We present and analyse quasi-simultaneous multi-epoch spectral energy distributions (SEDs) of seven reverberation-mapped active galactic nuclei (AGNs) for which accurate black hole mass measurements and suitable archival data are available from the 'AGNWatch' monitoring programs. We explore the potential of optical-UV and X-ray data, obtained within 2 d, to provide more accurate SED-based measurements of individual AGN and quantify the impact of source variability on key measurements typically used to characterize the black hole accretion process plus on bolometric correction factors at 5100 angstrom, 1350 angstrom and for the 2-10 keV X-ray band, respectively. The largest SED changes occur on long time-scales (greater than or similar to 1 year). For our small sample, the 1 mu m to 10 keV integrated accretion luminosity typically changes by 10 per cent on short time-scales (over 20 d), by similar to 30 per cent over a year, but can change by 100 per cent or more for individual AGN. The extreme ultraviolet (EUV) gap is the most uncertain part of the intrinsic SED, introducing a similar to 25 per cent uncertainty in the accretion-induced luminosity, relative to the model independent interpolation method that we adopt. That aside, our analysis shows that the uncertainty in the accretion-induced luminosity, the Eddington luminosity ratio and the bolometric correction factors can be reduced (by a factor of two or more) by use of the SEDs built from data obtained within 20 d. However, (M) over dot and eta are mostly limited by the unknown EUV emission and the unknown details of the central engine and our aspect angle.

During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert 1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far UV continuum and broad emission line variations decorrelated for similar to 60-70 days, starting similar to 75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterized by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission line flux deficits occurred for the high ionization, collisionally excited lines C IV and Si IV(+O IV]), and also He II(+O III]), while the anomaly in Ly alpha was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with E-ph > 54 eV relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon broad line region (BLR) clouds by a moving veil of material lying between the inner accretion disk and inner (BLR), perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies > 54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

Swift monitoring of NGC 4151 with an similar to 6 hr sampling over a total of 69 days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50 keV) and six in the ultraviolet (UV)/optical (1900-5500 angstrom). The three hardest X-ray bands (> 2.5 keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging similar to 3-4 days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by similar to 0.5-1 days. This combination of greater than or similar to 3 day lags between the X-rays and UV and less than or similar to 1 day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.

It has become clear in recent years that the study of interacting/merging galaxies plays an important role in understanding important astrophysical phenomena. This thesis presents an observational study of interacting/merging galaxies at radio frequencies. The observations have been carried out at extremely high resolution using very long baseline interferometry. The observations described here include: (1) A study of the high velocity Hi absorption associated with the peculiar galaxy NGC 1275; (2) A study of the radio continuum and Hi absorption of the ULIRG IRAS 172080014; (3) A study of the radio continuum and Hi absorption of the LIRG NGC 7674. Some of the most prominent results of these observations include: Detection of several narrow Hi absorption features in the high velocity system associated with NGC 1275. These Hi absorption lines were observed toward the strong radio nucleus 3C 84. The results indicate the existence of several Hi clouds with velocity differences and widths similar to those seen in Galactic neutral hydrogen absorption and similar to some of the Hi absorption seen in the Large Magellanic Cloud. The discovery of an extreme nuclear starburst region in the advanced merger system IRAS 172080014. Our results suggest a star formation rate of 84 M yr-1, and a supernova rate of 4 yr-1. Hi absorption is detected in multiple components with optical depths ranging between 0.3 and 2.5, and velocity widths between 58 and 232 km s-1. The detection of complex jet structures in the inner 1 kpc region of the galaxy NGC 7674. At full resolution, several compact sources are observed with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shock-like features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. At least eight Hi absorption lines are detected toward some of the continuum emission regions in NGC 7674. If the widest Hi feature in our observations is rotationally broadened by a central supermassive black hole, the implied dynamical mass is about 107 M.

We present a combined morphological and X-ray analysis of Was. 49, an isolated, dual-AGN system notable for the presence of a dominant AGN, Was 49b, in the disk of the primary galaxy, Was 49a, at a projected radial distance of 8. kpc from the nucleus. Using X-ray data from Chandra, the Nuclear Spectroscopic Telescope Array, and Swift, we find that this AGN has a bolometric luminosity of L-bol similar to 10(45) erg s(-1), with a black hole mass of M-BH = 1.3(-0.9)(+10)M(circle dot) . Despite the large mass, our analysis of optical data from the Discovery Channel Telescope shows that the supermassive black hole (SMBH) is hosted by a stellar counterpart with a mass of only 5.6(-2.6)(+4.9)M(circle dot), which makes the SMBH potentially larger than expected from SMBH-galaxy scaling relations, and the stellar counterpart exhibits a morphology that is consistent with dwarf elliptical galaxies. Our analysis of the system in the r and K bands indicates that Was. 49 is a minor merger, with the mass ratio of Was 49b to Was 49a between similar to 1:7 and similar to 1:15. This is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers and that minor mergers predominantly enhance AGN activity in the primary galaxy.

We investigate the origin of the parsec-scale radio emission from the changing-look active galactic nucleus (AGN) of Mrk 590, and examine whether the radio power has faded concurrently with the dramatic decrease in accretion rates observed between the 1990s and the present. We detect a compact core at 1.6 and 8.4 GHz using new Very Long Baseline Array observations, finding no significant extended, jet-like features down to similar to 1 pc scales. The flat spectral index (alpha(8.4)(1.6) = 0.03) and high brightness temperature (T-b similar to 10(8) K) indicate self-absorbed synchrotron emission from the AGN. The radio to X-ray luminosity ratio of log(L-R/L-X) similar to -5, similar to that in coronally active stars, suggests emission from magnetized coronal winds, although unresolved radio jets are also consistent with the data. Comparing new Karl G. Jansky Very Large Array measurements with archival and published radio flux densities, we find 46 per cent, 34 per cent, and (insignificantly) 13 per cent flux density decreases between the 1990s and the year 2015 at 1.4 GHz, 5 GHz and 8.4 GHz, respectively. This trend, possibly due to the expansion and fading of internal shocks within the radio-emitting outflow after a recent outburst, is consistent with the decline of the optical-UV and X-ray luminosities over the same period. Such correlated variability demonstrates the AGN accretion-outflow connection, confirming that the changing-look behaviour in Mrk 590 originates from variable accretion rates rather than dust obscuration. The present radio and X-ray luminosity correlation, consistent with low/hard state accretion, suggests that the black hole may now be accreting in a radiatively inefficient mode.

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 angstrom to 9157 angstrom) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination i = 36 degrees +/- 10 degrees, temperature T-1= (44 +/- 6) x 10(3) K at 1 light day from the black hole, and a temperature-radius slope (T proportional to r(-alpha)) of alpha = 0.99 +/- 0.03. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/L-Edd = 0.1.

We present a survey of 72 Seyfert galaxies and quasars observed by the Far Ultraviolet Spectroscopic Explorer (FUSE). Our survey is the largest to date searching for intrinsic UV absorption with high spectral resolution, and is the first step toward a more comprehensive study of intrinsic absorption in low-redshift AGN. We have determined that 72 of 253 available active galactic nuclei (AGN) are viable targets for detection of intrinsic absorption lines. We examined these spectra for signs of intrinsic absorption in the O VI doublet (lambda lamdba 1031.9, 1037.6) and Lyman beta (lambda 1025.7). The fraction of Seyfert 1 galaxies and low-redshift quasars at z<0.15 that show evidence of intrinsic UV absorption is ~50, which is slightly lower than Crenshaw et al. found (60%) based on a smaller sample of C IV absorption in Seyfert 1 galaxies observed with the Hubble Space Telescope (HST). The intrinsic absorption lines are mostly blueshifted with respect to the rest frame of the galaxy, indicating outflow of ionized gas from the AGN. With this new fraction we find a global covering factor of the absorbing gas with respect to the central nucleus of ~0.4. We also provide a deeper analysis of the intrinsic absorption features we found in 35 objects. We have characterized the relation between luminosity and velocity, and examined the relationships between equivalent width, full width at half maximum, velocity and continuum flux. The luminosity/velocity correlation has been explored previously by Laor & Brandt (2002), but at a significantly higher redshift and heavily weighted by Broad Absorption Line quasars. Our survey is for lower redshift and lower luminosity objects, mostly Seyfert galaxies. We have also explored each object with multiple observations for variablity in each of the aforementioned quantities and characterized the variation of equivalent width with continuum flux. Variability for low-z AGN has been seen in the past. In our survey, we find that variability of O VI (lambda lambda 1032, 1038) is less common than for the UV doublets of C IV and N V seen at longer wavelengths, because the O VI absorption is usually saturated. Lyman beta absorption variability is more frequent. In the target-by-target examination we find that Broad Absorption Line (BAL) features and Narrow Absorption Line (NAL) features are related, in that they follow a single relationship between the maximum outflow velocity and the AGN luminosity, and both can be exhibited in similar luminosity objects. IRAS F22456-5125 is one particular Seyfert galaxy that we have selected for modelling due to its interesting assortment of intrinsic absorption lines. It shows a system of five individual kinematic components of absorption features in both O VI lines and in several of the Lyman series lines. We find that each of the components are relatively simple to model and appear to be weak in the X-ray.

Post-starbursts (PSBs) are candidate for rapidly transitioning from starbursting to quiescent galaxies. We study the molecular gas evolution of PSBs at z similar to 0.03-0.2. We undertook new CO (2-1) observations of 22 Seyfert PSB candidates using the Arizona Radio Observatory Submillimeter Telescope. This sample complements previous samples of PSBs by including green-valley PSBs with Seyfert-like emission, allowing us to analyse for the first time the molecular gas properties of 116 PSBs with a variety of AGN properties. The distribution of molecular gas to stellar mass fractions in PSBs is significantly different from normal star-forming galaxies in the CO Legacy Database (COLD) GASS survey. The combined samples of PSBs with Seyfert-like emission line ratios have a gas fraction distribution that is even more significantly different and is broader (similar to 0.03-0.3). Most of them have lower gas fractions than normal star-forming galaxies. We find a highly significant correlation between the WISE 12 and 4.6 mu m flux ratios and molecular gas fractions in both PSBs and normal galaxies. We detect molecular gas in 27 per cent of our Seyfert PSBs. Taking into account the upper limits, the mean and the dispersion of the distribution of the gas fraction in our Seyfert PSB sample are much smaller (mu = 0.025, sigma = 0.018) than previous samples of Seyfert PSBs or PSBs in general (mu similar to 0.1-0.2, sigma similar to 0.1-0.2).

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured active galactic nuclei (AGNs) at high accretion luminosities, L-X(2-10 keV) greater than or similar to 10(44) erg s(-1), and out to redshift z approximate to 1.5. The sample covers an area of about 14 deg(2) and provides constraints on the space density of powerful AGNs over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, N-H approximate to 10(24) cm(-2). The fraction of obscured Compton-thin (N-H = 10(22) - 10(24) cm(-2)) AGNs is estimated to be approximate to 0.35 for luminosities L-X(2-10 keV) > 10(44) erg s(-1), independent of redshift. For less luminous sources, the fraction of obscured Compton-thin AGNs increases from 0.45 +/- 0.10 at z = 0.25 to 0.75 +/- 0.05 at z = 1.25. Studies that select AGNs in the infrared via template fits to the observed spectral energy distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGNs (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGNs as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGNs is primarily a function of redshift. This fraction increases from about 20-30 per cent at z = 0.25 to about 50-70 per cent at z = 1.5.