74 research outputs found
XMM-Newton view of MS0735+7421: the most energetic AGN outburst in a galaxy cluster
We discuss the possible cosmological effects of powerful AGN outbursts in
galaxy clusters by starting from the results of an XMM-Newton observation of
the supercavity cluster MS0735+7421.Comment: 6 pages, 5 figures. To appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
Heating Hot Atmospheres with Active Galactic Nuclei
High resolution X-ray spectroscopy of the hot gas in galaxy clusters has
shown that the gas is not cooling to low temperatures at the predicted rates of
hundreds to thousands of solar masses per year. X-ray images have revealed
giant cavities and shock fronts in the hot gas that provide a direct and
relatively reliable means of measuring the energy injected into hot atmospheres
by active galactic nuclei (AGN). Average radio jet powers are near those
required to offset radiative losses and to suppress cooling in isolated giant
elliptical galaxies, and in larger systems up to the richest galaxy clusters.
This coincidence suggests that heating and cooling are coupled by feedback,
which suppresses star formation and the growth of luminous galaxies. How jet
energy is converted to heat and the degree to which other heating mechanisms
are contributing, eg. thermal conduction, are not well understood. Outburst
energies require substantial late growth of supermassive black holes. Unless
all of the approximately 10E62 erg required to suppress star formation is
deposited in the cooling regions of clusters, AGN outbursts must alter
large-scale properties of the intracluster medium.Comment: 60 pages, 12 figures, to appear in 1997 Annual Reviews of Astronomy
and Astrophysics. This version supersedes the April 2007 version in Reviews
in Advance (references and minor corrections were added), and is similar to
the one scheduled to appear in Volume 45 of ARA
Close entrainment of massive molecular gas flows by radio bubbles in the central galaxy of Abell 1795
We present new ALMA observations tracing the morphology and velocity structure of the molecular gas in the central galaxy of the cluster Abell 1795. The molecular gas lies in two filaments that extend 5–7 kpc to the N and S from the nucleus and project exclusively around the outer edges of two inner radio bubbles. Radio jets launched by the central active galactic nucleus have inflated bubbles filled with relativistic plasma into the hot atmosphere surrounding the central galaxy. The N filament has a smoothly increasing velocity gradient along its length from the central galaxy’s systemic velocity at the nucleus to −370kms−1 −370kms−1 , the average velocity of the surrounding galaxies, at the furthest extent. The S filament has a similarly smooth but shallower velocity gradient and appears to have partially collapsed in a burst of star formation. The close spatial association with the radio lobes, together with the ordered velocity gradients and narrow velocity dispersions, shows that the molecular filaments are gas flows entrained by the expanding radio bubbles. Assuming a Galactic XCO factor, the total molecular gas mass is 3.2 ± 0.2 × 109 M⊙. More than half lies above the N radio bubble. Lifting the molecular clouds appears to require an infeasibly efficient coupling between the molecular gas and the radio bubble. The energy required also exceeds the mechanical power of the N radio bubble by a factor of 2. Stimulated feedback, where the radio bubbles lift low-entropy X-ray gas that becomes thermally unstable and rapidly cools in situ, provides a plausible model. Multiple generations of radio bubbles are required to lift this substantial gas mass. The close morphological association then indicates that the cold gas either moulds the newly expanding bubbles or is itself pushed aside and shaped as they inflate
Cold Feedback in Cooling-Flow Galaxy Clusters
We put forward an alternative view to the Bondi-driven feedback between
heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies
and clusters. We adopt the popular view that the heating is due to an active
galactic nucleus (AGN), i.e. a central black hole accreting mass and launching
jets and/or winds. We propose that the feedback occurs with the entire cool
inner region (5-30 kpc). A moderate cooling flow does exist here, and
non-linear over-dense blobs of gas cool fast and are removed from the ICM
before experiencing the next major AGN heating event. Some of these blobs may
not accrete on the central black hole, but may form stars and cold molecular
clouds. We discuss the conditions under which the dense blobs may cool to low
temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
AGN Heating through Cavities and Shocks
Three comments are made on AGN heating of cooling flows. A simple physical
argument is used to show that the enthalpy of a buoyant radio lobe is converted
to heat in its wake. Thus, a significant part of ``cavity'' enthalpy is likely
to end up as heat. Second, the properties of the repeated weak shocks in M87
are used to argue that they can plausibly prevent gas close to the AGN from
cooling. As the most significant heating mechanism at work closest to the AGN,
shock heating probably plays a critical role in the feedback mechanism. Third,
results are presented from a survey of AGN heating rates in nearby giant
elliptical galaxies. With inactive systems included, the overall AGN heating
rate is reasonably well matched to the total cooling rate for the sample. Thus,
intermittent AGN outbursts are energetically capable of preventing the hot
atmospheres of these galaxies from cooling and forming stars.Comment: 6 pages, 2 figures, for proceedings of "Heating vs. Cooling in
Galaxies and Clusters of Galaxies," eds H. Boehringer, P. Schuecker, G. W.
Pratt & A. Finoguenov, in Springer-Verlag series "ESO Astrophysics Symposia.
The outer halos of elliptical galaxies
Recent progress is summarized on the determination of the density
distributions of stars and dark matter, stellar kinematics, and stellar
population properties, in the extended, low surface brightness halo regions of
elliptical galaxies. With integral field absorption spectroscopy and with
planetary nebulae as tracers, velocity dispersion and rotation profiles have
been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the
outer edge at ~150 kpc. The results are generally consistent with the known
dichotomy of elliptical galaxy types, but some galaxies show more complex
rotation profiles in their halos and there is a higher incidence of
misalignments, indicating triaxiality. Dynamical models have shown a range of
slopes for the total mass profiles, and that the inner dark matter densities in
ellipticals are higher than in spiral galaxies, indicating earlier assembly
redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals
and comparison with dynamical mass determinations indicates that non-thermal
components to the pressure may be important in the inner ~10 kpc, and that the
properties of these systems are closely related to their group environments.
First results on the outer halo stellar population properties do not yet give a
clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances
indicate longer star formation histories pointing towards late accretion of the
halo. This is consistent with independent evidence for on-going accretion, and
suggests a connection to the observed size evolution of elliptical galaxies
with redshift.Comment: 8 pages. Invited review to appear in the proceedings of "Galaxies and
their Masks" eds. Block, D.L., Freeman, K.C. & Puerari, I., 2010, Springer
(New York
Chandra X-ray observations of the 3C295 cluster core
We examine the properties of the X-ray gas in the central regions of the
distant (z=0.46), X-ray luminous cluster of galaxies surrounding the powerful
radio source 3C 295, using observations made with the Chandra Observatory.
Between radii of 50-500 kpc, the cluster gas is approximately isothermal with
an emission-weighted temperature, kT ~5 keV. Within the central 50 kpc radius
this value drops to kT ~3.7 keV. The spectral and imaging Chandra data indicate
the presence of a cooling flow within the central 50 kpc radius of the cluster,
with a mass deposition rate of approximately 280 solar masses per year. We
estimate an age for the cooling flow of 1-2 Gyr, which is approximately one
thousand times older than the central radio source. We find no evidence in the
X-ray spectra or images for significant heating of the X-ray gas by the radio
source. We report the detection of an edge-like absorption feature in the
spectrum for the central 50 kpc region, which may be due to oxygen-enriched
dust grains. The implied mass in metals seen in absorption could have been
accumulated by the cooling flow over its lifetime. Combining the results on the
X-ray gas density profile with radio measurements of the Faraday rotation
measure in 3C295, we estimate the magnetic field strength in the region of the
cluster core to be B ~12 \muG.Comment: 27 pages, 16 figs, 5 tables. Accepted for publication in MNRA
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