Theses and Dissertations from UMD
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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 Growing Intermediate-Mass Black Holes with Gravitational Waves(2006-06-05) Gultekin, Kayhan; Miller, M. Coleman; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present results of numerical simulations of sequences of binary-single scattering events of black holes in dense stellar environments. The simulations cover a wide range of mass ratios from equal mass objects to 1000:10:10 solar masses and compare purely Newtonian simulations with a relativistic endpoint, simulations in which Newtonian encounters are interspersed with gravitational wave emission from the binary, and simulations that include the effects of gravitational radiation reaction by using equations of motion that include the 2.5-order post-Newtonian force terms, which are the leading-order terms of energy loss from gravitational waves. In all cases, the sequence is terminated when the binary's merger time due to gravitational radiation is less than the arrival time of the next interloper. We also examine the role of gravitational waves during an encounter and show that close approach cross-sections for three 1-solar-mass objects are unchanged from the purely Newtonian dynamics except for close approaches smaller than 0.00001 times the initial semimajor axis of the binary. We also present cross-sections for mergers resulting from gravitational radiation during three-body encounters for a range of binary semimajor axes and mass ratios including those of interest for intermediate-mass black holes (IMBHs). We find that black hole binaries typically merge with a very high eccentricity --- extremely high when gravitational waves are included during the encounter such that when the gravitational waves are detectable by LISA, most of the binaries will have eccentricities e > 0.9 though all will have circularized by the time they are detectable by LIGO. We also investigate the implications for the formation and growth of IMBHs and find that the inclusion of gravitational waves during the encounter results in roughly half as many black holes ejected from the host cluster for each black hole accreted onto the growing IMBH. The simulations show that the Miller & Hamilton model of IMBH formation is a viable method if it is modified to start with a larger seed mass.