G. Mark Voit

The baseline intracluster entropy profile from gravitational structure formation

The radial entropy profile of the hot gas in clusters of galaxies tends to follow a power law in radius outside of the cluster core. Here we present a simple formula giving both the normalization and slope for the power-law entropy profiles of clusters that form in the absence of non-gravitational processes such as radiative cooling and subsequent feedback. It is based on 71 clusters drawn from four separate cosmological simulations, two using smoothed particle hydrodynamics and two using adaptive-mesh refinement (AMR), and can be used as a baseline for assessing the impact of non-gravitational processes on the intracluster medium outside of cluster cores. All the simulations produce clusters with self-similar structure in which the normalization of the entropy profile scales linearly with cluster temperature, and these profiles are in excellent agreement outside of 0.2r 200 . Because the observed entropy profiles of clusters do not scale linearly with temperature, our models confirm that non-gravitational processes are necessary to break the self-similarity seen in the simulations. However, the core entropy levels found by the two codes used here significantly differ, with the AMR code producing nearly twice as much entropy at the centre of a cluster.

Modified Entropy Models for the Intracluster Medium

We present a set of cluster models that link the present-day properties of clusters to the processes that govern galaxy formation. These models treat the entropy distribution of the intracluster medium as its most fundamental property. Because convection strives to establish an entropy gradient that rises with radius, the observable properties of a relaxed cluster depend entirely on its dark-matter potential and the entropy distribution of its uncondensed gas. Guided by simulations, we compute the intracluster entropy distribution that arises in the absence of radiative cooling and supernova heating by assuming that the gas-density distribution would be identical to that of the dark matter. The lowest-entropy gas would then fall below a critical entropy threshold at which the cooling time equals a Hubble time. Radiative cooling and whatever feedback is associated with it must modify the entropy of that low-entropy gas, changing the overall entropy distribution function and thereby altering the observable properties of the cluster. Using some phenomenological prescriptions for entropy modification based on the existence of this cooling threshold, we construct a remarkably realistic set of cluster models. The surface-brightness profiles, masstemperature relation, and luminosity-temperature relation of observed clusters all naturally emerge from these models. By introducing a single adjustable parameter related to the amount of intracluster gas that can cool within a Hubble time, we can also reproduce the observed temperature gradients of clusters and the deviations of cooling-flow clusters from the standard luminosity-temperature relation. Subject headings: cosmology: theory — galaxies: clusters: general — galaxies: evolution — intergalactic medium — X-rays: galaxies: clusters

On the Origin of Intracluster Entropy

The entropy distribution of the intracluster medium and the shape of its confining potential well completely determine the X-ray properties of a relaxed cluster of galaxies, motivating us to explore the origin of intracluster entropy and to describe how it develops in terms of some simple models. We present an analytical model for smooth accretion, including both preheating and radiative cooling, that links a clusters entropy distribution to its mass accretion history and shows that smooth accretion overproduces the entropy observed in massive clusters by a factor of ~2-3, depending on the mass accretion rate. Any inhomogeneity in the accreting gas reduces entropy production at the accretion shock; thus, smoothing of the gas accreting onto a cluster raises its entropy level. Because smooth accretion produces more entropy than hierarchical accretion, we suggest that some of the observed differences between clusters and groups may arise because preheating smooths the smaller scale lumps of gas accreting onto groups more effectively than it smooths the larger scale lumps accreting onto clusters. This effect may explain why entropy levels at the outskirts of groups are ~2-3 times larger than expected from self-similar scaling arguments. The details of how the density distribution of accreting gas affects the entropy distribution of a cluster are complex, and we suggest how to explore the relevant physics with numerical simulations.

An Entropy Threshold for Strong Hα and Radio Emission in the Cores of Galaxy Clusters

Our Chandra X-Ray Observatory archival study of intracluster entropy in a sample of 222 galaxy clusters shows that H? and radio emission from the brightest cluster galaxy are much more pronounced when the clusters core gas entropy is 30 keV cm2. The prevalence of H? emission below this threshold indicates that it marks a dichotomy between clusters that can harbor multiphase gas and star formation in their cores and those that cannot. The fact that strong central radio emission also appears below this boundary suggests that AGN feedback turns on when the intracluster medium starts to condense, strengthening the case for AGN feedback as the mechanism that limits star formation in the universes most luminous galaxies.

An Observationally Motivated Framework for AGN Heating of Cluster Cores

The cooling flow problem is a long-standing puzzle that has received considerable recent attention, in part because the mechanism that quenches cooling flows in galaxy clusters is likely to be the same mechanism that sharply truncates the high end of the galaxy luminosity function. Most of the recent models for halting cooling in clusters have focused on AGN heating, but the actual heating mechanism has remained mysterious. Here we present a framework for AGN heating derived from a Chandra survey of gas entropy profiles within cluster cores. This set of observations strongly suggests that the inner parts of cluster cores are shock-heated every ~108 yr by intermittent AGN outbursts, driven by a kinetic power output of ~1045 ergs s-1 and lasting at least 107 yr. Beyond ~30 kpc these shocks decay to sound waves, releasing buoyant bubbles that heat the cores outer parts. Between heating episodes, cooling causes the core to relax toward an asymptotic pure cooling profile. The density distribution in this asymptotic profile is sufficiently peaked that the AGN shock does not cause a core entropy inversion, allowing the cluster core to retain a strong iron abundance gradient, as observed.

Entropy Profiles in the Cores of Cooling Flow Clusters of Galaxies

The X-ray properties of a relaxed cluster of galaxies are determined primarily by its gravitational potential well and the entropy distribution of its intracluster gas. That entropy distribution reflects both the accretion history of the cluster and the feedback processes that limit the condensation of intracluster gas. Here we present Chandra observations of the core entropy profiles of nine classic cooling flow clusters that appear relatively relaxed (at least outside the central 10-20 kpc) and contain intracluster gas with a cooling time less than a Hubble time. We show that those entropy profiles are remarkably similar, despite the fact that the clusters range over a factor of 3 in temperature. They typically have an entropy level of ?130 keV cm2 at 100 kpc that declines to a plateau ~10 keV cm2 at 10 kpc. Between these radii, the entropy profiles are r? with ? ? 1.0-1.3. The nonzero central entropy levels in these clusters correspond to a cooling time ~108 yr, suggesting that episodic heating on this timescale maintains the central entropy profile in a quasi-steady state. We show in an appendix that although disturbances and bubbles are visible in the central regions of these clusters, these phenomena do not strongly bias our entropy estimates.

The Second Most Distant Cluster of Galaxies in the Extended Medium Sensitivity Survey

We report on our ASCA, Keck, and ROSAT observations of MS 1137.5+6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have a full set of X-ray temperatures, optical velocity dispersions, and X-ray images for a complete, high-redshift sample of clusters of galaxies drawn from the EMSS. Our ASCA observations of MS 1137.5+6625 yield a temperature of 5.7 keV and a metallicity of 0.43 solar, with 90% confidence limits. Keck II spectroscopy of 22 cluster members reveals a velocity dispersion of 884 km s-1. This cluster is the most distant in the sample with a detected iron line. We also derive a mean abundance at z = 0.8 by simultaneously fitting X-ray data for the two z = 0.8 clusters, and obtain an abundance of ZFe = 0.33 ±. Our ROSAT observations show that MS 1137.5+6625 is regular and highly centrally concentrated. Fitting of a β model to the X-ray surface brightness yields a core radius of only 71 h-1 kpc (q0 = 0.1) with β = 0.70 ±. The gas mass interior to 0.5 h-1 Mpc is thus 1.2 ± ×1013 h-5/2 M☉ (q0 = 0.1). If the clusters gas is nearly isothermal and in hydrostatic equilibrium with the cluster potential, the total mass of the cluster within this same region is 2.1 ± ×1014 h-1 M☉, giving a gas fraction of 0.06 ± 0.04 h-3/2. This cluster is the highest redshift EMSS cluster showing evidence for a possible cooling flow (~20-400 M☉ yr-1). The velocity dispersion, temperature, gas fraction, and iron abundance of MS 1137.5+6625 are all statistically the same as those properties in lower redshift clusters of similar luminosity. With this clusters temperature now in hand, we derive a high-redshift temperature function for EMSS clusters at 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that the evolution of the temperature function is relatively modest. Supplementing our high-redshift sample with other data from the literature, we demonstrate that neither the cluster luminosity-temperature relation, nor cluster metallicities, nor the cluster gas fraction has detectably evolved with redshift. The very modest degree of evolution in the luminosity-temperature relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical density structure formation model is assumed.

A Deep Look at the Emission-Line Nebula in Abell 2597

The close correlation between cooling flows and emission-line nebulae in clusters of galaxies has been recognized for over a decade and a half, but the physical reason for this connection remains unclear. Here we present deep optical spectra of the nebula in Abell 2597, one of the nearest strong cooling-flow clusters. These spectra reveal the density, temperature, and metal abundances of the line-emitting gas. The abundances are roughly half-solar, and dust produces an extinction of at least a magnitude in V. The absence of [O III] ?4363 emission rules out shocks as a major ionizing mechanism, and the weakness of He II ?4686 rules out a hard ionizing source, such as an active galactic nucleus or cooling intracluster gas. Hot stars are therefore the best candidate for producing the ionization. However, even the hottest O stars cannot power a nebula as hot as the one we see. Some other nonionizing source of heat appears to contribute a comparable amount of power. We show that the energy flux from a confining medium can become important when the ionization level of a nebula drops to the low levels seen in cooling-flow nebulae. We suggest that this kind of phenomenon, in which energy fluxes from the surrounding medium augment photoelectric heating, might be the common feature underlying the diverse group of objects classified as LINERS.

CLASH-X: A comparison of lensing and X-ray techniques for measuring the mass profiles of galaxy clusters

We present profiles of temperature, gas mass, and hydrostatic mass estimated from new and archival X-ray observations of CLASH clusters. We compare measurements derived from XMM and Chandra observations with one another and compare both to gravitational lensing mass profiles derived with CLASH Hubble Space Telescope and Subaru Telescope lensing data. Radial profiles of Chandra and XMM measurements of electron density and enclosed gas mass are nearly identical, indicating that differences in hydrostatic masses inferred from X-ray observations arise from differences in gas-temperature measurements. Encouragingly, gas temperatures measured in clusters by XMM and Chandra are consistent with one another at ~100–200 kpc radii, but XMM temperatures systematically decline relative to Chandra temperatures at larger radii. The angular dependence of the discrepancy suggests that additional investigation on systematics such as the XMM point-spread function correction, vignetting, and off-axis responses is yet required. We present the CLASH-X mass-profile comparisons in the form of cosmology-independent and redshift-independent circular-velocity profiles. We argue that comparisons of circular-velocity profiles are the most robust way to assess mass bias. Ratios of Chandra hydrostatic equilibrium (HSE) mass profiles to CLASH lensing profiles show no obvious radial dependence in the 0.3–0.8 Mpc range. However, the mean mass biases inferred from the weak-lensing (WL) and SaWLens data are different. As an example, the weighted-mean value at 0.5 Mpc is 〈b〉 = 0.12 for the WL comparison and 〈b〉 = −0.11 for the SaWLens comparison. The ratios of XMM HSE mass profiles to CLASH lensing profiles show a pronounced radial dependence in the 0.3–1.0 Mpc range, with a weighted mean mass bias value rising to 〈b〉 gsim 0.3 at ~1 Mpc for the WL comparison and 〈b〉 ≈ 0.25 for the SaWLens comparison. The enclosed gas mass profiles from both Chandra and XMM rise to a value ≈1/8 times the total-mass profiles inferred from lensing at ≈0.5 Mpc and remain constant outside of that radius, suggesting that M_gas × 8 profiles may be an excellent proxy for total-mass profiles at ≳ 0.5 Mpc in massive galaxy clusters.

POLYCYCLIC AROMATIC HYDROCARBONS, IONIZED GAS, AND MOLECULAR HYDROGEN IN BRIGHTEST CLUSTER GALAXIES OF COOL-CORE CLUSTERS OF GALAXIES

We present measurements of 5-25 μm emission features of brightest cluster galaxies (BCGs) with strong optical emission lines in a sample of nine cool-core clusters of galaxies observed with the Infrared Spectrograph on board the Spitzer Space Telescope. These systems provide a view of dusty molecular gas and star formation, surrounded by dense, X-ray-emitting intracluster gas. Past work has shown that BCGs in cool-core clusters may host powerful radio sources, luminous optical emission-line systems, and excess UV, while BCGs in other clusters never show this activity. In this sample, we detect polycyclic aromatic hydrocarbons (PAHs), extremely luminous, rotationally excited molecular hydrogen line emission, forbidden line emission from ionized gas ([Ne II] and [Ne III]), and infrared continuum emission from warm dust and cool stars. We show here that these BCGs exhibit more luminous forbidden neon and H2 rotational line emission than star-forming galaxies with similar total infrared luminosities, as well as somewhat higher ratios of 70 μm/24 μm luminosities. Our analysis suggests that while star formation processes dominate the heating of the dust and PAHs, a heating process consistent with suprathermal electron heating from the hot gas, distinct from star formation, is heating the molecular gas and contributing to the heating of the ionized gas in the galaxies. The survival of PAHs and dust suggests that dusty gas is somehow shielded from significant interaction with the X-ray gas.