413 research outputs found
Heating the hot atmospheres of galaxy groups and clusters with cavities: the relationship between jet power and low-frequency radio emission
We present scaling relations between jet power and radio power measured using
the Giant Metrewave Radio Telescope (GMRT), Chandra and XMM-Newton, for a
sample of 9 galaxy groups combined with the Birzan et al. sample of clusters.
Cavity power is used as a proxy for mechanical jet power. Radio power is
measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz-10 GHz radio
luminosity is estimated from the GMRT 610-235 MHz spectral index. The use of
consistently analysed, high resolution low-frequency radio data from a single
observatory makes the radio powers for the groups more reliable than those used
by previous studies, and the combined sample covers 6-7 decades in radio power
and 5 decades in cavity power. We find a relation of the form Pjet proportional
to Lradio^~0.7 for integrated radio luminosity, with a total scatter of
sigma_Lrad=0.63 and an intrinsic scatter of sigma_i,Lrad=0.59. A similar
relation is found for 235 MHz power, but a slightly flatter relation with
greater scatter is found for 1.4 GHz power, suggesting that low-frequency or
broad band radio measurements are superior jet power indicators. We find our
low-frequency relations to be in good agreement with previous observational
results. Comparison with jet models shows reasonable agreement, which may be
improved if radio sources have a significant low-energy electron population. We
consider possible factors which could bias our results or render them more
uncertain, and find that correcting for such factors in those groups we are
able to study in detail leads to a flattening of the Pjet:Lradio relation.Comment: Accepted for publication in ApJ, 7 pages, 3 figure
A relationship between AGN jet power and radio power
Using Chandra X-ray and VLA radio data, we investigate the scaling
relationship between jet power, P_jet, and synchrotron luminosity, P_rad. We
expand the sample presented in Birzan et al. (2008) to lower radio power by
incorporating measurements for 21 gEs to determine if the Birzan et al. (2008)
P_jet-P_rad scaling relations are continuous in form and scatter from giant
elliptical galaxies (gEs) up to brightest cluster galaxies (BCGs). We find a
mean scaling relation of P_jet approximately 5.8x10^43 (P_rad/10^40)^(0.70)
erg/s which is continuous over ~6-8 decades in P_jet and P_rad with a scatter
of approximately 0.7 dex. Our mean scaling relationship is consistent with the
model presented in Willott et al. (1999) if the typical fraction of lobe energy
in non-radiating particles to that in relativistic electrons is > 100. We
identify several gEs whose radio luminosities are unusually large for their jet
powers and have radio sources which extend well beyond the densest parts of
their X-ray halos. We suggest that these radio sources are unusually luminous
because they were unable to entrain appreciable amounts of gas.Comment: Accepted for publication in the Astrophysical Journal; 8 pages, 3
color figures, 1 tabl
MHD Simulations of AGN Jets in a Dynamic Galaxy Cluster Medium
We present a pair of 3-d magnetohydrodynamical simulations of intermittent
jets from a central active galactic nucleus (AGN) in a galaxy cluster extracted
from a high resolution cosmological simulation. The selected cluster was chosen
as an apparently relatively relaxed system, not having undergone a major merger
in almost 7 Gyr. Despite this characterization and history, the intra-cluster
medium (ICM) contains quite active "weather". We explore the effects of this
ICM weather on the morphological evolution of the AGN jets and lobes. The
orientation of the jets is different in the two simulations so that they probe
different aspects of the ICM structure and dynamics. We find that even for this
cluster that can be characterized as relaxed by an observational standard, the
large-scale, bulk ICM motions can significantly distort the jets and lobes.
Synthetic X-ray observations of the simulations show that the jets produce
complex cavity systems, while synthetic radio observations reveal bending of
the jets and lobes similar to wide-angle tail (WAT) radio sources. The jets are
cycled on and off with a 26 Myr period using a 50% duty cycle. This leads to
morphological features similar to those in "double-double" radio galaxies.
While the jet and ICM magnetic fields are generally too weak in the simulations
to play a major role in the dynamics, Maxwell stresses can still become locally
significant.Comment: 20 pages, 14 figures, accepted for publication in the Astrophysical
Journa
A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst
We present the results of a Chandra study of the Hydra A galaxy cluster,
where a powerful AGN outburst created a large-scale cocoon shock. We
investigated possible azimuthal variations in shock strength and shape, finding
indications for a weak shock with a Mach number in the range ~1.2-1.3. We
measured the temperature change across the shock front. However, the detection
of a temperature rise in the regions immediately inside of the front is
complicated by the underlying temperature profile of the cluster atmosphere. We
measured the global temperature profile of the cluster up to 700 kpc, which
represents the farthest measurement obtained with Chandra for this cluster. A
"plateau" in the temperature profile in the range ~70-150 kpc indicates the
presence of cool gas, which is likely the result of uplift of material by the
AGN outburst. After masking the cool filaments visible in the hardness ratio
map, the plateau disappears and the temperature profile recovers a typical
shape with a peak around 190 kpc, just inside the shock front. However, it is
unlikely that such a temperature feature is produced by the shock as it is
consistent with the general shape of the temperature profiles observed for
relaxed galaxy clusters. We studied the spectral properties of the cool
filaments finding evidence that ~10^11 M_sun of low-entropy material has been
dredged up by the rising lobes from the central 30 kpc to the observed current
position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x
10^60 erg, which is comparable to the work required to inflate the cavities and
is ~25% of the total energy of the large-scale shock. Our results show that the
AGN feedback in Hydra A is acting not only by directly heating the gas, but
also by removing a substantial amount of potential fuel for the SMBH.Comment: 11 pages, 9 figures, accepted for publication in ApJ (version with
full resolution figures available at
http://www.bo.astro.it/~myriam/files/papers/gitti-hydra.pdf
Radio galaxy feedback in X-ray selected groups from COSMOS: the effect on the ICM
We quantify the importance of the mechanical energy released by
radio-galaxies inside galaxy groups. We use scaling relations to estimate the
mechanical energy released by 16 radio-AGN located inside X-ray detected galaxy
groups in the COSMOS field. By comparing this energy output to the host groups'
gravitational binding energy, we find that radio galaxies produce sufficient
energy to unbind a significant fraction of the intra-group medium. This
unbinding effect is negligible in massive galaxy clusters with deeper potential
wells. Our results correctly reproduce the breaking of self-similarity observed
in the scaling relation between entropy and temperature for galaxy groups.Comment: Accepted for publication in the Astrophysical Journal. 12 Page
The Growth of Black Holes and Bulges at the Cores of Cooling Flows
Central cluster galaxies (cDs) in cooling flows are growing rapidly through
gas accretion and star formation. At the same time, AGN outbursts fueled by
accretion onto supermassive black holes are generating X-ray cavity systems and
driving outflows that exceed those in powerful quasars. We show that the
resulting bulge and black hole growth follows a trend that is roughly
consistent with the slope of the local (Magorrian) relation between bulge and
black hole mass for nearby quiescent ellipticals. However, a large scatter
suggests that cD bulges and black holes do not always grow in lock-step. New
measurements made with XMM, Chandra, and FUSE of the condensation rates in
cooling flows are now approaching or are comparable to the star formation
rates, alleviating the need for an invisible sink of cold matter. We show that
the remaining radiation losses can be offset by AGN outbursts in more than half
of the systems in our sample, indicating that the level of cooling and star
formation is regulated by AGN feedback.Comment: 3 pages, 4 figures, to appear in the proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies," edited by H. Boehringer, P.
Schuecker, G. W. Pratt, and A. Finogueno
Cavities and shocks in the galaxy group HCG 62 as revealed by Chandra, XMM and GMRT data
We report on the results of an analysis of Chandra, XMM-Newton and new GMRT
data of the X-ray bright compact group of galaxies HCG 62, which is one of the
few groups known to possess clear, small X-ray cavities in the inner regions.
This is part of an ongoing X-ray/low-frequency radio study of 18 groups,
initially chosen for the availability of good-quality X-ray data and evidence
for AGN/hot gas interaction. At higher frequency (1.4 GHz), the HCG 62 cavity
system shows minimal if any radio emission, but the new GMRT observations at
235 MHz and 610 MHz clearly detect extended low-frequency emission from radio
lobes corresponding to the cavities. By means of the synergy of X-ray and
low-frequency radio observations, we compare and discuss the morphology,
luminosity and pressure of the gas and of the radio source. We find that the
radio source is radiatively inefficient, with a ratio of radio luminosity to
mechanical cavity power of , and that the radio pressure of the
lobes is about one order of magnitude lower than the X-ray pressure of the
surrounding thermal gas. Thanks to the high spatial resolution of the Chandra
surface brightness and temperature profiles, we also identify a shock front
located at 36 kpc to the south-west of the group center, close to the southern
radio lobe, with a Mach number and a total power which is about one
order of magnitude higher than the cavity power. Such a shock may have heated
the gas in the southern region, as indicated by the temperature map. The shock
may also explain the arc-like region of enriched gas seen in the iron abundance
map, as this may be produced by a non-Maxwellian electron distribution near its
front.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Revised version
including minor comments and expanded discussion (version with full
resolution figures available at
http://hea-www.harvard.edu/~mgitti/hcg62-gitti.pdf
Evidence for Rapidly Rotating Black Holes in FR I Radio Galaxies
We investigate the correlation between 151 MHz radio luminosity, L_151MHz,
and jet power, P_jet, for a sample of low-power radio galaxies, of which the
jet power is estimated from X-ray cavities. The jet power for a sample of FR I
radio galaxies is estimated with the derived empirical correlation. We find
that P_jet/L_Edd is positively correlated with Lx(2-10 keV)/L_Edd for FR Is,
where L_Edd is the Eddington luminosity and Lx(2-10 keV) is 2-10 keV X-ray
luminosity. We calculate the jet power of a hybrid model, as a variant of
Blandford-Znajek model proposed by Meier, based on the global solution of the
advection-dominated accretion flow (ADAF) surrounding a Kerr black hole (BH).
Our model calculations suggest that the maximal jet power is a function of mass
accretion rate and the black hole spin parameter j. The hard X-ray emission is
believed to be mainly from the ADAFs in FR Is, and the mass accretion rate is
therefore constrained with the X-ray emission in our ADAF model calculations.
We find that the dimensionless angular momentum of BH j>~0.9 is required in
order to reproduce the observed relation of P_jet/L_Edd-Lx(2-10 keV)/L_Edd for
FR Is. Our conclusion will be strengthened if part of the X-ray emission is
contributed by the jets. Our results suggest that BHs in FR I radio galaxies
are rapidly spinning, which are almost not affected by the uncertainty of the
black hole mass estimates.Comment: Accepted by Ap
Three-Dimensional Simulations of Bi-Directed Magnetohydrodynamic Jets Interacting with Cluster Environments
We report on a series of three-dimensional magnetohydrodynamic simulations of
active galactic nucleus (AGN) jet propagation in realistic models of magnetized
galaxy clusters. We are primarily interested in the details of energy transfer
between jets and the intracluster medium (ICM) to help clarify what role such
flows could have in the reheating of cluster cores. Our simulated jets feature
a range of intermittency behaviors, including intermittent jets that
periodically switch on and off and one model jet that shuts down completely,
naturally creating a relic plume. The ICM into which these jets propagate
incorporates tangled magnetic field geometries and density substructure
designed to mimic some likely features of real galaxy clusters. We find that
our jets are characteristically at least 60% efficient at transferring thermal
energy to the ICM. Irreversible heat energy is not uniformly distributed,
however, instead residing preferentially in regions very near the jet/cocoon
boundaries. While intermittency affects the details of how, when, and where
this energy is deposited, all of our models generically fail to heat the
cluster cores uniformly. Both the detailed density structure and nominally weak
magnetic fields in the ICM play interesting roles in perturbing the flows,
particularly when the jets are non-steady. Still, this perturbation is never
sufficient to isotropize the jet energy deposition, suggesting that some other
ingredient is required for AGN jets to successfully reheat cluster cores.Comment: 19 pages, 18 figures, Accepted for publication in the Astrophysical
Journa
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