arXiv subjects

David Merritt

Publications and source records attributed to David Merritt.

At least 145 records · Page 8Linked to original sources

arXiv2003

The Final Parsec Problem

The coalescence of massive black hole binaries is one of the main sources of low-frequency gravitational radiation that can be detected by LISA. When two galaxies containing massive black holes merge, a binary forms at the center of the new galaxy. We discuss the evolution of the binary after its separation decreases below one parsec. Whether or not stellar dynamical processes can drive the black holes to coalesce depends on the supply of stars that scatter against the binary. We discuss various mechanisms by which this supply can be replenished after the loss cone has been depleted.

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arXiv2003

Chaotic Loss Cones, Black Hole Fueling and the M-Sigma Relation

In classical loss cone theory, stars are supplied to a central black hole via gravitational scattering onto low angular momentum orbits. Higher feeding rates are possible if the gravitational potential near the black hole is non-axisymmetric and the orbits are chaotic. Motivated by recently published, self-consistent models, we evaluate rates of stellar capture and disruption in triaxial nuclei. Rates are found to substantially exceed those in collisionally-resupplied loss cones, as long as an appreciable fraction of the orbits are centrophilic. The mass captured by a black hole after a given time in a steep nucleus scales as the fifth power of the velocity dispersion, and the accumulated mass in 10^10 yr is of the correct order to reproduce the M-sigma relation. Triaxiality can solve the "final parsec problem" of decaying black hole binaries by increasing the flux of stars into the binary's loss cone.

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arXiv2003

Revised Rates of Stellar Disruption in Galactic Nuclei

We compute rates of tidal disruption of stars by supermassive black holes in galactic nuclei, using downwardly-revised black hole masses from the M-sigma relation. In galaxies with steep nuclear density profiles, which dominate the overall event rate, the disruption frequency varies inversely with assumed black hole mass. We compute a total rate for non-dwarf galaxies that is about a factor ten higher than in earlier studies. Disruption rates are predicted to be highest in nucleated dwarf galaxies, assuming that such galaxies contain black holes. Monitoring of a rich galaxy cluster for a few years could rule out the existence of intermediate mass black holes in dwarf galaxies.

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arXiv2003

Dark Matter Spikes and Indirect Detection

Annihilation radiation from supersymmetric particles at the Galactic center could be greatly enhanced if the dark matter density is peaked around the supermassive black hole. Arguments for and against the existence of density spikes are reviewed. Spikes are destroyed during mergers, and there is strong evidence for this effect in stellar density profiles. The dark matter spike at the Galactic center probably suffered this fate.

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arXiv2003

Single and Binary Black Holes and their Influence on Nuclear Structure

Massive central objects affect both the structure and evolution of galactic nuclei. Adiabatic growth of black holes generates power-law central density profiles with logarithmic slopes in the range from ~1.5 to ~2.5, in good agreement with the profiles observed in the nuclei of galaxies fainter than visual magnitude -20. However the shallow nuclear profiles of bright galaxies require a different explanation. Binary black holes are an inevitable result of galactic mergers, and the ejection of stars by a massive binary displaces a mass of order the binary's own mass, creating a core or shallow power-law cusp. This model is at least crudely consistent with core sizes in bright galaxies. Uncertainties remain about the effectiveness of stellar- and gas-dynamical processes at inducing coalescence of binary black holes, and uncoalesced binaries may be common in low-density nuclei.

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arXiv2002

Tracing Black Hole Mergers Through Radio Lobe Morphology

Binary supermassive black holes are produced by galactic mergers as the black holes from the two galaxies fall to the center of the merged system and form a bound pair. The two black holes will eventually coalesce in an enormous burst of gravitational radiation. Here we show that the orientation of a black hole's spin axis would change dramatically even in a minor merger, leading to a sudden flip in the direction of any associated jet. We identify the winged or X-type radio sources with galaxies in which this has occurred. The implied coalescence rate is similar to the overall galaxy merger rate, suggesting that the prospects are good for observing gravitational waves from coalescing supermassive black holes.

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arXiv2002

Instability of the Gravitational N-Body Problem in the Large-N Limit

We use a systolic N-body algorithm to evaluate the linear stability of the gravitational N-body problem for N up to 1.3 x 10^5, two orders of magnitude greater than in previous experiments. For the first time, a clear ~ln N-dependence of the perturbation growth rate is seen. The e-folding time for N = 10^5 is roughly 1/20 of a crossing time.

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arXiv2002

Supermassive Black Holes

After a brief historical introduction, we summarize current efforts and accomplishments in the study of supermassive black holes.

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arXiv2002

Dynamics of Dark Matter Cusps

Formation and disruption of dark-matter cusps are reviewed. Accumulation of baryons at the center of a halo can displace the dark matter, converting singular density cusps into low-density cores. The displaced mass can be of order 10 M_b with M_b the mass of the infalling population. If M_b is identified with the masses of the black holes currently observed at the centers of bright galaxies, predicted core radii are ~a few hundred pc. Other mechanisms, such as early mass outflow, may explain the large dark-matter cores in dwarf and low-surface-brightness galaxies. Predictions of dark matter annihilation radiation from the center of the Milky Way galaxy are shown to be strongly dependent on the galaxy's merger history.

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arXiv2002

Dark Matter Spikes and Annihilation Radiation from the Galactic Center

The annihilation rate of weakly interacting cold dark matter particles at the galactic center could be greatly enhanced by the growth of a density spike around the central supermassive black hole (SBH). Here we discuss the effects of hierarchical mergers on the central spike. Mergers between halos containing SBHs lead to the formation of SBH binaries which transfer energy to the dark matter particles, lowering their density. The predicted flux of annihiliation photons from the galactic center is several orders of magnitude smaller than in models that ignore the effects of SBHs and mergers. Measurement of the annihilation radiation could in principle be used to constrain the merger history of the galaxy.

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arXiv2002

Triaxial Black-Hole Nuclei

We demonstrate that the nuclei of galaxies containing supermassive black holes can be triaxial in shape. Schwarzschild's method was first used to construct self-consistent orbital superpositions representing nuclei with axis ratios of 1:0.79:0.5 and containing a central point mass representing a black hole. Two different density laws were considered, with power-law slopes of -1 and -2. We constructed two solutions for each power law: one containing only regular orbits and the other containing both regular and chaotic orbits. Monte-Carlo realizations of the models were then advanced in time using an N-body code to verify their stability. All four models were found to retain their triaxial shapes for many crossing times. The possibility that galactic nuclei may be triaxial complicates the interpretation of stellar-kinematical data from the centers of galaxies and may alter the inferred interaction rates between stars and supermassive black holes.

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arXiv2002

Galaxy Cores as Relics of Black Hole Mergers

We investigate the hypothesis that the cores of elliptical galaxies and bulges are created from the binding energy liberated by the coalescence of supermassive binary black holes during galaxy mergers. Assuming that the central density profiles of galaxies were initially steep power laws, we define the ``mass deficit'' as the mass in stars that had to be removed from the nucleus in order to produce the observed core. We use nonparametric deprojection to compute the mass deficit in a sample of 35 early-type galaxies with high-resolution imaging data. We find that the mass deficit correlates well with the mass of the nuclear black hole, consistent with the predictions of merger models. We argue that core sizes in halos of non-interacting dark matter particles should be comparable to those observed in the stars.

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arXiv2001

Rotational Brownian Motion of a Massive Binary

The orientation of a massive binary undergoes a random walk due to gravitational encounters with field stars. The rotational diffusion coefficient for a circular-orbit binary is derived via scattering experiments. The binary is shown to reorient itself by an angle of order (m/M)^1/2 during the time that its semi-major axis shrinks appreciably, where M is the binary mass and m the perturber mass. Implications for the orientations of rotating black holes are discussed.

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arXiv2001

Systolic and Hyper-Systolic Algorithms for the Gravitational N-Body Problem, with an Application to Brownian Motion

A systolic algorithm rhythmically computes and passes data through a network of processors. We investigate the performance of systolic algorithms for implementing the gravitational N-body problem on distributed-memory computers. Systolic algorithms minimize memory requirements by distributing the particles between processors. We show that the performance of systolic routines can be greatly enhanced by the use of non-blocking communication, which allows particle coordinates to be communicated at the same time that force calculations are being carried out. Hyper-systolic algorithms reduce the communication complexity at the expense of increased memory demands. As an example of an application requiring large N, we use the systolic algorithm to carry out direct-summation simulations using 10^6 particles of the Brownian motion of the supermassive black hole at the center of the Milky Way galaxy. We predict a 3D random velocity of 0.4 km/s for the black hole.

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arXiv2001

No Supermassive Black Hole in M33?

We analyze optical long-slit spectroscopy of the nucleus of M33 obtained from the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Rather than the steep rise expected within the radius of influence of a supermassive black hole, the velocity dispersion drops significantly within the inner parsec. Dynamical modelling yields an estimated upper limit of 3000 solar masses for the mass of a central compact object. This upper limit is however consistent within the uncertainties with the mass predicted by the M-sigma relation, which is between 2000 and 20,000 solar masses. We therefore can not conclude that the presence of a massive black hole in the nucleus of M33 would require a different formation mechanism from that of the black holes detected in galaxies with more luminous bulges.

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arXiv2001

Relationship of Black Holes to Bulges

Supermassive black holes appear to be uniquely associated with galactic bulges. The mean ratio of black hole mass to bulge mass was until recently very uncertain, with ground based, stellar kinematical data giving a value roughly an order of magnitude larger than other techniques. The discrepancy was resolved with the discovery of the M-sigma relation, which simultaneously established a tight corrrelation between black hole mass and bulge velocity dispersion, and confirmed that the stellar kinematical mass estimates were systematically too large due to failure to resolve the black hole's sphere of influence. There is now excellent agreement between the various techniques for estimating the mean black hole mass, including dynamical mass estimation in quiescent galaxies; reverberation mapping in active galaxies and quasars; and computation of the mean density of compact objects based on integrated quasar light. Implications of the M-sigma relation for the formation of black holes are discussed.

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arXiv2001

Galactic Mergers with Supermassive Black Holes

We present the results of N-body simulations of the accretion of high-density dwarf galaxies by low-density giant galaxies. Both galaxies contain power-law central density cusps and point masses representing supermassive black holes; the ratio of galaxy masses is 3:1. The cusp of the dwarf galaxy is always disrupted during the merger, leading to a remnant with a weak power law in the intrinsic density and a ``core'' in the projected density. Removing both black holes from the giant and dwarf galaxies allows the dwarf galaxy to remain intact and leads to a remnant with a high central density, contrary to what is observed. Our results support the hypothesis than the persistence of low-density cores in giant galaxies following mergers is a consequence of the existence of supermassive central black holes.

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arXiv2001

Non-integrable galactic dynamics

Orbital and self-consistent dynamics of non-integrable galaxy models are reviewed. Topics covered include torus construction; resonances; triaxial systems with central singularities; mixing and collisionless relaxation; and chaos in collisional systems.

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