19 research outputs found
Evolution of supermassive black holes
Supermassive black holes (SMBHs) are nowadays believed to reside in most
local galaxies, and the available data show an empirical correlation between
bulge luminosity - or stellar velocity dispersion - and black hole mass,
suggesting a single mechanism for assembling black holes and forming spheroids
in galaxy halos. The evidence is therefore in favour of a co-evolution between
galaxies, black holes and quasars. In cold dark matter cosmogonies, small-mass
subgalactic systems form first to merge later into larger and larger
structures. In this paradigm galaxy halos experience multiple mergers during
their lifetime. If every galaxy with a bulge hosts a SMBH in its center, and a
local galaxy has been made up by multiple mergers, then a black hole binary is
a natural evolutionary stage. The evolution of the supermassive black hole
population clearly has to be investigated taking into account both the
cosmological framework and the dynamical evolution of SMBHs and their hosts.
The seeds of SMBHs have to be looked for in the early Universe, as very
luminous quasars are detected up to redshift higher than z=6. These black holes
evolve then in a hierarchical fashion, following the merger hierarchy of their
host halos. Accretion of gas, traced by quasar activity, plays a fundamental
role in determining the two parameters defining a black hole: mass and spin. A
particularly intriguing epoch is the initial phase of SMBH growth. It is very
challenging to meet the observational constraints at z=6 if BHs are not fed at
very high rates in their infancy.Comment: Extended version of the invited paper to appear in the Proceedings of
the Conference "Relativistic Astrophysics and Cosmology - Einstein's Legacy
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
Future instrumentation for the study of the Warm-Hot Intergalactic Medium
We briefly review capabilities and requirements for future instrumentation in
UV- and X-ray astronomy that can contribute to advancing our understanding of
the diffuse, highly ionised intergalactic medium.Comment: 16 pages, 4 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 19; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Astronomical Distance Determination in the Space Age: Secondary Distance Indicators
The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations. © 2018, The Author(s)
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The Cambridge–Cambridge ROSAT Serendipity Survey – II. Classification of X-ray-luminous Galaxies
We present the results of an intermediate-resolution (1.5\AA) spectroscopic study of 17 X-ray luminous narrow emission-line galaxies previously identified in the Cambridge-Cambridge {\it ROSAT} Serendipity Survey and the {\it Einstein} Extended Medium Sensitivity Survey. Emission-line ratios reveal that the sample is composed of ten Seyfert and seven starburst galaxies. Measured linewidths for the narrow Hα emission lines lie in the range 170−460kms−1. Five of the objects show clear evidence for asymmetry in the [OIII]λ5007 emission-line profile. Broad Hα emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad Hβ emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is FWHM=3900±1750kms−1. Broad (FWHM=2200±600kms−1) Hα emission is also detected in three of the starburst galaxies, which could originate from stellar winds or supernovae remnants. The mean Balmer decrement for the sample is Hα/Hβ=3, consistent with little or no reddening for the bulk of the sample. There is no evidence for any trend with X-ray luminosity in the ratio of starburst galaxies to Seyfert galaxies. Based on our previous observations, it is therefore likely that both classes of object comprise ∼10per cent of the 2keV X-ray background.Astronom
Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. VII. Variability of the optical spectrum of NGC 5548 over 4 years
We report on the results of a continuation of a large monitoring program of optical spectroscopy of the Seyfert 1 galaxy NGC 5548. The new observations presented here were obtained between 1990 December and 1992 October, and extend the existing database to nearly 1400 days, dating back to 1988 December. The continuum variations are generally smooth and well-resolved, except during the third year of this 4 year project, when the variations were apparently more rapid and of lower amplitude than observed at other times. The broad Hβ emission line is found to vary in response to the continuum variations with a lag of about 18 days, but with some changes from year to year. The Hβ transfer functions for each of the 4 yr and for the entire 4 yr database are derived by using a maximum entropy method