Craig L. Sarazin

Chandra X-Ray Observations of the Hydra A Cluster: An Interaction between the Radio Source and the X-Ray-emitting Gas

We present Chandra X-ray observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the clusters X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, approximately 25-35 kpc in diameter, that are coincident with Hydra As radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest that the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is approximately 600 Myr in the inner 10 kpc. These properties are consistent with the presence of an approximately 34 M middle dot in circle yr-1 cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material that may be cooling at larger radii remains elusive. A bright, unresolved X-ray source is present in the BCGs nucleus, coincident with the radio core. Its X-ray spectrum is consistent with a power law absorbed by a foreground NH approximately 4x1022 cm-2 column of hydrogen. This column is roughly consistent with the hydrogen column seen in absorption toward the less, similar24 pc diameter VLBA radio source. Apart from the point source, no evidence for excess X-ray absorption above the Galactic column is found.

The Temperature structure of 30 nearby clusters observed with ASCA. Similarity of temperature profiles

We present an analysis of ASCAspatially resolved spectroscopic data for a nearly complete sample of bright clusters with redshifts between 0.04 and 0.09. Together with several clusters from our previously published ASCA observations, this sample consists of 26 objects with Te ∼> 4 keV for which we obtained projected temperature profiles and, when possible, crude two-dimensional temperature maps. The clusters are A85, A119, A399, A401, A478, A754, A780, A1650, A1651, A1795, A2029, A2065, A2142, A2256, A2319, A2597, A3112, A3266, A3376, A3395, A3558, A3571, A3667, A4059, Cygnus A, and Triangulum Australis. All clusters, with the possible exception of a few with insufficiently accurate data, are found to be nonisothermal with spatial temperature variations by a factor of 1.3–2. ASCAtemperature maps for many clusters reveal merger shocks. The most notable of these are A754, A2065, A3558, A3667, and Cygnus A; merging can also be inferred with lower confidence from the A85 and A119 temperature maps and from the A3395 and Triangulum Australis entropy maps. Including several mergers obvious from the images but for which our temperature maps are insufficiently accurate, about half of the sample shows signs of merging. Nearly all clusters show a significant radial temperature decline at large radii. For a typical 7 keV cluster, the observed temperature decline between 1 and 6 X-ray core radii (0.15 and 0.9h−1 Mpc) can be approximately quantified by a polytropic index of 1.2–1.3. Assuming such a polytropic temperature profile and hydrostatic equilibrium, the gravitating mass within 1 and within 6 core radii is approximately 1.35 and 0.7 times the isothermal β-model estimates, respectively. Most interestingly, we find that all temperature profiles (excluding those for the most asymmetric clusters) appear remarkably similar when the temperature is plotted against radius in units of the estimated virial radius. We compare the composite temperature profile to a host of published hydrodynamic simulations. The observed profiles appear steeper than predictions of most Lagrangian simulations (Evrard, Metzler, & Navarro 1996; Eke, Navarro, & Frenk 1997). The predictions for Ω = 1 cosmological models are most discrepant, while models with low Ω are closer to our data. We note, however, that at least one Ω = 1 Lagrangian simulation (Katz & White 1993) and the recent high-resolution Eulerian simulation (Bryan & Norman 1997) produced clusters with temperature profiles similar to or steeper than those observed. Our results thus provide a constraint for testing numerical simulations and discriminating among models of cluster formation. Subject headings: Cosmology — galaxies: clusters: individual — intergalactic medium — X-rays: galaxies

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Off-Axis Cluster Mergers: Effects of a Strongly Peaked Dark Matter Profile

We present a parameter study of offset mergers between clusters of galaxies. Using the Eulerian hydrodynamics/N-body code COSMOS, we simulate mergers between nonisothermal, hydrostatic clusters with a steep central dark matter density profile and a β-model gas profile. We constrain global properties of the model clusters using observed cluster statistical relationships. We consider impact parameters between 0 and 5 times the dark matter scale radius and mass ratios of 1 : 1 and 1 : 3. The morphological changes, relative velocities, and temperature jumps we observe agree with previous studies using the King profile for the dark matter. We observe a larger jump in X-ray luminosity (~4-10 times) than in previous work, and we argue that this increase is most likely a lower limit due to our spatial resolution. We emphasize that luminosity and temperature jumps due to mergers may have an important bearing on constraints on Ω derived from the observation of hot clusters at high redshift. Shocks are relatively weak in the cluster cores; hence, they do not significantly increase the entropy there. Instead, shocks create entropy in the outer regions, and this high-entropy gas is mixed with the core gas during later stages of the merger. Ram pressure initiates mixing by displacing the core gas from its potential center, causing it to become convectively unstable. The resulting convective plumes produce large-scale turbulent motions with eddy sizes up to several hundred kiloparsecs. This turbulence is pumped by dark matter-driven oscillations in the gravitational potential. Even after nearly a Hubble time these motions persist as subsonic turbulence in the cluster cores, providing 5%-10% of the support against gravity. The dark matter oscillations are also reflected in the extremely long time following a merger required for the remnant to reach virial equilibrium.

The heating of gas in a galaxy cluster by X-ray cavities and large-scale shock fronts.

Most of the baryons in galaxy clusters reside between the galaxies in a hot, tenuous gas. The densest gas in their centres should cool and accrete onto giant central galaxies at rates of 10–1,000 solar masses per year. No viable repository for this gas, such as clouds or new stars, has been found. New X-ray observations, however, have revealed far less cooling below X-ray temperatures than expected, altering the previously accepted picture of cooling flows. As a result, most of the gas must be heated to and maintained at temperatures above ∼2 keV (ref. 3). The most promising heating mechanism is powerful radio jets emanating from supermassive black holes in the central galaxies of clusters. Here we report the discovery of giant cavities and shock fronts in a distant (z = 0.22) cluster caused by an interaction between a radio source and the hot gas surrounding it. The energy involved is ∼6 × 1061 erg, the most powerful radio outburst known. This is enough energy to quench a cooling flow for several Gyr, and to provide ∼1/3 keV per particle of heat to the surrounding cluster.

The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation

We have measured half-light radii, rh, for thousands of globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC 4697. An analysis of the dependencies of the measured half-light radii on both the properties of the GCs themselves and their host galaxies reveals that, in analogy with GCs in the Galaxy but in a milder fashion, the average half-light radius increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average half-light radius decreases with the host galaxy color. We also show that there is no evidence for a variation of rh with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average rh depends on the color of GCs, with red GCs being ~17% smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed by Jordan. We discuss these findings in light of two simple pictures for the origin of the rh of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies on galaxy color, galactocentric radius, and underlying surface brightness, we show that the average GC half-light radii rh can be successfully used as a standard ruler for distance estimation. We outline the methodology, provide a calibration for its use, and discuss the prospects for this distance estimator with future observing facilities. We find rh = 2.7 ± 0.35 pc for GCs with (g - z) = 1.2 mag in a galaxy with color (g - z)gal = 1.5 mag and at an underlying surface z-band brightness of μz = 21 mag arcsec-2. Using this technique, we place an upper limit of 3.4 Mpc on the 1 σ line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the rh distribution for our sample galaxies and provide an analytic expression that successfully describes this distribution.

The Energy Spectrum of Primary Cosmic-Ray Electrons in Clusters of Galaxies and Inverse Compton Emission

Models for the evolution of the integrated energy spectrum of primary cosmic-ray electrons in clusters of galaxies have been calculated, including the effects of losses due to inverse Compton (IC), synchrotron and bremsstrahlung emission, and Coulomb losses to the intracluster medium (ICM). The combined timescale for these losses reaches a maximum of ~3 ? 109 yr for electrons with Lorentz factor ? ~ 300. A variety of models for the time evolution of particle injection are considered, including models in which the electrons are all produced at a single epoch in the past, models with continuous particle acceleration, and combinations of these. Analytical solutions are given for a number of limiting cases. Numerical solutions are given for more general cases. Only a cluster in which there has been a substantial injection of relativistic electrons since z 1 will have any significant population of primary cosmic-ray electrons at present. For models in which all of the electrons were injected in the past, there is a high-energy cutoff ?max to the present electron distribution. At low energies, at which Coulomb losses dominate, the electron distribution function N(?) tends to a constant value, independent of ?. On the other hand, if electrons are being accelerated at present, the energy distribution at high and low energies approaches steady state. If the electrons are injected with a power-law distribution, the steady state distribution is one power steeper at high energies and one power flatter at low energies. In models with large initial populations of particles, but also with significant rates of current particle injection, the electron distributions are a simple combination the behavior of the initial population models and the steady injection models. There is a steep drop in the electron population at ?max, but higher energy electrons are present at a rate determined by the current rate of particle injection. Increasing the ICM thermal gas density decreases the number of low-energy electrons (? 100). If the magnetic field is greater than generally expected, (B 3 ?G), synchrotron losses will reduce the number of high-energy electrons. A significant population of electrons with ? ~ 300, associated with the peak in the particle loss time, is a generic feature of the models, as long as there has been significant particle injection since z 1. The IC and synchrotron emission from these models was calculated. In models with steady particle injection with a power-law exponent p, the IC spectra relax into a steady state form. At low energies, the spectrum is a power law with ? ? -0.15, while at high energies ? ? -1.15. These two power laws meet at a knee at ? ~ 3 ? 1016 Hz. In models with no current particle injection, the cutoff in the electron distribution at high energies (? ? ?max) results in a rapid drop in the IC spectrum at high frequencies. In models in which the current rate of particle injection provides a small but significant fraction of the total electron energy, the spectra show an extended hump at low frequencies (? 1017 Hz), with a rapid fall off above ? ~ 1016 Hz. However, they also have an extended hard tail of emission at high frequencies, which has a power-law spectrum with a spectral index of ? ? -1.15. In the models, EUV and soft X-ray emission are nearly ubiquitous. This emission is produced by electrons with ? ~ 300, which have the longest loss times. The spectra are predicted to drop rapidly in going from the EUV to the X-ray band. The IC emission also extends down the UV, optical, and IR bands, with a fairly flat spectrum (-0.6 ? +0.3). At hard X-ray energies 20 keV, IC emission should become observable against the background of thermal X-ray emission. Such hard X-ray (HXR) emission is due to high-energy electrons (? ~ 104). The same electrons will produce diffuse radio emission (cluster radio halos) via synchrotron emission. Because of the short loss times of these particles, HXR and diffuse radio emission are expected only in clusters that have current (or very recent) particle acceleration. Assuming that the electrons are accelerated in ICM shocks, one would expect diffuse HXR/radio emission only in clusters that are currently undergoing large mergers. The luminosity of HXR emission is primarily determined by the current rates of particle acceleration in the clusters. The spectra in most models with significant HXR or radio emission are approximately power laws with ? ? -1.1. The IC spectra from the EUV to HXR are not generally fit by a single power law. Instead, there is a rapid falloff from EUV to X-ray energies, with a power-law tail extending into the HXR band.

An Infrared Survey of Brightest Cluster Galaxies. II. Why are Some Brightest Cluster Galaxies Forming Stars

Quillen et al. presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters. They found that at least half of these sources have signs of excess IR emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies. The gas depletion timescale is about 1 Gyr. The physical extent of the IR excess is consistent with that of the optical emission-line nebulae. This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates () estimated from the X-ray emission and the star formation rates estimated from the IR luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

What is a cool-core cluster? a detailed analysis of the cores of the X-ray flux-limited HIFLUGCS cluster sample

We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, some of them new. In order to identify the best parameter for characterizing cool-core clusters and quantify its relation to other parameters, we mainly use very high spatial resolution profiles of central gas density and temperature, and quantities derived from them. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, t cool , is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (t cool 50 h ―1 71 kpc) projected separation between their BCG and X-ray peak. In contrast, only two of the 56 clusters with a small separation between the BCG and X-ray peak (<50 h ―1 71 kpc) show large-scale radio emission. Based on this result, we argue that a large projected separation between the BCG and the X-ray peak is a good indicator of a major merger. The properties of weak cool-core clusters as an intermediate class of objects are discussed. Finally we describe individual properties of all 64 clusters in the sample.

CHANDRA X-RAY OBSERVATIONS OF THE X-RAY FAINT ELLIPTICAL GALAXY NGC 4697

A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ~80 are low-mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (~23% of the total count rate) is from interstellar gas. The X-ray-emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well fitted by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. NGC 4697 has a central X-ray source with a luminosity of LX = 8 ? 1038 ergs s-1, which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (?4 ? 10-8) of its Eddington luminosity. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is LX(LMXB, 0.3-10 keV)/LB = 8.1 ? 1029 ergs s-1 L, which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of 2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ~5 ? 1037 to ~2.5 ? 1039 ergs s-1. The luminosity function of the LMXBs has a knee at 3.2 ? 1038 ergs s-1, which is approximately the Eddington luminosity of a 1.4 M? neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.