Supermassive black holes (BHs) and their host-galaxies are thought to evolve in tandem, with the energy output from the rapidly-accreting BH regulating star formation and the growth of the BH itself. The goal of better understanding this process has led to much work focussing on the properties of quasars at high redshifts, $z\gtrsim 2$, when cosmic star formation and BH accretion both peaked. At these redshifts, however, ground-based statistical studies of the quasar population generally have no access to the rest-frame optical spectral region, which is needed to measure H$\beta$-based BH masses and narrow line region outflow properties. The cornerstone of this thesis has been a new near-infrared spectroscopic catalogue providing rest-frame optical data on 434 luminous quasars at redshifts $1.5 \lesssim z \lesssim 4$. At high redshift, $z \gtrsim 2$, quasar BH masses are derived using the velocity-width of the CIV broad emission-line, based on the assumption that the observed velocity-widths arise from virial-induced motions. However, CIV exhibits significant asymmetric structure which suggests that the associated gas is not tracing virial motions. By combining near-infrared spectroscopic data (covering the hydrogen Balmer lines) with optical spectroscopy from SDSS (covering CIV), we have quantified the bias in CIV BH masses as a function of the CIV blueshift. CIV BH masses are shown to be over-estimated by almost an order of magnitude at the most extreme blueshifts. Using the monotonically increasing relationship between the CIV blueshift and the mass ratio BH(CIV)/BH(H$\alpha$) we derive an empirical correction to all CIV BH-masses. The correction depends only on the CIV line properties and therefore enables the derivation of un-biased virial BH mass estimates for the majority of high-luminosity, high-redshift, spectroscopically confirmed quasars. Quasars driving powerful outflows over galactic scales is a central tenet of galaxy evolution models involving 'quasar feedback' and significant resources have been devoted to searching for observational evidence of this phenomenon. We have used [OIII] emission to probe ionised gas extended over kilo-parsec scales in luminous $z\gtrsim2$ quasars. Broad [OIII] velocity-widths and asymmetric structure indicate that strong outflows are prevalent in this population. We estimate the kinetic power of the outflows to be up to a few percent of the quasar bolometric luminosity, which is similar to the efficiencies required in recent quasar-feedback models. [OIII] emission is very weak in quasars with large CIV blueshifts, suggesting that quasar-driven winds are capable of sweeping away gas extended over kilo-parsec scales in the host galaxies. Using data from a number of recent wide-field photometric surveys, we have built a parametric SED model that is able to reproduce the median optical to infrared colours of tens of thousands of AGN at redshifts $1 < z < 3$. In individual objects, we find significant variation in the near-infrared SED, which is dominated by emission from hot dust. We find that the hot dust abundance is strongly correlated with the strength of outflows in the quasar broad line region, suggesting that the hot dust may be in a wind emerging from the outer edges of the accretion disc.

Philosophiae Doctor - PhD / The faint radio population consisting of star forming galaxies (SFG) and radio-quiet active galactic nuclei (AGN) is important in the study of galaxy evolution. However, the bulk of the faint population is below the detection threshold of the current radio surveys. I study this population through a Bayesian-stacking technique that I have adapted to probe the radio luminosity function (RLF) below the typical 5σ detection threshold. The technique works by fitting RLF models to radio flux densities extracted at the position of galaxies selected from an auxiliary catalogue. I test the technique by adding Gaussian noise (σ) to simulated data and the RLF models are in agreement with the simulated data for up to three orders of magnitude (3 dex) below the detection threshold (5σ). The source of radio emission from radio quiet quasars (subset of AGN) is widely debated. I apply the technique to 1.4-GHz flux densities from the Faint Images of the Radio Sky at Twenty-cm survey (FIRST) at the positions of the optical quasars from the Sloan Digital Sky Survey (SDSS). The RLF models are constrained to 2 dex below the FIRST detection threshold. I found that the radio luminosity where radio-quiet quasars emerge coincides with the luminosity where SFGs are expected to start to dominate the RLF. This Implies that the radio emission of radio-quiet quasars and radio-quiet AGN, in general, could have a significant contribution from star formation in the host galaxies.

It is now well established that a substantial fraction of all galaxy assembly occurs in intense bursts of star formation and black hole accretion, but the role of these two modes and how much they affect one another remains unclear. We thus investigate this in three complementary studies. In the first, we assemble a sample of 513 quasars identified by the Sloan Digital Sky Survey with detections by Herschel. These objects span a redshift range of 0 < z < 4, and their SEDs give a mean SFR of ~1000M☉/year. When comparing these SFRs to the intrinsic properties of the quasars, we find no clear connections between the quasars and the ongoing star formation events in their hosts. We then look for evidence of AGN feedback in broad absorption line (BAL) quasars, as such features are indicative of outflowing material. We find that high-ionization BAL quasars have indistinguishable properties to those of classical quasars. In our second study, which describes an iron low-ionization BAL quasar, SDSS J121441.42-000137.8, our results are again consistent with no feedback. Thus, it seems unlikely that feedback plays a dominant role in quenching star formation at the extreme SFRs seen in our BAL objects. We lastly study the host of an optically-bright quasar, SDSS J160705.16+355358.6, with evidence of an ongoing merger. We create the Point Spread Function (PSF) using a star that is in the same part of the field as our object, a method which is relatively unexplored. By subtracting the PSF, we are able to extract some of the host properties. We compare two PSF creation methods and find the empirical approach to be superior. Fits to the SEDs of the two galaxies are consistent with both falling on or above the main sequence of star formation. It is additionally plausible that these two galaxies could coalesce into a single massive quiescent galaxy by z ~ 2, and thus serve as progenitors to this class of galaxy that has proven challenging to our understanding of galaxy assembly. / Doctor of Philosophy / Quasars are among some of the brightest objects in the Universe and are powered by supermassive black holes that are rapidly accreting new material. The light from these distant objects can be detected across the electromagnetic spectrum, with each wavelength regime offering new insight into their properties. Further, if we look at their spectra, the features appear redshifted, i.e. they are at longer wavelengths compared to the expected values on Earth. More distant objects have higher redshifts. This, coupled with the constant speed of light, tells us that light from a quasar that has reached us on Earth must have been emitted many years ago; in other words, quasars offer glimpses into the past and can be used study how our Universe has assembled over time. Star formation and quasar activity in galaxies have been shown to coexist across all redshifts. This suggests a deep connection between a galaxy's stellar and black hole mass assemblies. Both peak at z ~ 2, implying that a substantial amount of all galaxy assembly took place in high-redshift, dusty bursts of star formation and quasar activity. This dust absorbs light originally emitted at optical/UV wavelengths and reradiates it in the infrared, making infrared wavelengths the perfect regime in which to investigate the connection between the two processes. In this dissertation, I have focused specifically on quasars with detections at both optical and far-infrared (FIR) wavelengths to determine what effect, if any, quasars have on the galaxies in which they reside. The optical emission of these systems describes the properties of the quasars, while the FIR estimates star formation rates (SFRs) in their hosts. Many astronomers invoke something called feedback, in which the quasar regulates the host star formation, to align theory with observations. We search for evidence of this process in the very bright quasars located within extremely star-forming systems. We, however, find no such evidence. This could imply that, at the high luminosities of our systems, feedback is not the dominant effect in regulating star formation, but perhaps some host self-regulation is instead. It could also imply that the feedback timescale is much shorter than that of either quasar or extreme star formation activity, making direct observations of feedback difficult.

This thesis presents a study of the sub-light year regions of Active Galactic Nuclei (AGN). These environments contain accretion discs that orbit a central super-massive black hole. The luminosity of the AGN inner regions varies over time across all wavelengths with variability at longer wavelengths lagging behind that at shorter wavelengths. Since the AGN themselves are too remote and too compact to resolve directly, I exploit these time lags to infer the physical characteristics of the accretion disc and surrounding gas clouds that emit broad emission lines. These characteristics include the inclination and temperature profile of the accretion disc, and the shape (or light curve) of the luminosity fluctuations that drive the accretion disc variability. This thesis details the work in the first author papers of Starkey et al. (2016, 2017), in which I detail the statistical code, CREAM (Continuum REverberting AGN Markov Chain Monte Carlo), that I developed to analyse AGN accretion disc variability. I apply the code to a set of AGN light curve observations of the Seyfert 1 galaxy NGC 5548 by the AGN STORM collaboration (De Rosa et al., 2015; Edelson et al., 2015; Fausnaugh et al., 2016a; Goad et al., 2016; Starkey et al., 2017). I also present work detailing my variability analysis of the Seyfert galaxies NGC 6814, NGC 2617, MCG 08-11-11 and NGC 4151. This work has contributed to the analysis presented in (Troyer et al. 2016, Fausnaugh et al. submitted). I also investigate the implications of a twin accretion disc structure (Nealon et al., 2015) on the disc time lag measurements across near UV and optical wavelengths. I finish by detailing a modification to CREAM that allows it to merge continuum light curves observed in a common filter, but taken by multiple telescopes with different calibration and instrumental effects to consider.

In the framework of active galactic nuclei (AGNs), a galaxy’s supermassive black hole is surrounded by a dusty torus whose clumpy configuration allows for either direct or obscured views toward the central engine. Viewing AGNs from different angles gives rise to a variety of AGN classifications; for example, the generic Type 1 AGN class requires the detection of optically broad emission lines, which arise from quickly moving material within the torus, whereas Type 2 AGNs lack these observations. While these viewing angles are not directly observable, synthetic torus models generated with CLUMPY provide a means to determine them along with other parameters that describe the nature and characteristics of the torus in general. Employing CLUMPY models with mid-infrared spectroscopic observations of a large sample of both Type 1 and Type 2 AGNs allows us to acquire a further understanding of the clumpy torus structure and its viewing angles.

Recent research has lead to the possible discovery of a new class of gamma-ray emitting ac- tive galactic nuclei (AGN). These objects – the very radio-loud narrow-line Seyfert 1 galaxies (NLSy1s)– demonstrate observational features suggesting that they are similar to blazars. One of the key characteristics of blazars is the presence of high-amplitude optical microvariability. While this phenomenon has been investigated in individual objects, no study of the intra-night variability of radio-loud NLSy1s as a class has previously been available. This dissertation presents a sys- tematic search for optical variability in a sample of 33 radio-loud NLSy1s. It was found that 26 objects demonstrated microvariations. However, only 9 objects did so with duty cycles comparable to blazars, and only 7 of these 9 objects — J0706+3901, J0849+5108, J0948+0022, J1246+0238, PKS 1502+036, J1644+2619, and IRAS 20181-2244 — demonstrated microvariability at compa- rable amplitudes. Two objects stand out as exceptional sources. J0849+5108 was found to have a duty cycle of ~90% and was observed to undergo an enormous 4-magnitude optical flare in a two-month time span. The object has not been reported to have undergone such an event since 1975. The second object, J0948+0022, is the class prototype. High cadence data indicates that J0948+0022 has a remarkably rapid doubling time scale of ~40 minutes, and it was seen to vary by over 0.9 magnitudes within an individual night. Attempts to correlate microvariability to radio loudness, gamma-ray loudness, and other parameters were largely unsuccessful. However, it was found that only radio-loud NLSy1s that were detected at gamma-ray energies demonstrated microvariability at blazar-like duty cycles. Additionally, an analysis of the frequency of microvariations at various amplitudes suggests that the sample of radio-loud NLSy1s presented in this study share a parent population identical to low energy peaked BL Lac-type (LBL) blazars. This is in agreement with the work of astronomers such as Abdo et al. 2009, who have created spectral energy distributions for a few radio-loud NLSy1s and found them to resemble those of LBLs. Blazar-like variability was found in multiple objects with radio loudnesses of log(R) < 2, suggesting that even moderately radio-loud NLSy1s may be blazar-like objects.

In this thesis I study the inner-most regions of Active Galactic Nuclei (AGN) using the reverberation mapping technique, and neutron star low-mass X-ray binaries in quiescence using X-ray observations. Using the 13-year optical monitoring data for the AGN NGC 5548, the luminosity dependence of the Hβ emitting radius was modelled using a delay map, finding that the radius scales with luminosity as predicted by recent theoretical models. Time-delays between the continuum at different wavelengths in AGN can be used to probe the accretion disc. Here, continuum time-delays in a sample of 14 AGN were used to measure the radial temperature profile of the accretion discs, determine the nuclear extinction, and measure distances to the objects. However, the distances measured correspond to a value for Hubble's constant that is a factor of ~2 lower than the accepted value. The implications of this on the thermal disc reprocessing model are discussed. I present two Chandra observations of the neutron star transient in the globular cluster NGC 6440 in quiescence, where the power-law component to the spectrum is seen to be variable between the observations, suggesting that there is ongoing residual accretion. From a Chandra observation of the globular cluster Terzan 1, I have identifed the likely quiescent counterpart to a transient previously observed in outburst, and discuss the other sources within the cluster. Using Chandra and XMM-Newton monitoring observations of two neutron star transients (KS 1731-260 and MXB 1659-29) in quiescence I have found that the neutron star crusts in both sources have now returned to thermal equilibrium with the core. These observations also indicate that the crusts in both sources may have a high thermal conductivity and that enhanced neutrino emission may be occurring in the core. Finally, the discovery of an X-ray transient with XMM-Newton is presented, and the other sources in this observation discussed.

Solving the cooling flow problem in cool-core galaxy clusters is critical to under- standing the largest structures in the universe. In addition, cool-core systems are the only places where we have observed direct evidence of AGN feedback, and thus provide the unique opportunity to test models of AGN feedback and various other physical processes. In this thesis we study the influence of momentum-driven AGN feedback on cool-core clusters using high-resolution adaptive mesh refinement (AMR) simulations. We find that run-away cooling first happens only in the central 50 pc region while no local instability develops outside the very center of the cluster. The gas is accreted onto the super-massive black hole (SMBH) which powers AGN jets at an increasing rate as the entropy continues to decrease in the core. The ICM first cools into clumps along the propagation direction of the AGN jets due to the non-linear perturbation. As the jet power increases, gas condensation occurs isotropically, forming spatially extended (up to a few tens kpc) structures that resemble the observed H&alpha; filaments in Perseus and many other cool-core cluster. Jet heating elevates the gas entropy and cooling time, halting clump formation. The cold gas that is not accreted onto the SMBH settles into a rotating disk. In the last few Gyr, the ICM cools onto the disk directly while the innermost region of the disk continues to accrete onto the SMBH, powering the AGN jets to achieve a thermal balance. The mass cooling rate averaged over 7 Gyr is &sim 30 solarmass/yr, an order of magnitude lower than the classic cooling flow value (which we obtain in runs without the AGN). Owing to its self-regulating mechanism, AGN feedback can successfully balance cooling with a wide range of model parameters. Besides suppressing cooling, our model produces cold structures in early stages (up to &sim 2 Gyr) that are in good agreement with the observations. However, the long-lived massive cold disk is unrealistic, suggesting that additional physical processes are still needed. Our recent investigation shows that star formation may play an important role.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 90-94).

Results from several studies are presented which detail explorations of the physical and spectral properties of low luminosity active galactic nuclei. An initial Sagittarius A* general relativistic magnetohydrodynamic simulation and Monte Carlo radiation transport model suggests accretion rate changes as the dominant flaring method. A similar study on M87 introduces new methods to the Monte Carlo model for increased consistency in highly energetic sources. Again, accretion rate variation seems most appropriate to explain spectral transients. To more closely resolve the methods of particle energization in active galactic nuclei accretion disks, a series of localized shearing box simulations explores the effect of numerical resolution on the development of current sheets. A particular focus on numerically describing converged current sheet formation will provide new methods for consideration of turbulence in accretion disks.