Astronomy Theses and Dissertations
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Item Case Studies in AGN Feedback(2022) Smith, Robyn N; Reynolds, Christopher S; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Galaxies in which the central supermassive black hole (SMBH) is actively accreting are referred to as active galactic nuclei (AGN) and are believed to play a crucial role in the evolution of both individual and clusters of galaxies. Empirically, the mass of the host galaxy and the mass of the SMBH are positively correlated. This is somewhat surprising given that the gravitational sphere of influence of the SMBH is orders or magnitude smaller than the host galaxy. The SMBH is believed to undergo periods of activity during which it is capable of powering galactic-scale outflows which in turn modulate star formation and therefore the overall mass of the host galaxy. Such processes are broadly referred to as feedback.Clusters of galaxies are the largest gravitationally bound systems in the universe. The intracluster medium (ICM) in relaxed clusters is strongly centrally peaked and suffi- ciently dense that it is expected to cool rapidly (in cosmological terms). Such cooling should create streams of cool gas flowing to the brightest cluster galaxy (BCG) which in turn should fuel high rates of star formation. Little evidence of either has been found giving rise to the ‘cooling flow problem’. AGN are again invoked to explain the absence of this cooling flow. The BCGs hosting AGN, often with powerful radio jets, are believed to inject energy into the ICM at a rate which can counteract the cooling. This cyclical nature of balancing the cooling is another form of AGN feedback. In this thesis, we present case studies of three AGN which provide unique insight into these feedback processes. Chapter 2 presents evidence for a relativistic X-ray driven outflow on accretion disk scales in an ultraluminous infrared galaxy known to host a galactic-scale molecular outflow. The observational properties which make a galaxy an ideal candidate for detection of large-scale outflows are intrinsically at odd with the properties which are ideal for detecting small-scale outflows. IRASF05189-2524, the subject of Chapter 2, is one of only a handful of galaxies for which positive detection of outflows on both small- and large-scale exist. Next, we turn our attention to AGN in BCGs and the cooling flow problem. Chapter 3 presents new Chandra observations of NGC 1275, the BCG in the famous Perseus Cluster. The high-cadence observing campaign finds X-ray variability on short intraweek timescales. The inclusion of archival observations reveals a general ‘harder when brighter’ trend. Examination of multiwavelength light curves finds a strongly correlated optical and γ-ray flare in late 2015 in which the optical emission leads the γ-ray emission by ~5 days. This robust (> 3σ) result is the first strong evidence of correlated emission with a time delay and is lends support to the idea that the γ-ray emission is produced by synchrotron self-Compton upscattering. In Chapter 4, we present new Chandra observations of the rare radio-quiet BCG quasar H1821+643. It is one of only two examples in the nearby universe of a highly luminous quasar with minimal radio jet activity at the center of a galaxy cluster. Despite observational challenges, we produce the first high-resolution spectrum of the quasar well-separated from the ICM in ~20 years. Our short-cadence observing campaign again reveals rapid variation on timescales corresponding to the light crossing time of the accretion disk. Although the flux varies, the spectrum is remarkably constant when compared to observations from previous decades. The result of this thesis is to add to the existing body of knowledge of AGN feedback on both galaxy and galaxy cluster scales. These three AGN presented various observing challenges which required a combination of non-standard observational techniques and data reduction methods in order to maximize results with current X-ray instrumentation.Item From Tantrums to Transformations: AGN Transients Discovered with the Zwicky Transient Facility(2021) Frederick, Sara; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation work has consisted of searches for extreme AGN-related outbursts during Phase I of the Zwicky Transient Facility (ZTF) survey, which has been a ground-breaking wide-field instrument for the real-time detection and regular cadence monitoring of transients in the Northern Sky. Transients found to be nuclear through photometric filtering were vetted by humans and coordinated for prompt follow-up with various rapid robotic, spectroscopic, and high energy resources, to understand the nature of the galaxy centers undergoing flares and the appearance of spectral features. Findings from this unprecedentedly high-volume data stream were often serendipitous, and led to surprising new avenues for study, including 1) the establishment of a new observational class of quiescent galaxies caught turning into quasars, 2) the discovery of a preponderance of smooth and high-amplitude optical transients hosted in NLSy1s, and 3) a framework for distinguishing extreme AGN variability from other transients in AGN. We present the results of these observations, including candidates for TDEs in AGN, changing-look AGN caught "turning-on", as well as members of the new emerging observational class of flares in Narrow-Line Seyfert 1 (NLSy1) galaxies associated with enhanced accretion (Trakhtenbrot et al. 2019). We compared the properties of these samples of flares to previously reported changing-look quasars and Seyfert galaxies, confirmed that they are a unique observational class of transients related to physical processes associated with the central supermassive black hole's accretion state, and considered the observations in the context of the physical interpretations for a range of related transients from the literature. With these unique sample sets, we also aim to understand why we have found certain galaxy types to preferentially host the sites of such rapid enhanced flaring activity, and attempt to map out the innermost environment of the accretion events. These pathfinding studies enabled with ZTF have the potential to guide how these exceptional moments of AGN evolution will be systematically discovered in future large area surveys such as the Vera C. Rubin Observatory.Item The Dynamics of Dense Stellar Systems with a Massive Black Hole(2011) Gill, Michael Allen; Miller, M. Coleman; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, we explore the dynamics of two similar types of dense stellar systems with a central black hole of mass much greater than a typical stellar object. In particular, we use numerical N-body simulations to examine the effects that the massive black hole (MBH) has on the surrounding stars and compact objects as they pertain to indirectly observable signals. The first systems we consider are the highly uncertain cusps likely comprised of primarily massive compact objects that surround the MBHs at the center of typical galaxies. The gradual inspiral of a compact object by emission of gravitational radiation, called an extreme mass-ratio inspiral (EMRI), will produce a signal that falls in the peak detection range of the space-bound laser interferometer space antenna (LISA). Despite a veritable gold mine of astrophysical data that could be gleaned from such a detection, previous investigations in the literature have left the predicted rate of these events uncertain by several orders of magnitude. We present direct N-body simulations of the innermost ≤ 100 objects with the inclusion of the first-order Post-Newtonian correction with the aim of reducing one of the key uncertainties in the dynamics of these systems - the efficiency of resonant relaxation. We find that relativistic pericenter precession prevents a significant enhancement of the EMRI rate; the rate we derive during this work is consistent with those derived in the literature from less direct methods. We do find, however, that our EMRI progenitors originate from much closer to the MBH than previous investigations have suggested was likely. Our second investigation delves into the possibility of finding intermediate-mass black holes (IMBHs), with masses ∼ 102−4 Msun, at the center of dense star clusters. Because of the substantial investment of telescope time needed to perform the multiyear proper motion studies that are likely needed to achieve a definitive detection, careful selection of candidate clusters is prudent. We provide a new observational signature of the presence of an IMBH in a dense star cluster - a quenching of mass segregation. Our ensemble of direct N-body simulations with N ≤ 32768 objects and highly varied initial conditions show that the existence of an IMBH with mass ∼ 1% of the total cluster mass limits the mass segregation in visible stars, as measured by the radial gradient in average stellar mass. This effect is consistently visible in systems that have had enough time to reach their equilibrium value of mass segregation, usually about 5 initial half-mass relaxation times. In practical terms, our method will apply to Galactic globular clusters that are fairly small, and that are unlikely to have lost a significant portion of their mass to Galactic tidal stripping. We apply this method to two of the ∼ 30 Galactic globular clusters that fit our conservative criteria for application of this method, NGC 2298 and NGC 6254(M10). Thanks to deep observations by the Hubble Space Telescope Advanced Camera for Surveys, data exist that are sufficient to allow a good comparison to our simulation data. We find that the degree of mass segregation we observe in NGC 2298 is clearly inconsistent with simulations harboring an IMBH at about the 3−σ level. In contrast, application of the method to NGC 6254 reveals a mass segregation profile that can only be explained by the presence of either an IMBH or a significant population of primordial binaries (≥ 5%). Unfortunately, a reliable measure of the binary fraction of NGC 6254 does not exist; however, NGC 6254 is a good candidate for follow-up proper motion studies.Item Black Hole Dynamics and Gravitational Radiation in Galactic Nuclei(2009) Lauburg, Vanessa; Miller, Michael C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we present new channels for the production of gravitational radiation sources: mergers of black holes in the nuclear star clusters found in many small galaxies, and mergers and tidal separations of black hole binaries in galaxies that host supermassive black holes. Mergers between stellar-mass black holes will be key sources of gravitational radiation for ground-based detectors. However, the rates of these events are highly uncertain, because we can not observe these binaries electromagnetically. In this work, we show that the nuclear star clusters found in the centers of small galaxies are conducive environments for black hole mergers. These clusters have large escape velocities, high stellar densities, and large numbers of black holes that will have multiple close encounters, which often lead to mergers. We present simulations of the three-body dynamics of black holes in this environment and estimate that, if many nuclear star clusters do not have supermassive black holes, tens of events per year will be detectable with Advanced LIGO. Larger galaxies that host supermassive black holes can produce extreme-mass ratio inspiral (EMRI) events, which are important sources for the future space-based detector, LISA. Here, we show that tidal separation of black hole binaries by supermassive black holes will produce a distinct class of EMRIs with near-zero eccentricities, and we estimate that rates from tidal separation could be comparable to or larger than those from the traditionally-discussed two-body capture formation scenario. Before tidal separation can occur, a binary encounters multiple stars as it sinks through the nucleus toward the supermassive black hole. In this region, velocities are high, and interactions with stars can destroy binaries through ionization. We investigate wide ranges in initial mass function and internal energy of the binaries, and find that tidal separations, mergers, and ionizations are all likely outcomes for binaries near the galactic center. Tidally separated binaries will contribute to the LISA detection rate, and mergers will produce tens of events per year for Advanced LIGO. We show, therefore, that galactic nuclei are promising hosts of gravitational wave sources for both LISA and LIGO.Item A Spectral Survey of Black Hole Spin in Active Galactic Nuclei(2007-09-20) Brenneman, Laura; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation explores the question of whether broad iron lines from the accretion disk can be used as viable diagnostic tools for constraining black hole spin. We begin by giving an overview of the importance of black hole angular momentum as a signature of General Relativity and as a means of testing this theory in the strong-field limit. We discuss the anatomy of the typical black hole/accretion disk system, focusing on the complex environments of active galactic nuclei, and in particular Seyfert-1 systems which we pursue in this work. After developing a robust technique for fitting the continuum and absorption parameters through a rigorous analysis of the XMM-Newton spectrum of the Sy-1 galaxy NGC 4593, we then discuss a new model we have developed that fits broad emission lines from the inner accretion disk. This model, kerrdisk, is fully relativistic and allows the black hole spin to be a free parameter in the fit. Using this model, we carefully analyze the 350 ks XMM-Newton spectrum of the Sy-1 source MCG--6-30-15, which has the broadest and best-studied iron line observed to date. Fitting for the black hole spin in this source, we conclude that a > 0.987 to 90% confidence. We then extend our source list to analyze the XMM-Newton spectra of nine other radio-quiet Sy-1 AGN that have previously been observed to harbor broad iron lines. We find that, given enough photons and a broad line indicative of an origin in the inner disk where relativistic effects are important, our new model enables us to place robust constraints on black hole spin. Four of our sampled AGN meet the criteria necessary to constrain spin. Those constraints are given, along with the full spectral fit to each source. Interestingly, the spins of these sources range from moderate (a ~ 0.5−0.7) to very high (a > 0.95), and we do not find any AGN consistent with non-rotating black holes. For those objects that had marginal spin constraints or none at all, we discuss the spectral fits and the probable reasons for the lack of robustness of our results. This is the first ever survey of black hole spin in type-1 AGN.