654 research outputs found
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
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
Long Gamma-Ray Bursts as standard candles
As soon as it was realized that long GRBs lie at cosmological distances,
attempts have been made to use them as cosmological probes. Besides their use
as lighthouses, a task that presents mainly the technological challenge of a
rapid deep high resolution follow-up, researchers attempted to find the Holy
Grail: a way to create a standard candle from GRB observables. We discuss here
the attempts and the discovery of the Ghirlanda correlation, to date the best
method to standardize the GRB candle. Together with discussing the promises of
this method, we will underline the open issues, the required calibrations and
how to understand them and keep them under control. Even though GRB cosmology
is a field in its infancy, ongoing work and studies will clarify soon if and
how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N.
Gehrels & J. Nouse
Study of the variability of Blazars gamma-ray emission
The gamma-ray emission of blazar jets shows a pronounced variability and this
feature provides limits to the size and to the speed of the emitting region. We
study the gamma-ray variability of bright blazars using data from the first 18
months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space
Telescope. From the daily light-curves of the blazars characterized by a
remarkable activity, we firstly determine the minimum variability time-scale,
giving an upper limit for the size of the emitting region of the sources,
assumed to be spheroidal blobs in relativistic motion. These regions must be
smaller than ~10^-3 parsec. Another interesting time-scale is the duration of
the outbursts. We conclude that they cannot correspond to radiation produced by
a single blob moving relativistically along the jet, but they are either the
signature of emission from a standing shock extracting energy from a modulated
jet, or the superposition of a number of flares occurring on a shorter
time-scale. We also derive lower limits on the bulk Lorentz factor needed to
make the emitting region transparent for gamma-rays interacting through
photon-photon collisions.Comment: 11 pages, 7 figures, accepted for publication on Advances in Space
Research. Poster presented at COSPAR 2010 (Bremen), event E11 (Time
variability at high energies: a probe of AGN physics
Instability of toroidal magnetic field in jets and plerions
Jets and pulsar-fed supernova remnants (plerions) tend to develop highly
organized toroidal magnetic field. Such a field structure could explain the
polarization properties of some jets, and contribute to their lateral
confinement. A toroidal field geometry is also central to models for the Crab
Nebula - the archetypal plerion - and leads to the deduction that the Crab
pulsar's wind must have a weak magnetic field. Yet this `Z-pinch' field
configuration is well known to be locally unstable, even when the magnetic
field is weak and/or boundary conditions slow or suppress global modes. Thus,
the magnetic field structures imputed to the interiors of jets and plerions are
unlikely to persist.
To demonstrate this, I present a local analysis of Z-pinch instabilities for
relativistic fluids in the ideal MHD limit. Kink instabilities dominate,
destroying the concentric field structure and probably driving the system
toward a more chaotic state in which the mean field strength is independent of
radius (and in which resistive dissipation of the field may be enhanced). I
estimate the timescales over which the field structure is likely to be
rearranged and relate these to distances along relativistic jets and radii from
the central pulsar in a plerion.
I conclude that a concentric toroidal field is unlikely to exist well outside
the Crab pulsar's wind termination shock. There is thus no dynamical reason to
conclude that the magnetic energy flux carried by the pulsar wind is much
weaker than the kinetic energy flux. Abandoning this inference would resolve a
long-standing puzzle in pulsar wind theory.Comment: 28 pages, plain TeX. Accepted for publication in Ap
Pathways to massive black holes and compact star clusters in pre-galactic dark matter haloes with virial temperatures > 10000K
Large dynamic range numerical simulations of atomic cooling driven collapse
of gas in pre-galactic DM haloes with T_vir ~ 10000 K show that the gas loses
90% and more of its angular momentum before rotational support sets in. In a
fraction of these haloes where the metallicity is low and UV radiation
suppresses H_2 cooling, conditions are thus very favourable for the rapid
build-up of massive black holes. Depending on the progression of metal
enrichment, the continued suppression of H_2 cooling by external and internal
UV radiation and the ability to trap the entropy produced by the release of
gravitational energy, the gas at the centre of the halo is expected to form a
supermassive star, a stellar-mass black hole accreting at super-Eddington
accretion rates or a compact star-cluster undergoing collisional run-away of
massive stars at its centre. In all three cases a massive black hole of
initially modest mass finds itself at the center of a rapid inflow of gas with
inflow rates of ~ 1 M_solar\yr. The massive black hole will thus grow quickly
to a mass of 10^5 to 10^6 M_solar until further inflow is halted either by
consumption of gas by star formation or by the increasing energy and momentum
feedback from the growing massive black hole. Conditions for the formation of
massive seed black holes in this way are most favourable in haloes with T_vir ~
15000 K and V_vir ~ 20 km\s with less massive haloes not allowing collapse of
gas by atomic cooling and more massive haloes being more prone to
fragmentation. This should imprint a characteristic mass on the mass spectrum
of an early population of massive black hole seeds in pre-galactic haloes which
will later grow into the observed population of supermassive black holes in
galactic bulges.Comment: 13 pages, 8 figures. Submitted to MNRA
The Merging History of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the
evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the
local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first
POPIII stars. As these pregalactic holes become incorporated through a series
of mergers into larger and larger halos, they sink to the center owing to
dynamical friction, accrete a fraction of the gas in the merger remnant to
become supermassive, form a binary system, and eventually coalesce. A simple
model in which the damage done to a stellar cusps by decaying BH pairs is
cumulative is able to reproduce the observed scaling relation between galaxy
luminosity and core size. An accretion model connecting quasar activity with
major mergers and the observed BH mass-velocity dispersion correlation
reproduces remarkably well the observed luminosity function of
optically-selected quasars in the redshift range 1<z<5. We finally asses the
potential observability of the gravitational wave background generated by the
cosmic evolution of SMBH binaries by the planned space-born interferometer
LISA.Comment: 4 pages, 2 figures, Contribute to "Multiwavelength Cosmology",
Mykonos, Greece, June 17-20, 200
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