Astronomy
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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 Two Dimensional Velocity Fields of Low Surface Brightness Galaxies(EDP Sciences, 2005) Kuzio de Naray, R.; McGaugh, S. S.; de Blok, W. J. G.; Bosma, A.We present high resolution two dimensional velocity fields from integral field spectroscopy along with derived rotation curves for nine low surface brightness galaxies. This is a positive step forward in terms of both data quality and number of objects studied. We fit NFW and pseudo-isothermal halo models to the observations. We find that the pseudo-isothermal halo better represents the data in most cases than the NFW halo, as the resulting concentrations are lower than would be expected for CDM.Item 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.Item Simulating Observations of Dark Matter Dominated Galaxies: Towards the Optimal Halo Profile(Blackwell, 2007) de Blok, W.J.G.; Bosma, Albert Bosma; McGaugh, StacyLow Surface Brightness (LSB) galaxies are dominated by dark matter, and their rotation curves thus reflect their dark matter distribution. Recent high-resolution rotation curves suggest that their dark matter mass-density distributions are dominated by a constant-density core. This seems inconsistent with the predictions of Cold Dark Matter (CDM) models which produce halos with compact density cusps and steep mass-density profiles. However, the observationally determined mass profiles may be affected by non-circular motions, asymmetries and offsets between optical and dynamical centres, all of which tend to lower the observed slopes. Here we determine the impact of each of these effects on a variety of halo models, and compare the results with observed mass-density profiles. Our simulations suggest that no single systematic effect can reconcile the data with the cuspy CDM halos. The data are best described by a model with a soft core with an inner power-law mass-density slope ⍺ = −0.2±0.2. However, no single universal halo profile provides a completely adequate description of the data.Item The AGN and Gas Disk in the Low Surface Brightness Galaxy PGC045080(Blackwell, 2007) Das, M.; Kantharia, N.; Ramya, S.; Prabhu, T. P.; McGaugh, S. S.; Vogel, S. N.We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disk and massive HI gas content. Radio continuum observations of the galaxy were carried out at 320 MHz, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disk at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with AGN activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the H flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centers can host significant AGN activity. We also mapped the HI gas disk and velocity field in PGC 045080. The HI disk extends well beyond the optical disk and appears warped. In the HI intensity maps, the disk appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disk but is fairly uniform in the other half. We derived the HI rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of 190km s−1.Item The Balance of Dark and Luminous Mass in Rotating Galaxies(Copyright 2005 The American Physical Society, 2005) McGaugh, Stacy S.baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for CDM galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.Item The pursuit of the whole NChilada: Virtual petaflops using multi-adaptive algorithms for gravitational systems(IBM, 2004) Lake, G.; Quinn, T.; Richardson, D. C.; Stadel, J.We describe the keys to meeting the challenges of N-body simulation: adaptive potential solvers, adaptive integration, and volume renormalization. With these techniques and a dedicated teraflop facility, simulation can keep pace with observations of the universe. We also describe some problems in simulating the formation and stability of planetary systems.Item Planetesimal clusters in a Keplerian disk I. gravitational evolution(EDP Sciences, 2002-08-30) Tanga, P.; Michel, P.; Richardson, D. C.It was recently demonstrated by numerical simulations that a turbulent flow in a rotating system is capable of efficiently concentrating passively advected particles having a density larger than the fluid – inside anti-cyclonic vortices. This process has important consequences on the distribution of solid particles in protoplanetary disks, since dust surface densities 1–2 orders of magnitude higher than the background are rapidly reached in vortex cores. However, until now, the role of self-gravitation of captured solids has been neglected. In this work we study the action of mutual gravitational interactions - after the gas has dissipated - over the dynamics of planetesimals inside clusters similar to those created in vortex cores. A comparison is made between the behavior of idealized clusters of planetesimals characterized by ad-hoc velocity profiles, and more complex initial conditions such as those obtained in previous hydrodynamical simulations. We show here that, within the explored interval of parameters, mutual scattering of particles can quickly disperse the cluster. Our results are demonstrated to be not dependent on the resolution employed. It can be concluded that if large planetesimals were formed inside vortex cores, they would be ejected by mutual perturbations.Item Gravitational instability and clustering in a disk of planetesimals(EDP Sciences, 2004-08-05) Tanga, P.; Weidenschilling, S. J.; Michel, P.; Richardson, D. C.For a long time, gravitational instability in the disk of planetesimals has been suspected to be the main engine responsible for the beginning of dust growth, its advantage being that it provides for rapid growth. Its real importance in planetary formation is still debated, mainly because the potential presence of turbulence can prevent the settling of particles into a gravitationally unstable layer. However, several mechanisms could yield strongly inhomogeneous distributions of solids in the disk: radial drift, trapping in vortices, perturbations by other massive bodies, etc. In this paper we present a numerical study of a gravitationally unstable layer. This allows us to go beyond the classical analytical study of linear perturbations, exploring a highly non-linear regime. A hierarchical growth of structure in the presence of dissipation (gas drag) can yield large, virialized clusters of planetesimals, the first time such clusters have been observed in the context of planetesimal disks.Item 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.