This dissertation presents three observational projects designed to characterize the outer disks of nearby galaxies (beyond the optical radius R₂₅). Until very recently, outer disks remained an elusive and poorly-understood component of disk galaxies. We first present a Large Binocular Telescope (LBT) optical imaging survey of nearby outer disks to examine the basic properties of this component. Our LBT observations indicate that most nearby galaxies host an outer disk with star formation occurring at a very low level. We detect hundreds of outer disk star clusters and show that they typically have masses ∼ 10² − 10⁴M⊙ and ages up to a Gyr. The clusters are born in groups that can remain clustered for a Gyr or more, while the clusters slowly evaporate stars into a diffuse stellar component. The clusters appear to form from localized overdensities in the gas distribution primarily associated with spiral structure. The clusters extend to 2R₂₅ in our sample. We find that some clusters may also reside well outside of their host galaxy’s gas disk. Our second project is a kinematic study of Hɑ knots in the outer disk of the large, isolated, face-on galaxy NGC 628, using Inamori Magellan Areal Camera and Spectrograph (IMACS) observations from the Magellan telescope. This galaxy shows a kinematically cold outer disk (velocity dispersion < 11 km s⁻¹) with a mass density ∑ = 7.5 M⊙ pc⁻². Our observations cannot exclude uniform star formation lasting a Hubble time in this outer disk and confirm that this component is an extension of the kinematically-cold inner disk. Our third project is a search for molecular emission in the outer disk of NGC 628, using the sensitive Atacama LargeMillimeter Array (ALMA) receiver on the Submillimeter Telescope (SMT). We did not detect emission from our outer disk pointings, though we are able to provide useful estimates for future ALMA observations of outer disk knots. Our SMT observations indicate that the H₂ / H I ratio is ∼ 100× lower in the outer disk than in the inner disk, which likely explains, at least in part, the trend towards smaller clusters and lower star formation rates at larger radii.

It has become increasingly clear that a complete picture of galaxy evolution requires a better understanding of the role of Active Galactic Nuclei (AGN). In particular, they could be responsible for regulating star formation and galaxy growth via feedback processes. There are also competing views about the main modes of stellar growth and supermassive black hole growth in galaxies that need to be resolved. With high infrared luminosities (thus star formation rates) and a frequent occurrence of AGN, galaxies selected in the far-infrared wavebands form an ideal sample to search for a connection between AGN and star formation. The first part of this thesis contains a detailed analysis of the molecular gas properties of nearby infrared luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). We find that the enhanced molecular gas density in the most IR-luminous systems can be explained by major galaxy mergers, and that AGN are more likely to reside in higher-density systems. While the frequent concurrence of AGN and galaxy mergers in ULIRGs was already established, this work provides a coherent framework that explains trends observed with five molecular gas tracers with a broad range of critical densities, and a comparison with simulations that reproduce observed molecular line ratios without invoking AGN-induced chemistry. The second part of the thesis presents an analysis of the AGN content of intermediate redshift galaxies (0.3<z<1). However, identifying complete AGN samples at these redshift is challenging because it is difficult to find X-ray weak or absorbed AGN. To alleviate this problem, we developed the Mass-Excitation (MEx) diagram, which is applicable out to redshift of 1 with existing optical spectra. It improves the overall AGN census by detecting AGN that are missed in even the most sensitive X-ray surveys. The new diagnostic was used to study the concurrence of star formation and AGN in 70 micron-selected galaxies from the Far-Infrared Deep Extragalactic Legacy survey. When multiple AGN diagnostics are combined, we find not only a high AGN fraction in FIR-selected galaxies (as high as for nearby FIR-selected galaxies), but a high incidence of X-ray absorbed AGN. These findings may have considerable implications for current views about the main mode of AGN growth.

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We investigate the possibility that present-day galaxies and their dark matter halos contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic disks, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host halos. In this context two types of dynamical processes are examined in more detail. We predict the mass and abundance of MBHs in present-day galactic halos and also estimate the MBH mass accretion rates considering two different accretion scenarios. On this basis we determine the bolometric, optical and X-ray luminosity functions for the accreting MBHs using thin disk and advection dominated accretion flow models. Our predicted MBH X-ray emissions are then compared with observations of ultra-luminous X-ray sources in galaxies. We find that the slope and normalisation of the predicted X-ray luminosity function are consistent with the observations. We also estimate the rate of gravitational wave events received from MBH mergers across all redshifts. At the end of their lives the first stars may explode in supernovae that are associated with gamma ray bursts (GRBs). Provided these are in principle detectable we have estimated the expected rate of events observed.

We perform a measurement of the mass-richness relation of the redMaPPer galaxy cluster catalogue using weak lensing data from the Sloan Digital Sky Survey (SDSS). We have carefully characterized a broad range of systematic uncertainties, including shear calibration errors, photo-z biases, dilution by member galaxies, source obscuration, magnification bias, incorrect assumptions about cluster mass profiles, cluster centring, halo triaxiality and projection effects. We also compare measurements of the lensing signal from two independently produced shear and photometric redshift catalogues to characterize systematic errors in the lensing signal itself. Using a sample of 5570 clusters from 0.1 <= z <= 0.33, the normalization of our power-law mass versus. relation is log(10)[M-200m/ h-M-1(circle dot)] = 14.344 +/- 0.021 (statistical) +/- 0.023 (systematic) at a richness lambda= 40, a 7 per cent calibration uncertainty, with a power-law index of 1.33(- 0.10)(+0.09) (1 sigma). The detailed systematics characterization in this work renders it the definitive weak lensing mass calibration for SDSS redMaPPer clusters at this time.

We present the first results from an ongoing survey to characterize the circumgalactic medium (CGM) of massive high-redshift galaxies detected as submillimeter galaxies (SMGs). We constructed a parent sample of 163 SMGQSO pairs with separations less than similar to 36" by cross-matching far-infrared-selected galaxies from Herschel with spectroscopically confirmed QSOs. The Herschel sources were selected to match the properties of the SMGs. We determined the sub-arcsecond positions of six Herschel sources with the Very Large Array and obtained secure redshift identification for three of those with near-infrared spectroscopy. The QSO sightlines probe transverse proper distances of 112, 157, and 198. kpc at foreground redshifts of 2.043, 2.515, and 2.184, respectively, which are comparable to the virial radius of the similar to 10(13) M circle dot halos expected to host SMGs. High-quality absorption-line spectroscopy of the QSOs reveals systematically strong H I Ly alpha absorption around all three SMGs, with rest-frame equivalent widths of similar to 2-3 A. However, none of the three absorbers exhibit compelling evidence for optically thick H I gas or metal absorption, in contrast to the dominance of strong neutral absorbers in the CGM of luminous z similar to 2 QSOs. The low covering factor of optically thick H I gas around SMGs tentatively indicates that SMGs may not have as prominent cool gas reservoirs in their halos as the coeval QSOs and that they may inhabit less massive halos than previously thought.

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Simple models for bright, helium producing phases in the lives of massive galaxies are used to investigate the distance out to which they could be seen as individual objects. Roughly speaking, objects radiating at effective temperatures of ..;40,000 o could be detected out to redshifts as large as 8 -+12. Such redshifts correspond to densities at which we might reasonably have expected galaxy condensation to occur, except possibly for the lowest part of the probable range of go-values. Such Objects ought to be bluer than ordinary "nearby" galaxies, and for open cosmological models would be expected to be much more numerous than ordinary galaxies; for closed models the numbers of bright and ordinary galaxies should be comparable. The feasibility of detecting such objects by ground -based measures of their integrated skybrightness in the L and M windows is discussed, and it appears that such a technique would be feasible and superior to direct photographic detection only for relatively low effective temperatures in the 20,000 to 1+0,000 range. The possibility of explaining the lack of general Ljy -c4 absorption in distant WO as due to a high degree of ionization brought about by W radiation from these bright galaxies is investigated. The conclusion is that this mechanism will not usually be adequate -- and when it is adequate, the objects causing the ionization should be detectable -- unless the current mean density of uncondensed gas is very low, of the order of 10 -7 particles /cm3 or less.

The efficiency of the Bowen fluorescence mechanism in models of planetary nebulae and Seyfert galaxy nuclei has been calculated by solving the equation of transfer for He II Ly -o( and the Bowen lines using the Feautrier method. The calculated efficiencies, which do not show significant differences between planetary nebulae and Seyfert galaxies, range from about 40% to 50% for realistic models. These values are somewhat higher than recent empirical determinations of Bowen conversion in planetary nebulae. Certain discrepancies between theoretical and observed line ratios are noted, however, which make the comparison with observation ambiguous. The efficiencies are shown to be rather insensitive to changes in various parameters. It is also noted that observations do not necessarily show that the Bowen lines are systematically weaker in Seyfert galaxy nuclei than in planetary nebulae.