363 research outputs found
Rotating Accretion Flows: From Infinity to the Black Hole
Accretion onto a supermassive black hole of a rotating inflow is a
particularly difficult problem to study because of the wide range of length
scales involved. There have been broadly utilized analytic and numerical
treatments of the global properties of accretion flows, but detailed numerical
simulations are required to address certain critical aspects. We use the ZEUS
code to run hydrodynamical simulations of rotating, axisymmetric accretion
flows with Bremsstrahlung cooling, considering solutions for which the
centrifugal balance radius significantly exceeds the Schwarzschild radius, with
and without viscous angular momentum transport. Infalling gas is followed from
well beyond the Bondi radius down to the vicinity of the black hole. We produce
a continuum of solutions with respect to the single parameter
Mdot_Bondi/Mdot_Edd, and there is a sharp transition between two general
classes of solutions at an Eddington ratio of Mdot_Bondi/Mdot_Edd ~ few x
10^(-2). Our high inflow solutions are very similar to the standard Shakura &
Sunyaev (1973) results. But our low inflow results are to zeroth order the
stationary Papaloizou and Pringle (1984) solution, which has no accretion. To
next order in the small, assumed viscosity they show circulation, with disk and
conical wind outflows almost balancing inflow. These solutions are
characterized by hot, vertically extended disks, and net accretion proceeds at
an extremely low rate, only of order alpha times the inflow rate. Our
simulations have converged with respect to spatial resolution and temporal
duration, and they do not depend strongly on our choice of boundary conditions.Comment: accepted for publication in Ap
The Tree-Particle-Mesh N-body Gravity Solver
The Tree-Particle-Mesh (TPM) N-body algorithm couples the tree algorithm for
directly computing forces on particles in an hierarchical grouping scheme with
the extremely efficient mesh based PM structured approach. The combined TPM
algorithm takes advantage of the fact that gravitational forces are linear
functions of the density field. Thus one can use domain decomposition to break
down the density field into many separate high density regions containing a
significant fraction of the mass but residing in a very small fraction of the
total volume. In each of these high density regions the gravitational potential
is computed via the tree algorithm supplemented by tidal forces from the
external density distribution. For the bulk of the volume, forces are computed
via the PM algorithm; timesteps in this PM component are large compared to
individually determined timesteps in the tree regions. Since each tree region
can be treated independently, the algorithm lends itself to very efficient
parallelization using message passing. We have tested the new TPM algorithm (a
refinement of that originated by Xu 1995) by comparison with results from
Ferrell & Bertschinger's P^3M code and find that, except in small clusters, the
TPM results are at least as accurate as those obtained with the
well-established P^3M algorithm, while taking significantly less computing
time. Production runs of 10^9 particles indicate that the new code has great
scientific potential when used with distributed computing resources.Comment: 24 pages including 9 figures, uses aaspp4.sty; revised to match
published versio
What does the local black hole mass distribution tell us about the evolution of the quasar luminosity function?
We present a robust method to derive the duty cycle of QSO activity based on
the empirical QSO luminosity function and on the present-day linear relation
between the masses of supermassive black holes and those of their spheroidal
host stellar systems. It is found that the duty cycle is substantially less
than unity, with characteristic values in the range .
Finally, we tested the expectation that the QSO luminosity evolution and the
star formation history should be roughly parallel, as a consequence of the
above--mentioned relation between BH and galaxy masses.Comment: 2 pages, to appear on ESO Astrophysics Symposia "The Mass of Galaxies
at Low and High Redshift", R. Bender and A. Renzini, ed
Reasoning From Fossils: Learning From the Local Black Hole Population About the Evolution of Quasars
We discuss a simple model for the growth of supermassive black holes (BHs) at
the center of spheroidal stellar systems. In particular, we assess the
hypotheses that (1) star formation in spheroids and BH fueling are proportional
to one another, and (2) the BH accretion luminosity stays near the Eddington
limit during luminous quasar phases. With the aid of this simple model, we are
able to interpret many properties of the QSO luminosity function, including the
puzzling steep decline of the characteristic luminosity from redshift z=2 to to
z=0: indeed the residual star formation in spheroidal systems is today limited
to a small number of bulges, characterized by stellar velocity dispersions a
factor of 2-3 smaller those of the elliptical galaxies hosting QSOs at z > 2. A
simple consequence of our hypotheses is that the redshift evolution of the QSO
emissivity and of the star formation history in spheroids should be roughly
parallel. We find this result to be broadly consistent with our knowledge of
the evolution of both the global star formation rate, and of the evolution of
the QSO emissivity, but we identify interesting discrepancies at both low and
high redshifts, to which we offer tentative solutions. Finally, our hypotheses
allow us to present a robust method to derive the duty cycle of QSO activity,
based on the observed QSO luminosity function, and on the present-day relation
between the masses of supermassive BHs and those of their spheroidal host
stellar systems. The duty cycle is found to be substantially less than unity,
with characteristic values in the range (3-6)x10^(-3), and we compute that the
average bolometric radiative efficiency is epsilon=0.07. Finally, we find that
the growth in mass of individual black holes at high redshift (z>2) can be
dominated by mergers, and is therefore not necessarily limited by accretion.Comment: Submitted to ApJ, 26 preprint pages with 3 figure
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