914 research outputs found
Free-free absorption effects on Eddington luminosity
In standard treatments the Eddington luminosity is calculated by assuming
that the electron-photon cross section is well described by the Thomson cross
section which is gray (frequency independent). Here we discuss some consequence
of the introduction of free-free opacity in the Eddington luminosity
computation: in particular, due to the dependence of free-free emission on the
square of the gas density, it follows that the associated absorption cross
section increases linearly with the gas density, so that in high density
environments Eddington luminosity is correspondingly reduced. We present a
summary of an ongoing exploration of the parameter space of the problem, and we
conclude that Eddington luminosity in high density environments can be lowered
by a factor of ten or more, making it considerably easier for black holes to
accelerate and eject ambient gas.Comment: 4 pages, to appear in "Plasmas in the Laboratory and in the Universe:
new insights and new challenges", G. Bertin, D. Farina, R. Pozzoli eds., AIP
Conference Proceeding
Active Galaxies and Radiative Heating
There is abundant evidence that heating processes in the central regions of
elliptical galaxies has both prevented large-scale cooling flows and assisted
in the expulsion of metal rich gas. We now know that each such spheroidal
system harbors in its core a massive black hole weighing approximately 0.13% of
the mass in stars and also know that energy was emitted by each of these black
holes with an efficiency exceeding 10% of its rest mass. Since, if only 0.5% of
that radiant energy were intercepted by the ambient gas, its thermal state
would be drastically altered, it is worth examining in detail the interaction
between the out-flowing radiation and the equilibrium or inflowing gas. On the
basis of detailed hydrodynamic computations we find that relaxation
oscillations are to be expected with the radiative feedback quite capable of
regulating both the growth of the central black hole and also the density and
thermal state of the gas in the galaxy. Mechanical input of energy by jets may
assist or dominate over these radiative effects. We propose specific
observational tests to identify systems which have experienced strong bursts of
radiative heating from their central black holes.Comment: 16 pages, 13 figures, in press on the "Philosophical Transactions of
the Royal Society". (Fig1.eps is a low-resolution version). Resized figures,
typos in Eq. (2.1) and (2.2) correcte
Phase mixing in MOND
Dissipationless collapses in Modified Newtonian Dynamics (MOND) have been
studied by using our MOND particle-mesh N-body code, finding that the projected
density profiles of the final virialized systems are well described by Sersic
profiles with index m<4 (down to m~2 for a deep-MOND collapse). The simulations
provided also strong evidence that phase mixing is much less effective in MOND
than in Newtonian gravity. Here we describe "ad hoc" numerical simulations with
the force angular components frozen to zero, thus producing radial collapses.
Our previous findings are confirmed, indicating that possible differences in
radial orbit instability under Newtonian and MOND gravity are not relevant in
the present context.Comment: 10 pages, 3 figures. To appear in the Proceedings of the
International Workshop "Collective Phenomena in Macroscopic Systems", G.
Bertin, R. Pozzoli, M. Rome, and K.R. Sreenivasan, eds., World Scientific,
Singapor
Feedback from Central Black Holes in Elliptical Galaxies: Two-dimensional Models Compared to One-dimensional Models
We extend the black hole (BH) feedback models of Ciotti, Ostriker, and Proga
to two dimensions. In this paper, we focus on identifying the differences
between the one-dimensional and two-dimensional hydrodynamical simulations. We
examine a normal, isolated galaxy subject to the cooling flow instability
of gas in the inner regions. Allowance is made for subsequent star formation,
Type Ia and Type II supernovae, radiation pressure, and inflow to the central
BH from mildly rotating galactic gas which is being replenished as a normal
consequence of stellar evolution. The central BH accretes some of the infalling
gas and expels a conical wind with mass, momentum, and energy flux derived from
both observational and theoretical studies. The galaxy is assumed to have low
specific angular momentum in analogy with the existing one-dimensional case in
order to isolate the effect of dimensionality. The code then tracks the
interaction of the outflowing radiation and winds with the galactic gas and
their effects on regulating the accretion. After matching physical modeling to
the extent possible between the one-dimensional and two-dimensional treatments,
we find essentially similar results in terms of BH growth and duty cycle
(fraction of the time above a given fraction of the Eddington luminosity). In
the two-dimensional calculations, the cool shells forming at 0.1--1 kpc from
the center are Rayleigh--Taylor unstable to fragmentation, leading to a
somewhat higher accretion rate, less effective feedback, and a more irregular
pattern of bursting compared to the one-dimensional case.Comment: 15 pages, 10 figures, ApJ 237:26. Updated to match published versio
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
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
A physically motivated toy model for the BH-spheroid coevolution
We present a summary of the results obtained with a time-dependent, one-zone
toy model aimed at exploring the importance of radiative feedback on the
co-evolution of massive black holes (MBHs) at the center of stellar spheroids
and their stellar and gaseous components. We consider cosmological infall of
gas as well as the mass and energy return for the evolving stellar population.
The AGN radiative heating and cooling are described by assuming photoionization
equilibrium of a plasma interacting with the average quasar SED. Our results
nicely support a new scenario in which the AGN accretion phase characterized by
a very short duty-cycle (and now common in the Universe) is due to radiative
feedback. The establishment of this phase is recorded as a fossil in the
Magorrian and Mbh-sigma relations.Comment: 2 pages. Proceedings of the MPA/MPE/ESO/USM Conference "Growing Black
Holes: accretion in a cosmological context", ESO Astrophysics Symposia, A.
Merloni, S. Nayakshin and R. Sunyaev ed
N-body simulations in modified Newtonian dynamics
We describe some results obtained with N-MODY, a code for N-body simulations
of collisionless stellar systems in modified Newtonian dynamics (MOND). We
found that a few fundamental dynamical processes are profoundly different in
MOND and in Newtonian gravity with dark matter. In particular, violent
relaxation, phase mixing and galaxy merging take significantly longer in MOND
than in Newtonian gravity, while dynamical friction is more effective in a MOND
system than in an equivalent Newtonian system with dark matter.Comment: 4 pages, no figures. To appear in EAS Publication Series (Proceedings
of Symposium 7 of the JENAM 2008, Vienna
- âŠ