Astronomy

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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.
Studies of SOHO Comets
(2008-08-20) Knight, Matthew Manning; A'Hearn, Michael F.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
We present a study of the Kreutz, Marsden, and Kracht comets observed by SOHO including photometric reductions and analysis, numerical modeling, and physical modeling. We analyze the results of our photometric study of more than 900 lightcurves of Kreutz comets observed by SOHO. We find that they do not have a bimodal distance of peak brightness as previously reported, but instead peak from 10.5-14 solar radii (prior to perihelion), suggesting there is a continuum of compositions rather than two distinct subpopulations. The lightcurves have two rates of brightening, typically ~r^-7.3 when first observed by SOHO then rapidly transitioning to ~r^-3.8 between 20-30 solar radii. It is unclear at what distance the steeper slope begins, but it likely does not extend much beyond the SOHO field of view. We derive nuclear sizes up to ~50 meters in radius for the SOHO observed comets, with a cumulative size distribution of N(>R)~R^-2.2 for comets larger than 5 meters in radius. This size distribution cannot explain the six largest members of the family seen from the ground, suggesting that either the family is not collisionally evolved or that the distribution is not uniform around the orbit. After correcting for the changing discovery circumstances, the flux of comets reaching perihelion has increased since 1996, and the increase is seen in comets of all sizes. We consider the Marsden and Kracht comets together due to their apparent dynamical linkage. Seasonal effects of the viewing geometry make it impossible to build a characteristic lightcurve of either group. Many are seen to survive perihelion and most reach a peak brightness within ~6 hours of perihelion with no preference for peaks before or after perihelion. Most are barely above the detection threshold, and the largest is probably smaller than 30 meters in radius. Our dynamical simulations suggest that the orbital distribution of the Kracht group can be produced by low velocity fragmentation events and close approaches to Jupiter over the last 50-250 years. We construct fragmentation trees for the Marsden and Kracht groups and predict that 7-8 comets in each group may be visible on their next perihelion passage.
Extragalactic X-ray Surveys of ULXs and AGNs
(2008-08-12) Winter, Lisa Marie; Reynolds, Christopher; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Extragalactic X-ray studies provide unique opportunities for studying accreting black holes. In particular, they are necessary for studying phenomena not easily selected or observed in other wavelengths. Among these objects, ultra-luminous X-ray sources (ULXs) emit the vast majority of their luminosity in the X-ray band and are very faint or confused in other wavebands. Similarly, heavily obscured active galactic nuclei (AGN) with absorbing columns > 10^{24} cm^{-2} are rarely detected in optical surveys, due to the extreme reddening. In my thesis, I study both phenomenon in the local universe. At ULX luminosities [L_X (0.3 - 10 keV) > 3 x 10^{39} erg s^{-1}], the European Space Agency's XMM-Newton satellite provides the spectral resolution and sensitivity necessary to study the population of local ULXs. Thus, we conducted an XMM-Newton archival study of a complete sample of the ULXs located within 8 Mpc. Our study confirmed key predictions of the intermediate mass black hole (IMBH) hypothesis for local ULXs. We then followed-up this study by investigating high signal-to-noise XMM-Newton observations of 14 ULX sources - studying their spectral shape, testing the validity of different accretion disk and power law models, and then using absorption of their spectra to measure the oxygen and iron abundances of the interstellar medium of their host galaxies. New breakthroughs are expected in the study of heavily obscured AGN from SWIFT. The SWIFT satellite, launched in 2004, has detected a sample of 153 AGN with the Burst Alert Telescope (BAT) in the first 9-months of data. The BAT is sensitive in the 14 - 195 keV band and the selected sources have an average redshift of ~ 0.03. Thus, it detects local AGN without bias towards all but the most obscured sources (n_H > 10^24 atoms cm^-2). The BAT AGN sources are the result of an all-sky survey with a flux limit of F_BAT > 10^{-11} erg s^{-1} cm^{-2}. I analyze and present the results of X-ray data from XMM-Newton, ASCA, as well as SWIFT's XRT (0.3 - 10 keV) and BAT, in order to understand the properties of obscured and unobscured AGN in the local universe. Among our results, we show that the new class of ``hidden''/buried AGN are a significant population of local AGN (~ 20%). We also find that our data supports the need for a modified AGN unified model -- one which includes a luminosity dependence.
Hydrodynamic Models of AGN Feedback in Cooling Core Clusters
(2008-05-20) Vernaleo, John C.; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
X-ray observations show that the Intra Cluster Medium (ICM) in many galaxy clusters is cooling at a rapid rate, often to the point that it should have radiated away all of its energy in less than the age of the cluster. There is however a very clear lack of enough cool end products of this gas in the centers of the clusters. Energetic arguments indicate that Active Galactic Nuclei (AGN) should be capable of heating the inner regions of clusters enough to offset the radiative cooling; truncating massive galaxy formation and solving the cooling flow problem. We present three sets of high resolution, ideal hydrodynamic simulations with the ZEUS code to test this AGN heating paradigm. For the first set of simulations, we study the dependence of the interaction between the AGN jets and the ICM on the parameters of the jets themselves. We present a parameter survey of two-dimensional (axisymmetric) models of back-to-back jets injected into a cluster atmosphere. We follow the passive evolution of the resulting structures. These simulations fall into roughly two classes, cocoon-bounded and non-cocoon bounded. We find that the cocoon-bounded sources inject significantly more entropy into the core regions of the ICM atmosphere, even though the efficiency with which the energy is thermalized is independent of the morphological class. In all cases, a large fraction of the energy injected by the jet ends up as gravitational potential energy due to the expansion of the atmosphere. For the second set, we present three-dimensional simulations of jetted AGN that act in response to cooling-mediated accretion of an ICM atmosphere. We find that our models are incapable of producing a long term balance of heating and cooling; catastrophic cooling can be delayed by the jet action but inevitably takes hold. At the heart of the failure of these models is the formation of a low density channel through which the jet can freely flow, carrying its energy out of the cooling core. Finally, we present a set of simulations with both feedback and precessing jets. The addition of jet precession is not sufficient to couple the jets to the ICM energetically although it can deposit a large amount of energy in sound waves. These sound waves are lost to the system in ideal hydrodynamics, but ultimately may provide a powerful heating mechanism for clusters cores by AGN when additional physical effects are taken into account.
A Decade of Rossi X-ray Timing Explorer Seyfert Observations:An RXTE Seyfert Spectral Database
(2008-04-24) Mattson, Barbara; Weaver, Kimberly; Reynolds, Christopher; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
With over forty years of X-ray observations, we should have a grasp on the X-ray nature of active galactic nuclei (AGN). The unification model of Antonucci and Miller (1985) offered a context for understanding observations by defining a "typical" AGN geometry, with observed spectral differences explained by line-of-sight effects. However, the emerging picture is that the central AGN is more complex than unification alone can describe. We explore the unified model with a systematic X-ray spectral study of bright Seyfert galaxies observed by the Rossi X-Ray Timing Explorer (RXTE) over its first 10 years. We develop a spectral-fit database of 821 time-resolved spectra from 39 Seyfert galaxies fitted to a model describing the effects of an X-ray power-law spectrum reprocessed and absorbed by material in the central AGN region. We observe a relationship between radio and X-ray properties for Seyfert 1s, with the spectral parameters differing between radio-loud and radio-quiet Seyfert 1s. We also find a complex relationship between the Fe K equivalent width (EW) and the power-law photon index (Gamma) for the Seyfert 1s, with a correlation for the radio-loud sources and an anti-correlation for the radio-quiet sources. These results can be explained if X-rays from the relativistic jet in radio-loud sources contribute significantly to the observed spectrum. We observe scatter in the EW-Gamma relationship for the Seyfert 2s, suggesting complex environments that unification alone cannot explain. We see a strong correlation between Gamma and the reflection fraction (R) in the Seyfert 1 and 2 samples, but modeling degeneracies are present, so this relationship cannot be trusted as instructive of the AGN physics. For the Seyfert 1 sample, we find an anticorrelation between EW and the 2 to 10 keV luminosity (L_X), also known as the X-ray Baldwin effect. This may suggest that higher luminosity sources contain less material or may be due to a time-lag effect. We do not observe the previously reported relationship between Gamma and the ratio of L_X to the Eddington luminosity.
The Star Formation History of Low Surface Brightness Galaxies
(2007-11-14) Kim, Ji Hoon; McGaugh, Stacy S; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The star formation histories of low surface brightness galaxies are interesting but poorly constrained. These objects tend to be rather blue, contradicting the initial impression that they may simply be faded remnants of higher surface brightness galaxies whose star formation has finished. Other scenarios span a broad range: a young mean age, less dust, a lower metallicity, perhaps even a variable IMF. Distinguishing between these scenarios requires sufficient information to build stellar population synthesis models which, if not unique, at least exclude certain possibilities. The total stellar mass (M*) of a galaxy is most closely traced with the Ks-band light. Considering that this mass must form over a Hubble time, this in effect gives a measure of the time averaged star formation rate ( ~M* Ho). H alpha emission traces the location of star formation, and also provides a fairly robust quantitative measure of its current rate. We have obtained near-infrared broadband photometry and H alpha photometry of a large sample of low surface brightness galaxies to measure the current and the time-averaged star formation rate in order to constrain their star formation histories. The current star formation rates of LSBGs generally are higher than their past star formation rate, suggesting that the mean age of their stellar population is relatively young. This may stem from either a late epoch of formation or a sluggish evolution. In the latter case, the star formation efficiency may be an increasing function of time, perhaps due in part to the slow build up of metals and dust. Nevertheless, star formation remains sporadic and is generally not well organized across the disk. We find a strong correlation between the ratio of current to past average star formation rate and the gas mass fraction. Galaxies with large reservoirs of gas have relatively high current SFRs. There is a conspicuous absence of high gas mass fraction, low SFR galaxies, suggesting that the observed trend is not driven by bursts of star formation with short duty cycles.
Dust Structure and Composition Within Molecular Clouds and Cores
(2007-10-02) Chapman, Nicholas; Mundy, Lee; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
We observed three molecular clouds and four isolated cores at wavelengths from 3.6-24 microns. The clouds we observed were Ophiuchus, Perseus, and Serpens and the cores were L204C-2, L1152, L1155C-2, and L1228. Our goal was to use these deep infrared data to map changes in the extinction law and the dust properties throughout the observed regions. In our clouds, we found the lowest density regions have an IRAC extinction law similar to the one observed in the diffuse ISM. At higher extinctions, there is evidence for grain growth because the extinction law flattens compared to the diffuse ISM law and becomes more consistent with a model utilizing larger dust grains. In the densest regions of Serpens and Perseus, Ak > 2, it appears icy mantles are forming on the dust grains. We detected one low extinction region in Perseus with an anomalous extinction law that is not explained by current ideas about grain growth or the formation of ices onto dust grains. The extinction law in the cores shows only a slight flattening of the extinction law with increased extinction. Even at the lowest extinctions, the extinction law is more consistent with a dust model containing grain growth, rather than with the diffuse ISM. Two of the four cores have evidence for ices forming the densest regions. Molecular outflows appear to have an impact on the dust grains in two of our cores: L1152 and L1228. In both our clouds and cores, the extinction law at 24 microns is almost always higher than the value predicted by current dust models, but is consistent with other observations. We find some evidence for the 24 micron extinction law decreasing as the extinction increases. Overall, there are relatively few stars with detections >3 sigma at 24 microns. More observations are needed to understand the nature of the extinction law at this wavelength.
Gas Kinematics and Dynamics: Spiral Structure and Cloud Formation in Disk Galaxies
(2007-09-25) Shetty, Rahul; Ostriker, Eve C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Star formation in disk galaxies occurs as a result of various physical mechanisms acting in the interstellar medium. Molecular clouds, the sites of star formation, grow from the diffuse interstellar medium due to a combination of gas self-gravity, galactic rotation, and magnetohydrodynamic effects. Observations have suggested that spiral arms promote star formation by compressing gas as it flows through the arms. When the density is sufficiently high, gravitational instability causes gas to collapse and form clouds. After the formation of stars within such clouds, the subsequent evolution of the cloud and the surrounding ISM is dramatically altered. Feedback effects such as stellar winds, ionizing radiation, and supernova explosions inject energy into the surrounding medium; these processes may halt the collapse, and perhaps even destroy the natal clouds. In this thesis, we study the flow of gas through the spiral arms of the grand-design galaxy M51; additionally, through numerical simulations, we model the growth of clouds in spiral arms and investigate the effect of feedback on cloud formation and disk dynamics. We use both observational and numerical methodologies to study gas kinematics and dynamics in spiral galaxies. Using CO and H alpha velocity fields we study spiral arm streaming in M51. With numerical simulations, we investigate gravitational instability in disk galaxies, which leads to the growth of clouds. In order to study the subsequent evolution of the gaseous disk, we include feedback effects that return dense cloud gas back into the surrounding ISM. We find that the simple description of a stationary spiral pattern in M51 is inaccurate. Our numerical models suggest that sheared features can grow regardless of the presence of grand-design spiral structure, but that spiral perturbations cause arm clouds to grow, along with distinct spiral substructure. We find that feedback can significantly affect the evolution of the gaseous disk. We suggest that the disk thickness is important in setting the rate at which stars form. The turbulence scale also needs to be considered, both for the growth of clouds and stars, as well as for the evolution of any large-scale spiral pattern.
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.
Resonant and Secular Orbital Interations
(2007-08-01) Zhang, Ke; Hamilton, Douglas P; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In stable solar systems, planets remain in nearly elliptical orbits around their stars. Over longer timescales, however, their orbital shapes and sizes change due to mutual gravitational perturbations. Orbits of satellites around a planet vary for the same reason. Because of their interactions, the orbits of planets and satellites today are different from what they were earlier. In order to determine their original orbits, which are critical constraints on formation theories, it is crucial to understand how orbits evolve over the age of the Solar System. Depending on their timescale, we classify orbital interactions as either short-term (orbital resonances) or long-term (secular evolution). My work involves examples of both interaction types. Resonant history of the small Neptunian satellites In satellite systems, tidal migration brings satellite orbits in and out of resonances. During a resonance passage, satellite orbits change dramatically in a very short period of time. We investigate the resonant history of the six small Neptunian moons. In this unique system, the exotic orbit of the large captured Triton (with a circular, retrograde, and highly tilted orbit) influences the resonances among the small satellites very strongly. We derive an analytical framework which can be applied to Neptune's satellites and to similar systems. Our numerical simulations explain the current orbital tilts of the small satellites as well as constrain key physical parameters of both Neptune and its moons. Secular orbital interactions during eccentricity damping Long-term periodic changes of orbital shape and orientation occur when two or more planets orbit the same star. The variations of orbital elements are superpositions of the same number of fundamental modes as the number of planets in the system. We investigate how this effect interacts with other perturbations imposed by external disturbances, such as the tides and relativistic effects. Through analytical studies of a system consisting of two planets, we find that an external perturbation exerted on one planet affects the other indirectly. We formulate a general theory for how both orbits evolve in response to an arbitrary externally-imposed slow change in eccentricity.

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