26 research outputs found
Outflows of stars due to quasar feedback
Quasar feedback outflows are commonly invoked to drive gas out of galaxies in
the early gas-rich epoch to terminate growth of galaxies. Here we present
simulations that show that AGN feedback may drive not only gas but also stars
out of their host galaxies under certain conditions. The mechanics of this
process is as following: (1) AGN-driven outflows accelerate and compress gas
filling the host galaxy; (2) the accelerated dense shells become
gravitationally unstable and form stars on radial trajectories. For the
spherically symmetric initial conditions explored here, the black hole needs to
exceed the host's M_sigma mass by a factor of a few to accelerate the shells
and the new stars to escape velocities. We discuss potential implications of
these effects for the host galaxies: (i) radial mixing of bulge stars with the
rest of the host; (ii) contribution of quasar outflows to galactic fountains as
sources of high-velocity clouds; (iii) wholesale ejection of hyper velocity
stars out of their hosts, giving rise to type II supernovae on galactic
outskirts, and contributing to reionization and metal enrichment of the
Universe; (iv) bulge erosion and even complete destruction in extreme cases
resulting in overweight or bulgeless SMBHs.Comment: 6 pages, 3 figures; accepted for publication in MNRA
GRANAT/WATCH catalogue of cosmic gamma-ray bursts: December 1989 to September 1994
We present the catalogue of gamma-ray bursts observed with the WATCH all-sky
monitor on board the GRANAT satellite during the period December 1989 to
September 1994. The cosmic origin of 95 bursts comprising the catalogue is
confirmed either by their localization with WATCH or by their detection with
other GRB experiments. For each burst its time history and information on its
intensity in the two energy ranges 8-20 keV and 20-60 keV are presented. Most
events show hardening of the energy spectrum near the burst peak. In part of
the bursts an X-ray precursor or a tail is seen at 8-20 keV. We have determined
the celestial positions of the sources of 47 bursts. Their localization regions
(at 3-sigma confidence level) are equivalent in area to circles with radii
ranging from 0.2 to 1.6 deg. The burst sources appear isotropically distributed
on the sky on large angular scales.Comment: 18 pages (including 3 tables and 7 figures), LaTeX, l-aa style.
Accepted by Astronomy and Astrophysics Suppl. Serie
The Accretion Disc Particle Method for Simulations of Black Hole Feeding and Feedback
Black holes grow by accreting matter from their surroundings. However,
angular momentum provides an efficient natural barrier to accretion and so only
the lowest angular momentum material will be available to feed the black holes.
The standard sub-grid model for black hole accretion in galaxy formation
simulations - based on the Bondi-Hoyle method - does not account for the
angular momentum of accreting material, and so it is unclear how representative
the black hole accretion rate estimated in this way is likely to be. In this
paper we introduce a new sub-grid model for black hole accretion that naturally
accounts for the angular momentum of accreting material. Both the black hole
and its accretion disc are modelled as a composite accretion disc particle. Gas
particles are captured by the accretion disc particle if and only if their
orbits bring them within its accretion radius R_acc, at which point their mass
is added to the accretion disc and feeds the black hole on a viscous timescale
t_visc. The resulting black hole accretion rate (dM/dt)_BH powers the accretion
luminosity L_acc ~ (dM/dt)_BH, which drives black hole feedback. Using a series
of controlled numerical experiments, we demonstrate that our new accretion disc
particle method is more physically self-consistent than the Bondi-Hoyle method.
We also discuss the physical implications of the accretion disc particle method
for systems with a high degree of rotational support, and we argue that the
M_BH-sigma relation in these systems should be offset from the relation for
classical bulges and ellipticals, as appears to be observed.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure
Ibuprofen controlled release from E-beam treated polycaprolactone electrospun scaffolds
Synthetic biodegradable polymers are considered to be a highly suitable materials for the targeted drug delivery devices creating. Especially promising is the use of the electrospinning technique, which makes it possible to obtain materials with a high surface-to-volume ratio that provides active diffusion of the drug into the body tissues. In order to control the rate of polymer degradation and drug release from polymer scaffolds surface modification techniques are widely used. This study was focused on the investigation of ibuprofen-loaded poly (e-caprolactone) electrospun fibrous scaffolds and modification of theirs surface. Scaffolds with two ibuprofen concentrations were obtained: 5 wt./wt. % and 10 wt./wt.%. The modification was conducted by the pulsed electron beam irradiation. The sustained release of the model drug over a period of one day from both non-treated and treated samples was demonstrated. It was shown, that treatment leads to an increase in drug release rate and does not change surface morphology of scaffolds and fibers diameter distribution
Simulations of momentum feedback by black hole winds
The observed super-massive black hole (SMBH) mass -- galaxy velocity
dispersion () correlation may be established when
winds/outflows from the SMBH drive gas out of the potential wells of classical
bulges. Here we present numerical simulations of this process in a static
isothermal potential. Simple spherically symmetric models of SMBH feedback at
the Eddington luminosity can successfully explain the and
nuclear cluster mass correlations, as well as why larger
bulges host SMBHs while smaller ones host nuclear star clusters. However these
models do not specify how SMBHs feed on infalling gas whilst simultaneously
producing feedback that drives gas out of the galaxy.
More complex models with rotation and/or anisotropic feedback allow SMBHs to
feed via a disc or regions not exposed to SMBH winds, but in these more
realistic cases it is not clear why a robust relation
should be established. In fact, some of the model predictions contradict
observations. For example, an isotropic SMBH wind impacting on a disc (rather
than a shell) of aspect ratio requires the SMBH mass to be larger
by a factor , which is opposite to what is observed. We conclude that
understanding how a SMBH feeds is as important a piece of the puzzle as
understanding how its feedback affects its host galaxy.
Finally, we note that in aspherical cases the SMBH outflows induce
differential motions in the bulge. This may pump turbulence that is known to
hinder star formation in star forming regions. SMBH feedback thus may not only
drive gas out of the bulge but also reduce the fraction of gas turned into
stars.Comment: 17 pages, to appear in MNRA
The profile of a narrow line after single scattering by Maxwellian electrons: relativistic corrections to the kernel of the integral kinetic equation
The frequency distribution of photons in frequency that results from single
Compton scattering of monochromatic radiation on thermal electrons is derived
in the mildly relativistic limit. Algebraic expressions are given for (1) the
photon redistribution function, K(nu,Omega -> nu',Omega'), and (2) the spectrum
produced in the case of isotropic incident radiation, P(nu -> nu'). The former
is a good approximation for electron temperatures kT_e < 25 keV and photon
energies hnu < 50 keV, and the latter is applicable when hnu(hnu/m_ec^2) < kT_e
< 25 keV, hnu < 50 keV. Both formulae can be used for describing the profiles
of X-ray and low-frequency lines upon scattering in hot, optically thin
plasmas, such as present in clusters of galaxies, in the coronae of accretion
disks in X-ray binaries and AGNs, during supernova explosions, etc. Both
formulae can also be employed as the kernels of the corresponding integral
kinetic equations (direction-dependent and isotropic) in the general problem of
Comptonization on thermal electrons. The K(nu,Omega -> nu',Omega') kernel, in
particular, is applicable to the problem of induced Compton interaction of
anisotropic low-frequency radiation of high brightness temperature with free
electrons in the vicinity of powerful radiosources and masers.
Fokker-Planck-type expansion (up to fourth order) of the integral kinetic
equation with the P(nu -> nu') kernel derived here leads to a generalization of
the Kompaneets equation. We further present (1) a simpler kernel that is
necessary and sufficient to derive the Kompaneets equation and (2) an
expression for the angular function for Compton scattering in a hot plasma,
which includes temperature and photon energy corrections to the Rayleigh
angular function.Comment: 29 pages, 17 figures, accepted for publication in ApJ, uses
emulateapj.sty, corrects misprints in previous astro-ph versio
Are SMBHs shrouded by "super-Oort" clouds of comets and asteroids?
The last decade has seen a dramatic confirmation that an in situ star
formation is possible inside the inner parsec of the Milky Way. Here we suggest
that giant planets, solid terrestrial-like planets, comets and asteroids may
also form in these environments, and that this may have observational
implications for Active Galactic Nuclei (AGN). Like in debris discs around main
sequence stars, collisions of large solid objects should initiate strong
fragmentation cascades. The smallest particles in such a cascade - the
microscopic dust - may provide a significant opacity. We put a number of
observational and physical constraints on AGN obscuring torii resulting from
such fragmentation cascades. We find that torii fed by fragmenting asteroids
disappear at both low and high AGN luminosities. At high luminosities, , where is the Eddington limit, the AGN radiation
pressure blows out the microscopic dust too rapidly. At low luminosities, on
the other hand, the AGN discs may avoid gravitational fragmentation into stars
and solids. We also note that these fragmentation cascades may be responsible
for astrophysically "large" dust particles of approximately micrometer sizes
that were postulated by some authors to explain unusual absorption properties
of the AGN torii.Comment: a typo in the title correcte
Fermi Bubbles in the Milky Way: the closest AGN feedback laboratory courtesy of Sgr A*?
Deposition of a massive ( to 10^5 \msun) giant molecular cloud (GMC)
into the inner parsec of the Galaxy is widely believed to explain the origin of
over a hundred unusually massive young stars born there Myr ago. An
unknown fraction of that gas could have been accreted by Sgr A*, the
supermassive black hole (SMBH) of the Milky Way. It has been recently suggested
that two observed -ray-emitting bubbles emanating from the very center
of our Galaxy were inflated by this putative activity of Sgr A*. We run a suite
of numerical simulations to test whether the observed morphology of the bubbles
could be due to the collimation of a wide angle outflow from Sgr A* by the
disc-like Central Molecular Zone (CMZ), a well known massive repository of
molecular gas in the central pc. We find that an Eddington-limited
outburst of Sgr A* lasting Myr is required to reproduce the
morphology of the {\it Fermi} bubbles, suggesting that the GMC mass was \sim
10^5 \msun and it was mainly accreted by Sgr A* rather than used to make
stars. We also find that the outflow from Sgr A* enforces strong angular
momentum mixing in the CMZ disc, robustly sculpting it into a much narrower
structure -- a ring -- perhaps synonymous with the recently reported "Herschel
ring". In addition, we find that Sgr A* outflow is likely to have induced
formation of massive star-forming GMCs in the CMZ. In this scenario, the Arches
and Quintuplet clusters, the two observed young star clusters in the central
tens of parsecs of the Galaxy, and also GMCs such as Sgr B2, owe their
existence to the recent Sgr A* activity.Comment: 18 pages, 10 figures, submitted to MNRA
Phenological shifts of abiotic events, producers and consumers across a continent
Ongoing climate change can shift organism phenology in ways that vary depending on species, habitats and climate factors studied. To probe for large-scale patterns in associated phenological change, we use 70,709 observations from six decades of systematic monitoring across the former Union of Soviet Socialist Republics. Among 110 phenological events related to plants, birds, insects, amphibians and fungi, we find a mosaic of change, defying simple predictions of earlier springs, later autumns and stronger changes at higher latitudes and elevations. Site mean temperature emerged as a strong predictor of local phenology, but the magnitude and direction of change varied with trophic level and the relative timing of an event. Beyond temperature-associated variation, we uncover high variation among both sites and years, with some sites being characterized by disproportionately long seasons and others by short ones. Our findings emphasize concerns regarding ecosystem integrity and highlight the difficulty of predicting climate change outcomes. The authors use systematic monitoring across the former USSR to investigate phenological changes across taxa. The long-term mean temperature of a site emerged as a strong predictor of phenological change, with further imprints of trophic level, event timing, site, year and biotic interactions.Peer reviewe