1,274 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
Cosmological Simulations Using Special Purpose Computers: Implementing P3M on Grape
An adaptation of the Particle-Particle/Particle-Mesh (P3M) code to the
special purpose hardware GRAPE is presented. The short range force is
calculated by a four chip GRAPE-3A board, while the rest of the calculation is
performed on a Sun Sparc 10/51 workstation. The limited precision of the GRAPE
hardware and algorithm constraints introduce stochastic errors of the order of
a few percent in the gravitational forces. Tests of this new P3MG3A code show
that it is a robust tool for cosmological simulations. The code currently
achieves a peak efficiency of one third the speed of the vectorized P3M code on
a Cray C-90 and significant improvements are planned in the near future.
Special purpose computers like GRAPE are therefore an attractive alternative to
supercomputers for numerical cosmology.Comment: 9 pages (ApJS style); uuencoded compressed PostScript file (371 kb)
Also available by anonymous 'ftp' to astro.Princeton.EDU [128.112.24.45] in:
summers/grape/p3mg3a.ps (668 kb) and WWW at:
http://astro.Princeton.EDU/~library/prep.html (as POPe-600) Send all
comments, questions, requests, etc. to: [email protected]
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