arXiv subjects

D. Merritt

Publications and source records attributed to D. Merritt.

22 records · Page 2Linked to original sources

arXiv2002

A Self-Consistent Study of Triaxial Black-Hole Nuclei

We construct models of triaxial galactic nuclei containing central black holes using the method of orbital superposition, then verify their stability by advancing N-body realizations of the models forward in time. We assume a power-law form for the stellar density, rho ~ 1/r and 1/r^2; these correspond approximately to the nuclear density profiles of bright and faint galaxies respectively. Equidensity surfaces are ellipsoids with fixed axis ratios. The central black hole is represented by a Newtonian point mass. We consider three triaxial shapes: almost prolate, almost oblate and maximally triaxial. Two kinds of orbital solution are attempted for each mass model: the first including only regular orbits, the second including chaotic orbits as well. We find that stable configurations exist in the maximally triaxial and nearly-oblate cases; however steady-state solutions in the nearly-prolate geometry could not be found. A large fraction of the mass, of order 50% or more, could be assigned to the chaotic orbits without inducing evolution. Our results demonstrate that triaxiality may persist even within the sphere of influence of the central black hole, and that chaotic orbits may constitute an important building block of galactic nuclei.

astro-ph↗

arXiv2001

Orbital Dynamics of Triaxial Black-Hole Nuclei

Orbital motion in triaxial nuclei with central point masses, representing supermassive black holes, is investigated. The stellar density is assumed to follow a power law, rho ~ 1/r^gamma, with gamma=1 or gamma=2. At low energies the motion is essentially regular; the major families of orbits are the tubes and the pyramids. Pyramid orbits are similar to box orbits but have their major elongation parallel to the short axis of the figure. A number of regular orbit families associated with resonances also exist, most prominently the banana orbits, which are also elongated with the short axis. At a radius where the enclosed stellar mass is a few times the black hole mass, the pyramid orbits become stochastic. The energy of transition to this ``zone of chaos'' is computed as a function of gamma and of the shape of the stellar figure; it occurs at lower energies in more elongated potentials. Our results suggest that supermassive black holes may place tight constraints on departures from triaxiality in galactic nuclei, both by limiting the allowed shapes of regular orbits and by inducing chaos.

astro-ph↗

arXiv2000

Dynamics of Black-Hole Nuclei

Space-based observations are beginning to yield detailed information about the stellar kinematics at the very centers of galaxies, within the sphere of gravitational influence of the black hole. The structure and dynamics of these regions is probably determined in part by the infall and coalescence of black holes during galaxy mergers. A goal of N-body simulations is to reproduce the kinematics near the black holes as well as the relations that exist between the nuclear and global properties of galaxies. However, the problem is computationally difficult due to the wide range of length and time scales, and no single N-body code can efficiently follow the evolution from kiloparsec to sub-parsec scales. We review existing N-body work on this problem and present the first, fully self-consistent merger simulations of galaxies containing dense stellar cusps and black holes.

astro-ph↗

arXiv2000

The Nuclear Dynamics of M32. I. Data and Stellar Kinematics

We have obtained optical long-slit spectroscopy of the nucleus of M32 using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. The stellar rotation velocity and velocity dispersion, as well as the full line-of-sight velocity distribution (LOSVD), were determined as a function of position along the slit using two independent spectral deconvolution algorithms. We see three clear kinematical signatures of the nuclear black hole: a sudden upturn, at ~0.3 arc seconds from the center, in the stellar velocity dispersions; a flat or rising rotation curve into the center; and strong, non-Gaussian wings on the central LOSVD. The central velocity dispersion is ~130 km/s (Gaussian fit) or ~175 km/s (corrected for the wings). Both the velocity dispersion spike and the shape of the central LOSVD are consistent with the presence of a supermassive compact object in M32 with a mass in the range 2-5 x 10^6 solar masses. These data are a significant improvement on previous stellar kinematical data, making M32 the first galaxy for which the imprint of the black hole's gravitation on the stellar velocities has been observed with a resolution comparable to that of gas-dynamical studies.

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