rot and gas velocity dispersion sg) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by z similar to 2, massive star-forming galaxies (log M-*/M-circle dot less than or similar to 10.2) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully-Fisher relation. These massive galaxies have values of V-rot sg that are factors of 2-5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low-mass galaxies (log M-*/M-circle dot less than or similar to 10.2) at this epoch are still in the early stages of disk assembly: their kinematics are often dominated by gas velocity dispersion and they fall from the Tully-Fisher relation to significantly low values of V-rot. This "kinematic downsizing" implies that the process(es) responsible for disrupting disks at z similar to 2 have a stronger effect and/or are more active in low-mass systems. In conclusion, we find that the period of rapid stellar mass growth at z similar to 2 is coincident with the nascent assembly of low-mass disks and the assembly and settling of high-mass disks.

In this paper, we investigate the relationship between star formation and structure, using a mass-complete sample of 27,893 galaxies at 0.5. <. z. <. 2.5 selected from 3D-HST. We confirm that star-forming galaxies are larger than quiescent galaxies at fixed stellar mass (M*). However, in contrast with some simulations, there is only a weak relation between star formation rate (SFR) and size within the star-forming population: when dividing into quartiles based on residual offsets in SFR, we find that the sizes of star-forming galaxies in the lowest quartile are 0.27. +/-. 0.06 dex smaller than the highest quartile. We show that 50% of star formation in galaxies at fixed M. takes place within a narrow range of sizes (0.26 dex). Taken together, these results suggest that there is an abrupt cessation of star formation after galaxies attain particular structural properties. Confirming earlier results, we find that central stellar density within a 1 kpc fixed physical radius is the key parameter connecting galaxy morphology and star formation histories: galaxies with high central densities are red and have increasingly lower SFR/M., whereas galaxies with low central densities are blue and have a roughly constant (higher) SFR/M. at a given redshift. We find remarkably little scatter in the average trends and a strong evolution of > 0.5 dex in the central density threshold correlated with quiescence from z.similar to. 0.7-2.0. Neither a compact size nor high-n are sufficient to assess the likelihood of quiescence for the average galaxy; instead, the combination of these two parameters together with M* results in a unique quenching threshold in central density/velocity.

In this paper we use ASPECS, the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field in band. 3 and band. 6, to place blind constraints on the CO luminosity function and the evolution of the cosmic molecular gas density as a function of redshift up to z similar to 4.5. This study is based on galaxies that have been selected solely through their CO emission and not through any other property. In all of the redshift bins the ASPECS measurements reach the predicted "knee" of the CO luminosity function (around 5 x 10(9) K km s(-1) pc(2)). We find clear evidence of an evolution in the CO luminosity function with respect to z similar to 0, with more CO-luminous galaxies present at z similar to 2. The observed galaxies at z similar to 2 also appear more gas-rich than predicted by recent semi-analytical models. The comoving cosmic molecular gas density within galaxies as a function of redshift shows a drop by a factor of 3-10 from z similar to 2 to z similar to 0 (with significant error bars), and possibly a decline at z > 3. This trend is similar to the observed evolution of the cosmic star formation rate density. The latter therefore appears to be at least partly driven by the increased availability of molecular gas reservoirs at the peak of cosmic star formation (z similar to 2).

We present new deep photometry of the rich globular cluster (GC) systems around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143 (Abell 2147), obtained with the Hubble Space Telescope (HST) ACS and WFC3 cameras. For comparison, we also present new reductions of similar HST/ACS data for the Coma supergiants NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to the radial limits of our data, comprising from 12,000 to 23,000 clusters per galaxy). The metallicity distribution functions (MDFs) of the GCs can still be matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes are separated by similar or equal to 0.8 dex, but the internal dispersions of each mode are so large that the total MDF becomes very broad and nearly continuous from [Fe/H] similar or equal to-2.4 to solar. There are, however, significant differences between galaxies in the relative numbers of metal-rich clusters, suggesting that they underwent significantly different histories of mergers with massive gas-rich halos. Last, the proportion of metal-poor GCs rises especially rapidly outside projected radii R >= 4 R-eff, suggesting the importance of accreted dwarf satellites in the outer halo. Comprehensive models for the formation of GCs as part of the hierarchical formation of their parent galaxies will be needed to trace the systematic change in structure of the MDF with galaxy mass, from the distinctly bimodal form in smaller galaxies up to the broad continuum that we see in the very largest systems.

We present the Magellan/FIRE detection of highly ionized C IV lambda 1550 and O III]lambda 1666 in a deep infrared spectrum of the z = 6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously known Ly alpha. No corresponding emission is detected at the expected location of He II lambda 1640. The upper limit on He II, paired with detection of O III] and C IV, constrains possible ionization scenarios. Production of C IV and O III] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (similar to 0.5 Ryd). As a pure recombination line, He II emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He II flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general z = 2-3 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.

Lenticular and elliptical galaxies, collectively referred to as "early-type galaxies" (ETGs), are commonly thought to represent the end-points of galaxy evolution. Lying in the red sequence of galaxies, these objects are defined by their mostly old stellar populations and by their "red and dead" appearance in optical observations. Much progress in understanding these objects has been made with integral-field spectroscopy in recent years, with results repeatedly pointing to a link between early-type galaxies and high-redshift spiral galaxies. However, the exact nature of this link remains unclear, with a wide variety of evolution scenarios likely required to fully explain the range of observed early-type galaxy properties. In my study, I analysed observations of twelve early-type galaxies taken with the Mitchell Integral-Field Spectrograph at McDonald Observatory, Texas. These galaxies have previously been found to contain detectable quantities of neutral hydrogen gas, with ten out of the twelve displaying large-scale hydrogen disks. I extracted line-of-sight kinematics of the stellar and ionised gas components of these galaxies, and I used various modelling approaches to constrain their stellar population parameters as well as their three-dimensional mass structure in terms of both dark and visible components. An important feature of this study is the wide field of view of the spectroscopic observations, which reach beyond two half-light radii for almost all of the sample; this remains rare for integral-field unit (IFU) studies of ETGs, and so sets this study apart from most earlier works. The gas-rich nature of the sample is likewise novel. I find all aspects of my analysis to yield a consistent view of these galaxies' evolution, in which one or more gaseous interaction events served to shape them into their observed forms. I find these galaxies to contain low dark matter fractions on average within the inner half-light radius, and I also find mass modelling to favour near-isothermal total density profiles over much of the sample.

Mode of access: World Wide Web. / Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references (leaves 180-192). / Electronic reproduction. / Also available by subscription via World Wide Web / xviii, 192 leaves, bound col. ill. 29 cm