690 research outputs found
Quasars at z=6: the survival of the fittest
The Sloan Digital Sky survey detected luminous quasars at very high redshift,
z>6. Follow-up observations indicated that at least some of these quasars are
powered by supermassive black holes (SMBHs) with masses in excess of billion
solar masses. SMBHs, therefore, seem to have already existed when the Universe
was less than 1 Gyr old, and the bulk of galaxy formation still has to take
place. We investigate in this paper to which extent accretion and dynamical
processes influence the early growth of SMBHs. We assess the impact of (i)
black hole mergers, (ii) the influence of the merging efficiency and (iii) the
negative contribution due to dynamical effects which can kick black holes out
of their host halos (gravitational recoil). We find that if accretion is always
limited by the Eddington rate via a thin disc, the maximum radiative efficiency
allowed to reproduce the LF at z=6 is of order 12%, when the adverse effect of
the gravitational recoil is taken into consideration. Dynamical effects cannot
be neglected in studies of high-redshift SMBHs. If black holes can accrete at
super-critical rate during an early phase, reproducing the observed SMBH mass
values is not an issue, even in the case that the recoil velocity is in the
upper limits range, as the mass ratios of merging binaries are skewed towards
low values, where the gravitational recoil effect is very mild. We propose that
SMBH growth at early times is very selective, and efficient only for black
holes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure
Radiatively-Driven Outflows and Avoidance of Common-Envelope Evolution in Close Binaries
Recent work on Cygnus X-2 suggests that neutron-star or black-hole binaries
survive highly super-Eddington mass transfer rates without undergoing
common-envelope evolution. We suggest here that the accretion flows in such
cases are radiation pressure-dominated versions of the "ADIOS" picture proposed
by Blandford and Begelman (1999), in which almost all the mass is expelled from
large radii in the accretion disk. We estimate the maximum radius from which
mass loss is likely to occur, and show that common-envelope evolution is
probably avoided in any binary in which a main-sequence donor transfers mass on
a thermal timescale to a neutron star or black hole, even though the mass
transfer rate may reach values of 0.001 solar masses per year. This conclusion
probably applies also to donors expanding across the Hertzsprung gap, provided
that their envelopes are radiative. SS433 may be an example of a system in this
state.Comment: 4 pages, submitted to Astrophysical Journal Letters, 26 March 199
Turbulent mixing layers in the interstellar medium of galaxies
We propose that turbulent mixing layers are common in the interstellar medium (ISM). Injection of kinetic energy into the ISM by supernovae and stellar winds, in combination with density and temperature inhomogeneities, results in shear flows. Such flows will become turbulent due to the high Reynolds number (low viscosity) of the ISM plasma. These turbulent boundary layers will be particularly interesting where the shear flow occurs at boundaries of hot (approximately 10(exp 6) K) and cold or warm (10(exp 2) - 10(exp 4) K) gas. Mixing will occur in such layers producing intermediate-temperature gas at T is approximately equal to 10(exp 5.0) - 10(exp 5.5) that radiates strongly in the optical, ultraviolet, and EUV. We have modeled these layers under the assumptions of rapid mixing down to the atomic level and steady flow. By including the effects of non-equilibrium ionization and self-photoionization of the gas as it cools after mixing, we predict the intensities of numerous optical, infrared, and ultraviolet emission lines, as well as absorption column densities of C 4, N 5, Si 4, and O 6
IUE absorption studies of broad- and narrow-line gas in Seyfert galaxies
The interstellar medium of a galaxy containing an active nucleus may be profoundly affected by the high energy (X-ray, EUV) continuum flux emanating from the central source. The energetic source may photoionize the interstellar medium out to several kiloparsecs, thereby creating a global H II region. The International Ultraviolet Explorer (IUE) satellite has attempted to observe in several Seyfert galaxies (NGC 3516, NGC 4151, NGC 1068, 3C 120) the narrow absorption lines expected from such global H II regions. Instead, in two of the galaxies (NGC 3516, NGC 4151) broad, variable absorption lines at C IV lambda 1550, N V lambda 1240, and Si IV lambda 1400 were found, as well as weaker absorption features at O I lambda 1302 and C II lambda 1335. These features swamp any possible global H II region absorption. Such broad absorption features have previously been observed in IUE data, but their origin is still not well understood
Mechanical heating by active galaxies
Jets and winds are significant channels for energy loss from accreting black
holes. These outflows mechanically heat their surroundings, through shocks as
well as gentler forms of heating. We discuss recent efforts to understand the
nature and distribution of mechanical heating by central AGNs in clusters of
galaxies, using numerical simulations and analytic models. Specifically, we
will discuss whether the relatively gentle `effervescent heating' mechanism can
compensate for radiative losses in the central regions of clusters, and account
for the excess entropy observed at larger radii.Comment: 10 pages, no figures. Submitted to Philosophical Transactions of the
Royal Society (Series A: Mathematical, Physical and Engineering Sciences),
proceedings of the Poyal Society Discussion Meeting on the Impact of Active
Galaxies on the Universe at Large, London, February 16-17, 200
Constraints on First-Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background
We examine the constraints on high-redshift star formation, ultraviolet and
X-ray pre-ionization, and the epoch of reionization at redshift z_r, inferred
from the recent WMAP-5 measurement, tau_e = 0.084 +/- 0.016, of the electron
scattering optical depth of the cosmic microwave background (CMB). Half of this
scattering can be accounted for by the optical depth, tau_e = 0.04-0.05, of a
fully ionized intergalactic medium (IGM) at z < z_GP = 6-7, consistent with
Gunn-Peterson absorption in neutral hydrogen. The required additional optical
depth, Delta-tau_e = 0.03 +/- 0.02 at z > z_GP, constrains the ionizing
contributions of first light sources. WMAP-5 also measured a significant
increase in small-scale power, which lowers the required efficiency of star
formation and ionization from mini-halos. Early massive stars (UV radiation)
and black holes (X-rays) can produce a partially ionized IGM, adding to the
residual electrons left from incomplete recombination. Inaccuracies in
computing the ionization history, x_e(z), and degeneracies in cosmological
parameters (Omega_m, Omega_b, sigma_8, n_s) add systematic uncertainty to the
measurement and modeling of . From the additional optical depth from
sources at z > z_GP, we limit the star-formation efficiency, the rate of
ionizing photon production for Pop III and Pop II stars, and the photon escape
fraction, using standard histories of baryon collapse, minihalo star formation,
and black-hole X-ray preionization.Comment: Greatly revised version, based on WMAP-5 results and new models.
Accepted for ApJ (2008
Hierarchical build-up of galactic bulges and the merging rate of supermassive binary black holes
The hierarchical build-up of galactic bulges should lead to the build-up of
present-day supermassive black holes by a mixture of gas accretion and merging
of supermassive black holes. The tight relation between black hole mass and
stellar velocity dispersion is thereby a strong argument that the supermassive
black holes in merging galactic bulges do indeed merge. Otherwise the ejection
of supermassive black holes by gravitational slingshot would lead to excessive
scatter in this relation. At high redshift the coalescence of massive black
hole binaries is likely to be driven by the accretion of gas in the major
mergers signposted by optically bright QSO activity. If massive black holes
only form efficiently by direct collapse of gas in deep galactic potential
wells with v_c > 100 km/s as postulated in the model of Kauffmann & Haehnelt
(2000) LISA expects to see event rates from the merging of massive binary black
holes of about 0.1-1 yr^{-1} spread over the redshift range 0 < z < 5. If,
however, the hierarchical build-up of supermassive black holes extends to
pre-galactic structures with significantly shallower potential wells event
rates may be as high as 10-100 yr^{-1} and will be dominated by events from
redshift z > 5.Comment: 8 pages, 4 postscript figures. Proceedings of the 4th International
LISA Symposium, Penn State University, 19-24 July 2002, ed. L S Fin
Super-Eddington Atmospheres that Don't Blow Away
We show that magnetized, radiation dominated atmospheres can support steady
state patterns of density inhomogeneity that enable them to radiate at far
above the Eddington limit, without suffering mass loss. The inhomogeneities
consist of periodic shock fronts bounding narrow, high-density regions,
interspersed with much broader regions of low density. The radiation flux
avoids the regions of high density, which are therefore weighed down by
gravity, while gas in the low-density regions is slammed upward into the shock
fronts by radiation force. As the wave pattern moves through the atmosphere,
each parcel of matter alternately experiences upward and downward forces, which
balance on average. Magnetic tension shares the competing forces between
regions of different densities, preventing the atmosphere from blowing apart.
We calculate the density structure and phase speed of the wave pattern, and
relate these to the wavelength, the density contrast, and the factor by which
the net radiation flux exceeds the Eddington limit. In principle, this factor
can be as large as the ratio of magnetic pressure to mean gas pressure, or the
ratio of radiation pressure to gas pressure, whichever is smaller. Although the
magnetic pressure must be large compared to the mean gas pressure in order to
support a large density contrast, it need not be large compared to the
radiation pressure. These highly inhomogeneous flows could represent the
nonlinear development of the "photon bubble" instability discovered by Gammie.
We briefly discuss the applicability of these solutions to astrophysical
systems.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journa
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