9,389 research outputs found
Dynamical Cusp Regeneration
After being destroyed by a binary supermassive black hole, a stellar density
cusp can regrow at the center of a galaxy via energy exchange between stars
moving in the gravitational field of the single, coalesced hole. We illustrate
this process via high-accuracy N-body simulations. Regeneration requires
roughly one relaxation time and the new cusp extends to a distance of roughly
one-fifth the black hole's influence radius, with density rho ~ r^{-7/4}; the
mass in the cusp is of order 10% the mass of the black hole. Growth of the cusp
is preceded by a stage in which the stellar velocity dispersion evolves toward
isotropy and away from the tangentially-anisotropic state induced by the
binary. We show that density profiles similar to those observed at the center
of the Milky Way and M32 can regenerate themselves in several Gyr following
infall of a second black hole; the presence of density cusps at the centers of
these galaxies can therefore not be used to infer that no merger has occurred.
We argue that Bahcall-Wolf cusps are ubiquitous in stellar spheroids fainter
than M_V ~ -18.5 that contain supermassive black holes, but the cusps have not
been detected outside of the Local Group since their angular sizes are less
than 0.1". We show that the presence of a cusp implies a lower limit of
\~10^{-4} per year on the rate of stellar tidal disruptions, and discuss the
consequences of the cusps for gravitational lensing and the distribution of
dark matter on sub-parsec scales.Comment: Accepted for publication in The Astrophysical Journa
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.Comment: 10 pages, 4 postscript figure
Time-Dependent Models for Dark Matter at the Galactic Center
The prospects of indirect detection of dark matter at the galactic center
depend sensitively on the mass profile within the inner parsec. We calculate
the distribution of dark matter on sub-parsec scales by integrating the
time-dependent Fokker-Planck equation, including the effects of
self-annihilations, scattering of dark matter particles by stars, and capture
in the supermassive black hole. We consider a variety of initial dark matter
distributions, including models with very high densities ("spikes") near the
black hole, and models with "adiabatic compression" of the baryons. The
annihilation signal after 10 Gyr is found to be substantially reduced from its
initial value, but in dark matter models with an initial spike,
order-of-magnitude enhancements can persist compared with the rate in
spike-free models, with important implications for indirect dark matter
searches with GLAST and Air Cherenkov Telescopes like HESS and CANGAROO.Comment: Four page
The Low End of the Supermassive Black Hole Mass Function: Constraining the Mass of a Nuclear Black Hole in NGC 205 via Stellar Kinematics
Hubble Space Telescope (HST) images and spectra of the nucleated dwarf
elliptical galaxy NGC 205 are combined with 3-integral axisymmetric dynamical
models to constrain the mass (M_BH) of a putative nuclear black hole. This is
only the second attempt, after M33, to use resolved stellar kinematics to
search for a nuclear black hole with mass below 10^6 solar masses. We are
unable to identify a best-fit value of M_BH in NGC 205; however, the data
impose a upper limit of 2.2x10^4 M_sun (1sigma confidence) and and upper limit
of 3.8x10^4 M_sun (3sigma confidence). This upper limit is consistent with the
extrapolation of the M_BH-sigma relation to the M_BH < 10^6 M_sunregime. If we
assume that NGC 205 and M33 both contain nuclear black holes, the upper limits
on M_BH in the two galaxies imply a slope of ~5.5 or greater for the M_BH-sigma
relation. We use our 3-integral models to evaluate the relaxation time (T_r)
and stellar collision time (T_coll) in NGC 205; T_r~10^8 yr or less in the
nucleus and T_coll~10^11 yr. The low value of T_r is consistent with core
collapse having already occurred, but we are unable to draw conclusions from
nuclear morphology about the presence or absence of a massive black hole.Comment: Latex emulateapj, 15 pages, 16 figures, Version accepted for
Publication in ApJ, 20 July 2005, v628. Minor changes to discussion
Statistical mechanics of collisionless orbits. II. Structure of halos
In this paper, we present the density, \rho, velocity dispersion, \sigma, and
\rho/\sigma^3 profiles of isotropic systems which have the energy distribution,
N(E)\propto[\exp(\phi_0-E)-1], derived in Paper I. This distribution, dubbed
DARKexp, is the most probable final state of a collisionless self-gravitating
system, which is relaxed in terms of particle energies, but not necessarily in
terms of angular momentum. We compare the DARKexp predictions with the results
obtained using the extended secondary infall model (ESIM). The ESIM numerical
scheme is optimally suited for the purpose because (1) it relaxes only through
energy redistribution, leaving shell/particle angular momenta unaltered, and
(2) being a shell code with radially increasing shell thickness it has very
good mass resolution in the inner halo, where the various theoretical
treatments give different predictions. The ESIM halo properties, and especially
their energy distributions, are very well fit by DARKexp, implying that the
techniques of statistical mechanics can be used to explain the structure of
relaxed self-gravitating systems.Comment: 17 pages, 8 figure
Tax-Free Corporate Acquisitions - The Law and the Proposed Regulations
The quest for interpretation is indeed an ancient one. Our life path is pervaded by a search for meanings.
It has been said of the law that it is sometimes better to have a bad rule than to have no rule. I suppose the rationale is that unsatisfactory certainties at least permit action, and are susceptible to a change for the better, the very badness of the rule serving to accelerate the equitable resolution. Being neither philosopher nor historian, I do not know whether that patience which awaits ultimate improvement is always a virtue
Merced College: An Online Library Presence
Merced College has a number of courses being taught on line and that the number continues to grow. The college is currently implementing online courses that meet the requirements of California SB520, passed in May of 2013, requiring all California students to have access to online classes at any California community college. This applies only if the classes they need are not available at their home institution. Added to this is the requirement from the accrediting agency for California Community Colleges that all students, both distance and face-to-face have equal access to library resources. As more and more classes go online, the need for a library presence in Blackboard, the current Learning Management System, also grows. The goal of this project was to explore how an online library presence would meet the needs of both online students and online faculty. Ultimately, the first two modules of an information literacy course were created that would to meet those needs. The modules were designed to stand alone as individual modules, but could be combined into a larger information literacy course if required by an online instructor.
The stakeholders in this project were Merced College online faculty, current and potential online Merced College students and Merced College librarians. Feedback from all the stakeholders was collected and used to plan improvements to these future modules. There are ongoing discussions with the stakeholders about creating more modules to meet the continuing information needs of Merced College students and instructors
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