1,245 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
On the motion and radiation of charged particles in strong electromagnetic waves. 1 - Motion in plane and spherical waves
Motion and radiation of charged particles in strong electromagnetic waves in plane and spherical wave
Evidence of widespread degradation of gene control regions in hominid genomes
Although sequences containing regulatory elements located close to protein-coding genes are often only weakly conserved during evolution, comparisons of rodent genomes have implied that these sequences are subject to some selective constraints. Evolutionary conservation is particularly apparent upstream of coding sequences and in first introns, regions that are enriched for regulatory elements. By comparing the human and chimpanzee genomes, we show here that there is almost no evidence for conservation in these regions in hominids. Furthermore, we show that gene expression is diverging more rapidly in hominids than in murids per unit of neutral sequence divergence. By combining data on polymorphism levels in human noncoding DNA and the corresponding human¿chimpanzee divergence, we show that the proportion of adaptive substitutions in these regions in hominids is very low. It therefore seems likely that the lack of conservation and increased rate of gene expression divergence are caused by a reduction in the effectiveness of natural selection against deleterious mutations because of the low effective population sizes of hominids. This has resulted in the accumulation of a large number of deleterious mutations in sequences containing gene control elements and hence a widespread degradation of the genome during the evolution of humans and chimpanzees
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
Topology of Large-Scale Structure by Galaxy Type: Hydrodynamic Simulations
The topology of large scale structure is studied as a function of galaxy type
using the genus statistic. In hydrodynamical cosmological CDM simulations,
galaxies form on caustic surfaces (Zeldovich pancakes) then slowly drain onto
filaments and clusters. The earliest forming galaxies in the simulations
(defined as ``ellipticals") are thus seen at the present epoch preferentially
in clusters (tending toward a meatball topology), while the latest forming
galaxies (defined as ``spirals") are seen currently in a spongelike topology.
The topology is measured by the genus (= number of ``donut" holes - number of
isolated regions) of the smoothed density-contour surfaces. The measured genus
curve for all galaxies as a function of density obeys approximately the
theoretical curve expected for random-phase initial conditions, but the early
forming elliptical galaxies show a shift toward a meatball topology relative to
the late forming spirals. Simulations using standard biasing schemes fail to
show such an effect. Large observational samples separated by galaxy type could
be used to test for this effect.Comment: Princeton University Observatory, submitted to The Astrophysical
Journal, figures can be ftp'ed from ftp://astro.princeton.edu/cen/TOP
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