227 research outputs found
Cause and Effect of Feedback: Multiphase Gas in Cluster Cores Heated by AGN Jets
Multiwavelength data indicate that the X-ray emitting plasma in the cores of
galaxy clusters is not cooling catastrophically. To large extent, cooling is
offset by heating due to active galactic nuclei (AGN) via jets. The cool-core
clusters, with cooler/denser plasmas, show multiphase gas and signs of some
cooling in their cores. These observations suggest that the cool core is
locally thermally unstable while maintaining global thermal equilibrium. Using
high-resolution, three-dimensional simulations we study the formation of
multiphase gas in cluster cores heated by highly-collimated bipolar AGN jets.
Our key conclusion is that spatially extended multiphase filaments form only
when the instantaneous ratio of the thermal instability and free-fall
timescales (t_TI/t_ff) falls below a critical threshold of \approx 10. When
this happens, dense cold gas decouples from the hot ICM phase and generates
inhomogeneous and spatially extended Halpha filaments. These cold gas clumps
and filaments `rain' down onto the central regions of the core, forming a cold
rotating torus and in part feeding the supermassive black hole. Consequently,
the self-regulated feedback enhances AGN heating and the core returns to a
higher entropy level with t_TI/t_ff > 10. Eventually the core reaches
quasi-stable global thermal equilibrium, and cold filaments condense out of the
hot ICM whenever t_TI/t_ff \lesssim 10. This occurs despite the fact that the
energy from AGN jets is supplied to the core in a highly anisotropic fashion.
The effective spatial redistribution of heat is enabled in part by the
turbulent motions in the wake of freely-falling cold filaments. Increased AGN
activity can locally reverse the cold gas flow, launching cold filamentary gas
away from the cluster center. Our criterion for the condensation of spatially
extended cold gas is in agreement with observations and previous idealized
simulations.Comment: Submitted to ApJ; 16 pages, 11 figure
AGN heating, thermal conduction and Sunyaev-Zeldovich effect in galaxy groups and clusters
(abridged) We investigate in detail the role of active galactic nuclei on the
physical state of the gas in galaxy groups and clusters, and the implications
for anisotropy in the CMB from Sunyaev-Zeldovich effect. We include the effect
of thermal conduction, and find that the resulting profiles of temperature and
entropy are consistent with observations. Unlike previously proposed models,
our model predicts that isentropic cores are not an inevitable consequence of
preheating. The model also reproduces the observational trend for the density
profiles to flatten in lower mass systems. We deduce the energy E_agn required
to explain the entropy observations as a function of mass of groups and
clusters M_cl and show that E_agn is proportional to M_cl^alpha with alpha~1.5.
We demonstrate that the entropy measurements, in conjunction with our model,
can be translated into constraints on the cluster--black hole mass relation.
The inferred relation is nonlinear and has the form M_bh\propto M_cl^alpha.
This scaling is an analog and extension of a similar relation between the black
hole mass and the galactic halo mass that holds on smaller scales. We show that
the central decrement of the CMB temperature is reduced due to the enhanced
entropy of the ICM, and that the decrement predicted from the plausible range
of energy input from the AGN is consistent with available data of SZ decrement.
We show that AGN heating, combined with the observational constraints on
entropy, leads to suppression of higher multipole moments in the angular power
spectrum and we find that this effect is stronger than previously thought.Comment: accepted for publication in The Astrophysical Journa
Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz
We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at
230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. This
confirms the previously reported detection with the JCMT 14-m antenna. Our high
resolution observations demonstrate that the polarization does not arise from
dust but from a synchrotron source associated with Sgr A*. We see no change in
the polarization position angle and only a small change in the polarization
fraction in four observations distributed over 60 days. We find a position
angle 139 +/- 4 degrees that differs substantially from what was found in
earlier JCMT observations at the same frequency. Polarized dust emission cannot
account for this discrepancy leaving variability and observational error as the
only explanations. The BIMA observations alone place an upper limit on the
magnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observations
when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM
=-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faraday
screen. Barring a special geometry or a high number of field reversals, this RM
rules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement is
inconsistent with high accretion rates necessary in standard advection
dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low
accretion rates as a major factor in the overall faintness of Sgr A*.Comment: accepted for publication in ApJ, 18 pages, 4 figure
Models of the ICM with Heating and Cooling: Explaining the Global and Structural X-ray Properties of Clusters
(Abridged) Theoretical models that include only gravitationally-driven
processes fail to match the observed mean X-ray properties of clusters. As a
result, there has recently been increased interest in models in which either
radiative cooling or entropy injection play a central role in mediating the
properties of the intracluster medium. Both sets of models give reasonable fits
to the mean properties of clusters, but cooling only models result in fractions
of cold baryons in excess of observationally established limits and the
simplest entropy injection models do not treat the "cooling core" structure
present in many clusters and cannot account for entropy profiles revealed by
recent X-ray observations. We consider models that marry radiative cooling with
entropy injection, and confront model predictions for the global and structural
properties of massive clusters with the latest X-ray data. The models
successfully and simultaneously reproduce the observed L-T and L-M relations,
yield detailed entropy, surface brightness, and temperature profiles in
excellent agreement with observations, and predict a cooled gas fraction that
is consistent with observational constraints. The model also provides a
possible explanation for the significant intrinsic scatter present in the L-T
and L-M relations and provides a natural way of distinguishing between clusters
classically identified as "cooling flow" clusters and dynamically relaxed
"non-cooling flow" clusters. The former correspond to systems that had only
mild levels (< 300 keV cm^2) of entropy injection, while the latter are
identified as systems that had much higher entropy injection. This is borne out
by the entropy profiles derived from Chandra and XMM-Newton.Comment: 20 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Shallow Dark Matter Cusps in Galaxy Clusters
We study the evolution of the stellar and dark matter components in a galaxy
cluster of from to the present epoch using
the high-resolution collisionless simulations of Ruszkowski & Springel (2009).
At the dominant progenitor halos were populated with spherical model
galaxies with and without accounting for adiabatic contraction. We apply a
weighting scheme which allows us to change the relative amount of dark and
stellar material assigned to each simulation particle in order to produce
luminous properties which agree better with abundance matching arguments and
observed bulge sizes at . This permits the study of the effect of initial
compactness on the evolution of the mass-size relation. We find that for more
compact initial stellar distributions the size of the final Brightest Cluster
Galaxy grows with mass according to , whereas for more extended
initial distributions, . Our results show that collisionless
mergers in a cosmological context can reduce the strength of inner dark matter
cusps with changes in logarithmic slope of 0.3 to 0.5 at fixed radius. Shallow
cusps such as those found recently in several strong lensing clusters thus do
not necessarily conflict with CDM, but may rather reflect on the initial
structure of the progenitor galaxies, which was shaped at high redshift by
their formation process.Comment: 8 pages, 4 figures, submitted to MNRA
Feedback under the microscope: thermodynamic structure and AGN driven shocks in M87
(abridged) Using a deep Chandra exposure (574 ks), we present high-resolution
thermodynamic maps created from the spectra of 16,000 independent
regions, each with 1,000 net counts. The excellent spatial resolution of
the thermodynamic maps reveals the dramatic and complex temperature, pressure,
entropy and metallicity structure of the system. Excluding the 'X-ray arms',
the diffuse cluster gas at a given radius is strikingly isothermal. This
suggests either that the ambient cluster gas, beyond the arms, remains
relatively undisturbed by AGN uplift, or that conduction in the intracluster
medium (ICM) is efficient along azimuthal directions. We confirm the presence
of a thick (40 arcsec or 3 kpc) ring of high pressure gas at a
radius of 180 arcsec (14 kpc) from the central AGN. We verify that
this feature is associated with a classical shock front, with an average Mach
number M = 1.25. Another, younger shock-like feature is observed at a radius of
40 arcsec (3 kpc) surrounding the central AGN, with an estimated
Mach number M > 1.2. As shown previously, if repeated shocks occur every
10 Myrs, as suggested by these observations, then AGN driven weak shocks
could produce enough energy to offset radiative cooling of the ICM. A high
significance enhancement of Fe abundance is observed at radii 350 - 400 arcsec
(27 - 31 kpc). This ridge is likely formed in the wake of the rising bubbles
filled with radio-emitting plasma that drag cool, metal-rich gas out of the
central galaxy. We estimate that at least solar masses of
Fe has been lifted and deposited at a radius of 350-400 arcsec; approximately
the same mass of Fe is measured in the X-ray bright arms, suggesting that a
single generation of buoyant radio bubbles may be responsible for the observed
Fe excess at 350 - 400 arcsec.Comment: 18 pages, 16 figures. Accepted to MNRA
Rapidly Evolving Circularly Polarized Emission during the 1994 Outburst of GRO J1665-40
We report the detection of circular polarization during the 1994 outburst of
the Galactic microquasar GRO J1655-40. The circular polarization is clearly
detected at 1.4 and 2.4GHz, but not at 4.8 and 8.4GHz, where its magnitude
never exceeds 5 mJy. Both the sign and magnitude of the circular polarization
evolve during the outburst. The time dependence and magnitude of the polarized
emission can be qualitatively explained by a model based on synchrotron
emission from the outbursts, but is most consistent with circular polarization
arising from propagation effects through the relativistic plasma surrounding
the object.Comment: 8 pages, 3 figs., A&A accepte
Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters
Future high-resolution microwave background measurements hold the promise of
detecting galaxy clusters throughout our Hubble volume through their
Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number
density of galaxy clusters is highly sensitive to cluster mass through
fluctuations in the matter power spectrum, as well as redshift through the
comoving volume and the growth factor. This sensitivity in principle allows
tight constraints on such quantities as the equation of state of dark energy
and the neutrino mass. We evaluate the ability of future cluster surveys to
measure these quantities simultaneously when combined with PLANCK-like CMB
data. Using a simple effective model for uncertainties in the cluster mass-SZ
flux relation, we evaluate systematic shifts in cosmological constraints from
cluster SZ surveys. We find that a systematic bias of 10% in cluster mass
measurements can give rise to shifts in cosmological parameter estimates at
levels larger than the statistical errors. Systematic errors are
unlikely to be detected from the mass and redshift dependence of cluster number
counts alone; increasing survey size has only a marginal effect. Implications
for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted
versio
Average Heating Rate of Hot Atmospheres in Distant Clusters by Radio AGN: Evidence for Continuous AGN Heating
We examine atmospheric heating by radio active galactic nuclei (AGN) in
distant X-ray clusters by cross correlating clusters selected from the 400
Square Degree (400SD) X-ray Cluster survey with radio sources in the NRAO VLA
Sky Survey. Roughly 30% of the clusters show radio emission above a flux
threshold of 3 mJy within a projected radius of 250 kpc. The radio emission is
presumably associated with the brightest cluster galaxy. The mechanical jet
power for each radio source was determined using scaling relations between
radio power and cavity (mechanical) power determined for nearby clusters,
groups, and galaxies with hot atmospheres containing X-ray cavities. The
average jet power of the central radio AGN is approximately \ergs. We find no significant correlation between radio power, hence
mechanical jet power, and the X-ray luminosities of clusters in the redshift
range 0.1 -- 0.6. This implies that the mechanical heating rate per particle is
higher in lower mass, lower X-ray luminosity clusters. The jet power averaged
over the sample corresponds to an atmospheric heating of approximately 0.2 keV
per particle within R. Assuming the current AGN heating rate does not
evolve but remains constant to redshifts of 2, the heating rate per particle
would rise by a factor of two. We find that the energy injected from radio AGN
contribute substantially to the excess entropy in hot atmospheres needed to
break self-similarity in cluster scaling relations. The detection frequency of
radio AGN is inconsistent with the presence of strong cooling flows in 400SD
clusters, but does not exclude weak cooling flows. It is unclear whether
central AGN in 400SD clusters are maintained by feedback at the base of a
cooling flow. Atmospheric heating by radio AGN may retard the development of
strong cooling flows at early epochs.Comment: ApJ in pres
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