39 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
LISA observations of massive black hole mergers: event rates and issues in waveform modelling
The observability of gravitational waves from supermassive and
intermediate-mass black holes by the forecoming Laser Interferometer Space
Antenna (LISA), and the physics we can learn from the observations, will depend
on two basic factors: the event rates for massive black hole mergers occurring
in the LISA best sensitivity window, and our theoretical knowledge of the
gravitational waveforms. We first provide a concise review of the literature on
LISA event rates for massive black hole mergers, as predicted by different
formation scenarios. Then we discuss what (in our view) are the most urgent
issues to address in terms of waveform modelling. For massive black hole binary
inspiral these include spin precession, eccentricity, the effect of high-order
Post-Newtonian terms in the amplitude and phase, and an accurate prediction of
the transition from inspiral to plunge. For black hole ringdown, numerical
relativity will ultimately be required to determine the relative quasinormal
mode excitation, and to reduce the dimensionality of the template space in
matched filtering.Comment: 14 pages, 2 figures. Added section with conclusions and outlook.
Matches version to appear in the proceedings of 10th Annual Gravitational
Wave Data Analysis Workshop (GWDAW 10), Brownsville, Texas, 14-17 Dec 200
Hawking radiation and thermodynamics of dynamical black holes in phantom dominated universe
The thermodynamic properties of dark energy-dominated universe in the
presence of a black hole are investigated in the general case of a varying
equation-of-state-parameter . We show that all the thermodynamics
quantities are regular at the phantom divide crossing, and particularly the
temperature and the entropy of the dark fluid are always positive definite. We
also study the accretion process of a phantom fluid by black holes and the
conditions required for the validity of the generalized second law of
thermodynamics. As a results we obtain a strictly negative chemical potential
and an equation-of-state parameter Comment: 22 pages,3 figure
Intermediate and extreme mass-ratio inspirals â astrophysics, science applications and detection using LISA
Black hole binaries with extreme (gtrsim104:1) or intermediate (~102â104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
Very Large Baseline Array observations of Mrk 6 : probing the jet-lobe connection
We present the results of high-resolution VLBI (very long baseline interferometry) observations at 1.6 and 4.9 GHz of the radio-loud Seyfert galaxy, Mrk 6. These observations are able to detect a compact radio core in this galaxy for the first time. The core has an inverted spectral index (α1.6 4.9 = +1.0 ± 0.2) and a brightness temperature of 1 Ă 108 K. Three distinct radio components, which resemble jet elements and/or hotspots, are also detected. The position angles of these elongated jet elements point not only to a curved jet in Mrk 6, but also towards a connection between the AGN and the kpc-scale radio lobes/bubbles in this galaxy. Firmer constraints on the star formation rate provided by new Herschel observations (SFR <0.8 Mâ yr-1) make the starburst-wind-powered bubble scenario implausible. From plasma speeds, obtained via prior Chandra X-ray observations, and ram pressure balance arguments for the interstellar medium and radio bubbles, the north-south bubbles are expected to take 7.5 Ă 106 yr to form, and the east-west bubbles 1.4 Ă 106 yr. We suggest that the jet axis has changed at least once in Mrk 6 within the last â107 yr. A comparison of the nuclear radio-loudness of Mrk 6 and a small sample of Seyfert galaxies with a subset of low-luminosity FR I radio galaxies reveals a continuum in radio properties.Peer reviewe
The Formation and Evolution of the First Massive Black Holes
The first massive astrophysical black holes likely formed at high redshifts
(z>10) at the centers of low mass (~10^6 Msun) dark matter concentrations.
These black holes grow by mergers and gas accretion, evolve into the population
of bright quasars observed at lower redshifts, and eventually leave the
supermassive black hole remnants that are ubiquitous at the centers of galaxies
in the nearby universe. The astrophysical processes responsible for the
formation of the earliest seed black holes are poorly understood. The purpose
of this review is threefold: (1) to describe theoretical expectations for the
formation and growth of the earliest black holes within the general paradigm of
hierarchical cold dark matter cosmologies, (2) to summarize several relevant
recent observations that have implications for the formation of the earliest
black holes, and (3) to look into the future and assess the power of
forthcoming observations to probe the physics of the first active galactic
nuclei.Comment: 39 pages, review for "Supermassive Black Holes in the Distant
Universe", Ed. A. J. Barger, Kluwer Academic Publisher
High redshift quasars and the supermassive black hole mass budget: constraints on quasar formation models
We investigate the constraints on models of supermassive black hole (SMBH)
and quasar formation obtainable from two recent observational developments: the
discovery of luminous quasars at z~6, and estimates of the local mass density
of SMBHs. If ~90 per cent of this mass was accreted at redshifts z<3, as
suggested by the observed quasar luminosity functions, these joint constraints
pose a challenge for models, which must account for the observed luminous
quasar population at z~6 within a very limited `mass budget'. We investigate a
class of models based within the hierarchical structure formation scenario, in
which major mergers lead to black hole formation and fuelling, and the
resulting quasars shine at their Eddington-limited rate until their fuel is
exhausted. We show that the simplest such model, in which a constant fraction
of the gas within the halo is accreted in each major merger, cannot satisfy
both constraints simultaneously. When this model is normalized to reproduce the
number density of luminous quasars at z~6, the mass budget is grossly exceeded
due to an overabundance of lower mass SMBHs. We explore a range of
modifications to the simple model designed to overcome this problem. We show
that both constraints can be satisfied if the gas accretion fraction scales as
a function of the halo virial velocity. Similar scalings have been proposed in
order to reproduce the local M-sigma relation. Successful models can also be
constructed by restricting the formation of seed black holes to redshifts above
z~11.5 or to haloes above a velocity threshold ~55 km/s, or assuming that only
a fraction of major mergers result in formation of a seed SMBH. (abridged)Comment: 19 pages, 6 figures, 1 table. v2: Corrected references. v3: Extended
Section 5.1, corrected Section 3.2, various other corrections and additions
suggested by referee. Accepted by MNRAS in this for