106 research outputs found
Emission Lines as a Tool in Search for Supermassive Black Hole Binaries and Recoiling Black Holes
Detection of electromagnetic (EM) counterparts of pre-coalescence binaries
has very important implications for our understanding of the evolution of these
systems as well as the associated accretion physics. In addition, a combination
of EM and gravitational wave signatures observed from coalescing supermassive
black hole binaries (SBHBs) would provide independent measurements of redshift
and luminosity distance, thus allowing for high precision cosmological
measurements. However, a statistically significant sample of these objects is
yet to be attained and finding them observationally has proven to be a
difficult task. Here we discuss existing observational evidence and how further
advancements in the theoretical understanding of observational signatures of
SBHBs before and after the coalescence can help in future searches.Comment: 8 pages, 8 figures, submitted to the New Astronomy Reviews as a part
of the SCSLSA-7 proceeding
Gravitational recoil: effects on massive black hole occupation fraction over cosmic time
We assess the influence of massive black hole (MBH) ejections from galaxy
centres, due to the gravitational radiation recoil, along the cosmic merger
history of the MBH population. We discuss the 'danger' of the recoil for MBHs
as a function of different MBH spin/orbit configurations and of the host halo
cosmic bias, and on how that reflects on the 'occupation fraction' of MBHs. We
assess ejection probabilities for mergers occurring in a gas-poor environment,
where the MBH binary coalescence is driven by stellar dynamical processes, and
the spin/orbit configuration is expected to be isotropically distributed. We
contrast this case with the 'aligned' case. The latter is the most realistic
situation for 'wet', gas-rich mergers, which are the expectation for
high-redshift galaxies. We find that if all halos at z>5-7 host a MBH, the
probability of the Milky Way (or similar size galaxy) to host a MBH today is
less than 50%, unless MBHs form continuously in galaxies. The 'occupation
fraction' of MBHs, intimately related to halo bias and MBH formation
efficiency, plays a crucial role in increasing the retention fraction. Small
halos, with shallow potential wells and low escape velocities, have a high
ejection probability, but the MBH merger rate is very low along their galaxy
formation merger hierarchy: MBH formation processes are likely inefficient in
such shallow potential wells. Recoils can decrease the overall frequency of
MBHs in small galaxies to ~60%, while they have little effect on the frequency
of MBHs in large galaxies (at most a 20% effect).Comment: Accepted for publication in MNRA
Imprints of recoiling massive black-holes on the hot gas of early type galaxies
Anisotropic gravitational radiation from a coalescing black hole binary is
known to impart recoil velocities of up to ~1000 km/s to the remnant black
hole. In this context, we study the motion of a recoiling black hole inside a
galaxy modelled as an Hernquist sphere, and the signature that the hole
imprints on the hot gas, using N-body/SPH simulations. Ejection of the black
hole results in a sudden expansion of the gas ending with the formation of a
gaseous core, similarly to what is seen for the stars. A cometary tail of
particles bound to the black hole is initially released along its trail. As the
black hole moves on a return orbit, a nearly spherical swarm of hot gaseous
particles forms at every apocentre: this feature can live up to ~ 100 Myr. If
the recoil velocity exceeds the sound speed initially, the black hole shocks
the gas in the form of a Mach cone in density near each super-sonic pericentric
passage. We find that the X-ray fingerprint of a recoiling black hole can be
detected in Chandra X-ray maps out to a distance of Virgo. For exceptionally
massive black holes the Mach cone and the wakes could be observed out to a few
hundred of Mpc. Detection of the Mach cone is found to become of twofold
importance: i) as a probe of high-velocity recoils and ii) as an assessment of
the scatter of the mass-sigma relation at large black hole masses.Comment: 9 pages, 8 figures, new simulations added, accepted for publication
in MNRA
The evolution of massive black hole seeds
We investigate the evolution of high redshift seed black hole masses at late
times and their observational signatures. The massive black hole seeds studied
here form at extremely high redshifts from the direct collapse of pre-galactic
gas discs. Populating dark matter halos with seeds formed in this way, we
follow the mass assembly of these black holes to the present time using a
Monte-Carlo merger tree. Using this machinery we predict the black hole mass
function at high redshifts and at the present time; the integrated mass density
of black holes and the luminosity function of accreting black holes as a
function of redshift. These predictions are made for a set of three seed models
with varying black hole formation efficiency. Given the accuracy of current
observational constraints, all 3 models can be adequately fit. Discrimination
between the models appears predominantly at the low mass end of the present day
black hole mass function which is not observationally well constrained.
However, all our models predict that low surface brightness, bulgeless galaxies
with large discs are least likely to be sites for the formation of massive seed
black holes at high redshifts. The efficiency of seed formation at high
redshifts has a direct influence on the black hole occupation fraction in
galaxies at z=0. This effect is more pronounced for low mass galaxies. This is
the key discriminant between the models studied here and the Population III
remnant seed model. We find that there exists a population of low mass galaxies
that do not host nuclear black holes. Our prediction of the shape of the black
hole mass - velocity dispersion relation at the low mass end is in agreement
with the recent observational determination from the census of low mass
galaxies in the Virgo cluster.Comment: MNRAS in pres
Formation of galactic nuclei with multiple supermassive black holes at high redshifts
We examine the formation of groups of multiple supermassive black holes
(SMBHs) in gas-poor galactic nuclei due to the high merger rate of galaxies at
high redshifts. We calculate the relative likelihood of binary, triple, and
quadruple SMBH systems, by considering the timescales for relevant processes
and combining merger trees with N-body simulations for the dynamics of stars
and SMBHs in galactic nuclei. Typical haloes today with mass M have an average mass M at
, while rare haloes with current mass M
have an average mass M at that redshift.
These cluster-size haloes are expected to host single galaxies at . We
expect about 30% galaxies within haloes with present-day mass M to contain more than two SMBHs at redshifts . For larger present-day haloes, with
M, this fraction is almost 60%. The existence of multiple SMBHs at high
redshifts can potentially explain the mass deficiencies observed in the cores
of massive elliptical galaxies, which are up to 5 times the mass of their
central BHs. Multiple SMBHs would also lead to an enhanced rate of tidal
disruption of stars, modified gravitational wave signals compared to isolated
BH binaries, and slingshot ejection of SMBHs from galaxies at high speeds in
excess of 2000 km s.Comment: 18 pages, 14 figures, 2 tables; submitted to MNRAS; comments welcom
Growing the first bright quasars in cosmological simulations of structure formation
We employ cosmological hydrodynamical simulations to study the growth of
massive black holes (BHs) at high redshifts subject to BH merger recoils from
gravitational wave emission. We select the most massive dark matter halo at z=6
from the Millennium simulation, and resimulate its formation at much higher
resolution including gas physics and a model for BH seeding, growth and
feedback. Assuming that the initial BH seeds are relatively massive, of the
order of 10^5 Msun, and that seeding occurs around z~15 in dark matter haloes
of mass 10^9-10^10 Msun, we find that it is possible to build up supermassive
BHs (SMBHs) by z=6 that assemble most of their mass during extended
Eddington-limited accretion periods. The properties of the simulated SMBHs are
consistent with observations of z=6 quasars in terms of the estimated BH masses
and bolometric luminosities, the amount of star formation occurring within the
host halo, and the presence of highly enriched gas in the innermost regions of
the host galaxy. After a peak in the BH accretion rate at z=6, the most massive
BH has become sufficiently massive for the growth to enter into a much slower
phase of feedback-regulated accretion. We explore the full range of expected
recoils and radiative efficiencies, and also consider models with spinning BHs.
In the most `pessimistic' case where BH spins are initially high, we find that
the growth of the SMBHs can be potentially hampered if they grow mostly in
isolation and experience only a small number of mergers. Whereas BH kicks can
expel a substantial fraction of low mass BHs, they do not significantly affect
the build up of the SMBHs. On the contrary, a large number of BH mergers has
beneficial consequences for the growth of the SMBHs by considerably reducing
their spin. [Abridged]Comment: 26 pages, 19 figures, minor revisions, MNRAS accepte
Endothelial dysfunction and vascular disease
The endothelium can evoke relaxations (dilatations) of the underlying vascular smooth muscle, by releasing vasodilator substances. The best characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO). The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDHF-mediated responses). Endothelium-dependent relaxations involve both pertussis toxin-sensitive G i (e.g. responses to serotonin and thrombin) and pertussis toxin-insensitive G q (e.g. adenosine diphosphate and bradykinin) coupling proteins. The release of NO by the endothelial cell can be up-regulated (e.g. by oestrogens, exercise and dietary factors) and down-regulated (e.g. oxidative stress, smoking and oxidized low-density lipoproteins). It is reduced in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively loose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and causing endothelium-dependent hyperpolarizations), endothelial cells also can evoke contraction (constriction) of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factor (EDCF). Most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells. EDCF-mediated responses are exacerbated when the production of NO is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients. © 2008 Scandinavian Physiological Society.postprin
Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)3 Project
Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the (AC)3 project has been established in 2016. It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, ship-borne, and satellite data of physical, chemical, and meteorological properties of the Arctic atmosphere, cryosphere, and upper ocean that are available for the Arctic climate research community. Short-term changes and indications of long-term trends in Arctic climate parameters have been detected using existing and new data
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