577 research outputs found
Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and Clusters
The radiative cooling timescales at the centers of hot atmospheres
surrounding elliptical galaxies, groups, and clusters are much shorter than
their ages. Therefore, hot atmospheres are expected to cool and to form stars.
Cold gas and star formation are observed in central cluster galaxies but at
levels below those expected from an unimpeded cooling flow. X-ray observations
have shown that wholesale cooling is being offset by mechanical heating from
radio active galactic nuclei. Feedback is widely considered to be an important
and perhaps unavoidable consequence of the evolution of galaxies and
supermassive black holes. We show that cooling X-ray atmospheres and the
ensuing star formation and nuclear activity are probably coupled to a
self-regulated feedback loop. While the energetics are now reasonably well
understood, other aspects of feedback are not. We highlight the problems of
atmospheric heating and transport processes, accretion, and nuclear activity,
and we discuss the potential role of black hole spin. We discuss X-ray imagery
showing that the chemical elements produced by central galaxies are being
dispersed on large scales by outflows launched from the vicinity of
supermassive black holes. Finally, we comment on the growing evidence for
mechanical heating of distant cluster atmospheres by radio jets and its
potential consequences for the excess entropy in hot halos and a possible
decline in the number of distant cooling flows.Comment: Accepted for publication in New Journal of Physics Focus Issue on
Clusters of Galaxie
The role of cooling flows in galaxy formation
The present structure of galaxies is governed by the radiative dissipation of
the gravitational and supernova energy injected during formation. A crucial
aspect of this process is whether the gas cools as fast as it falls into the
gravitational potential well. If it does then rapid normal star formation is
assumed to ensue. If not, and the gas can still cool by the present time, then
the situation resembles that of a cooling flow, such as commonly found in
clusters of galaxies. The cooled matter is assumed to accumulate as very cold
clouds and/or low mass stars, i.e. as baryonic dark matter. In this paper we
investigate the likelihood of a cooling flow phase during the hierarchical
formation of galaxies. We concentrate on the behaviour of the gas, using a
highly simplified treatment of the evolution of the dark matter potential
within which the gas evolves. We assume that normal star formation is limited
by how much gas the associated supernovae can unbind and allow the gas profile
to flatten as a consequence of supernova energy injection. We find that cooling
flows are an important phase in the formation of most galaxies with total (dark
plus luminous) masses approxgt 10^12 Msun , creating about 20 per cent of the
total dark halo in a galaxy such as our own and a smaller but comparable
fraction of an elliptical galaxy of similar mass. The onset of a cooling flow
determines the upper mass limit for the formation of a visible spheroid from
gas, setting a characteristic mass scale for normal galaxies. We argue that
disk formation requires a cooling flow phase and that dissipation in the
cooling flow phase is the most important factor in the survival of normal
galaxies during subsequent hierarchical mergers.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Fuelling quasars with hot gas
We consider a model for quasar formation in which massive black holes are
formed and fuelled largely by the accretion of hot gas during the process of
galaxy formation. In standard hierarchical collapse models, objects about the
size of normal galaxies and larger form a dense hot atmosphere when they
collapse. We show that if such an atmosphere forms a nearly "maximal" cooling
flow, then a central black hole can accrete at close to its Eddington limit.
This leads to exponential growth of a seed black hole, resulting in a quasar in
some cases. In this model, the first quasars form soon after the first
collapses to produce hot gas. The hot gas is depleted as time progresses,
mostly by cooling, so that the accretion rate eventually falls below the
threshold for advection-dominated accretion, at which stage radiative
efficiency plummets and any quasar turns off. A simple implementation of this
model, incorporated into a semi-analytical model for galaxy formation,
over-produces quasars when compared with observed luminosity functions, but is
consistent with models of the X-ray Background which indicate that most
accretion is obscured. It produces few quasars at high redshift due to the lack
of time needed to grow massive black holes. Quasar fuelling by hot gas provides
a minimum level, sufficient to power most quasars at redshifts between one and
two, to which other sources of fuel can be added. The results are sensitive to
feedback effects, such as might be due to radio jets and other outflows.Comment: 12 pages, 6 figures, MN Latex style, accepted for publication in
MNRA
On the soft X-ray spectrum of cooling flows
Strong evidence for cooling flows has been found in low resolution X-ray
imaging and spectra of many clusters of galaxies. However high resolution X-ray
spectra of several clusters from the Reflection Grating Spectrometer (RGS) on
XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling
flow. The main problem is a lack of the emission lines expected from gas
cooling below 1--2 keV. Lines from gas at about 2--3 keV are observed, even in
a high temperature cluster such as A 1835, indicating that gas is cooling down
to about 2--3 keV, but is not found at lower temperatures. Here we discuss
several solutions to the problem; heating, mixing, differential absorption and
inhomogeneous metallicity. Continuous or sporadic heating creates further
problems, including the targetting of the heat at the cooler gas and also the
high total energy required. So far there is no clear observational evidence for
widespread heating, or shocks, in cluster cores, except in radio lobes which
occupy only part of the volume. The implied ages of cooling flows are short, at
about 1 Gyr. Mixing. or absorption, of the cooling gas are other possibilities.
Alternatively, if the metals in the intracluster medium are not uniformly
spread but are clumped, then little line emission is expected from the gas
cooling below 1 keV. The low metallicity part cools without line emission
whereas the strengths of the soft X-ray lines from the metal-rich gas depend on
the mass fraction of that gas and not on the abundance, since soft X-ray line
emission dominates the cooling function below 2 keV.Comment: 5 pages, with 2 figures, submitted to MNRA
The effect of supernova heating on cluster properties and constraints on galaxy formation models
Models of galaxy formation should be able to predict the properties of
clusters of galaxies, in particular their gas fractions, metallicities, X-ray
luminosity-temperature relation, temperature function and mass-deposition-rate
function. Fitting these properties places important constaints on galaxy
formation on all scales. By following gas processes in detail, our
semi-analytic model (based on that of Nulsen & Fabian 1997) is the only such
model able to predict all of the above cluster properties. We use realistic gas
fractions and gas density profiles, and as required by observations we break
the self-similarity of cluster structure by including supernova heating of
intracluster gas, the amount of which is indicated by the observed
metallicities. We also highlight the importance of the mass-deposition-rate
function as an independent and very sensitive probe of cluster structure.Comment: 5 pages, 4 figures, accepted for publication in MNRAS as a lette
Model-independent X-ray mass determinations
A new method is introduced for making X-ray mass determinations of spherical
clusters of galaxies. Treating the distribution of gravitating matter as
piecewise constant and the cluster atmosphere as piecewise isothermal, X-ray
spectra of a hydrostatic atmosphere are determined up to a single overall
normalizing factor. In contrast to more conventional approaches, this method
relies on the minimum of assumptions, apart from the conditions of hydrostatic
equilibrium and spherical symmetry. The method has been implemented as an XSPEC
mixing model called CLMASS, which was used to determine masses for a sample of
nine relaxed X-ray clusters. Compared to conventional mass determinations,
CLMASS provides weak constraints on values of M_500, reflecting the quality of
current X-ray data for cluster regions beyond r_500. At smaller radii, where
there are high quality X-ray spectra inside and outside the radius of interest
to constrain the mass, CLMASS gives confidence ranges for M_2500 that are only
moderately less restrictive than those from more familiar mass determination
methods. The CLMASS model provides some advantages over other methods and
should prove useful for mass determinations in regions where there are high
quality X-ray data.Comment: 12 pages, 8 figures, accepted for publication in Ap
The soft X-ray background: evidence for widespread disruption of the gas halos of galaxy groups
Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be
accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4
\bgunit\ for the remaining background at 0.25 keV. However, the XRB from the
gas halos of groups of galaxies, with gas removal due to cooling accounted for,
exceeds this upper limit by an order of magnitude if non-gravitational heating
is not included. We calculate this using simulations of halo merger trees and
realistic gas density profiles, which we require to reproduce the observed gas
fractions and abundances of X-ray clusters. In addition, we find that the
entire mass range of groups, from to \Ms,
contributes to the 0.25 keV background in this case. In a further study, we
reduce the luminosities of groups by maximally heating their gas halos while
maintaining the same gas fractions. This only reduces the XRB by a factor of 2
or less. We thus argue that most of the gas associated with groups must be
outside their virial radii. This conclusion is supported by X-ray studies of
individual groups. The properties of both groups and X-ray clusters can be
naturally explained by a model in which the gas is given excess specific
energies of keV/particle by non-gravitational heating. With this
excess energy, the gas is gravitationally unbound from groups, but recollapses
with the formation of a cluster of temperature \ga 1 keV. This is similar to
a model proposed by Pen, but is contrary to the evolution of baryons described
by Cen \& Ostriker. (abridged)Comment: 14 pages, 14 figures, submitted to MNRA
Stripped elliptical galaxies as probes of ICM physics: II. Stirred, but mixed? Viscous and inviscid gas stripping of the Virgo elliptical M89
Elliptical galaxies moving through the intra-cluster medium (ICM) are
progressively stripped of their gaseous atmospheres. X-ray observations reveal
the structure of galactic tails, wakes, and the interface between the galactic
gas and the ICM. This fine-structure depends on dynamic conditions (galaxy
potential, initial gas contents, orbit in the host cluster), orbital stage
(early infall, pre-/post-pericenter passage), as well as on the still
ill-constrained ICM plasma properties (thermal conductivity, viscosity,
magnetic field structure). Paper I describes flow patterns and stages of
inviscid gas stripping. Here we study the effect of a Spitzer-like temperature
dependent viscosity corresponding to Reynolds numbers, Re, of 50 to 5000 with
respect to the ICM flow around the remnant atmosphere. Global flow patterns are
independent of viscosity in this Reynolds number range. Viscosity influences
two aspects: In inviscid stripping, Kelvin-Helmholtz instabilities (KHIs) at
the sides of the remnant atmosphere lead to observable horns or wings.
Increasing viscosity suppresses KHIs of increasing length scale, and thus
observable horns and wings. Furthermore, in inviscid stripping, stripped
galactic gas can mix with the ambient ICM in the galaxy's wake. This mixing is
suppressed increasingly with increasing viscosity, such that viscously stripped
galaxies have long X-ray bright, cool wakes. We provide mock X-ray images for
different stripping stages and conditions. While these qualitative results are
generic, we tailor our simulations to the Virgo galaxy M89 (NGC 4552), where
Re~ 50 corresponds to a viscosity of 10% of the Spitzer level. Paper III
compares new deep Chandra and archival XMM-Newton data to our simulations.Comment: ApJ in press. 16 pages, 16 figures. Text clarified, conclusions
unchange
- …