Cool Core Research Articles

A statistically selected Chandra sample of 20 galaxy clusters - I. Temperature and cooling time profiles

We present an analysis of 20 galaxy clusters observed with the Chandra X-ray satellite, focusing on the temperature structure of the intracluster medium and the cooling time of the gas. Our sample is drawn from a flux-limited catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their large angular size compared to the Chandra field of view. We describe a quantitative measure of the impact of central cooling, and find that the sample comprises nine clusters possessing cool cores (CCs) and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the CC clusters, which obey a nearly universal radial scaling in temperature of the form T ∝ r ∼0.4 , within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in CC clusters (tcool ∝ r ∼1.3 ), reaching tcool < 1 Gyr in all cases, although surprisingly low central cooling times (<5 Gyr) are found in many of the non-CC systems. The scatter between the cooling time profiles of all the clusters is found to be remarkably small, implying a universal form for the cooling time of gas at a given physical radius in virialized systems, in agreement with recent previous work. Our results favour cluster merging as the primary factor in preventing the formation of

Effects of Mergers and Core Structure on the Bulk Properties of Nearby Galaxy Clusters

We use morphological measurements and the scatter of clusters about observed and simulated scaling relations to examine the impact of merging and core-related phenomena on the structure of galaxy clusters. All relations constructed from emission-weighted mean temperature and intracluster medium mass, X-ray luminosity, isophotal size, or near-IR luminosity show a separation between cool core (CC) and non-cool core (NCC) clusters. We attribute this partially to a temperature bias in CC clusters, and partially to other cool core-related structural changes. We attempt to minimize CC/NCC separation in scaling relations by applying a uniform scale factor to CC cluster temperatures and determining the scale factor for each relation that minimizes the separation between CC and NCC populations, and by introducing central surface brightness as a third parameter in relations. The latter approach reduces scatter in relations more than temperature scaling. We compare the scatter within subsamples split by CC/NCC and morphological merger indicators. CC clusters and clusters with less substructure generally exhibit higher scatter about relations. The larger structural variations in CC clusters exit well outside the core, suggesting that a process more global than core radiative instability is at work. Simulations without cooling mechanisms also show no correlation between substructure and larger scatter about relations, indicating that any merger-related scatter increases are subtle. The results indicate that cool core related phenomena, not merging processes, are the primary contributor to scatter in scaling relations. Our analysis does not appear to support the scenario in which clusters evolve cool cores over time unless they experience major mergers. (Abridged)

Simulation Training for Surgical Residents: Cool Computers or Core Curriculum?

Simulation Training for Surgical Residents: Cool Computers or Core Curriculum?

ChandraReveals Galaxy Cluster with the Most Massive Nearby Cooling Core: RXC J1504.1−0248

A Chandra follow-up observation of an X-ray-luminous galaxy cluster with a compact appearance, RXC J1504.1-0248, discovered in our REFLEX Cluster Survey, reveals an object with one of the most prominent cluster cooling cores. A β-model fit to the X-ray surface brightness profile gives a core radius of ~30 h kpc, which is much smaller than the cooling radius with ~140 kpc. As a consequence, more than 70% of the high X-ray luminosity of Lbol = 4.3 × 1045 h ergs s-1 of this cluster is radiated inside the cooling radius. A simple modeling of the X-ray morphology of the cluster leads to a formal mass deposition rate within the classical cooling flow model of 1500-1900 M☉ yr-1 (for h100 = 0.7; 2300-3000 M☉ yr-1 for h100 = 0.5). The center of the cluster is marked by a giant elliptical galaxy, which is also a known radio source. Thus, it is very likely that we are observing one of the interaction systems where the central cluster active galactic nucleus (AGN) is heating the cooling core region in a self-regulated way to prevent a massive cooling of the gas, similar to several such cases studied in detail in more nearby clusters. The interest raised by this system is then due to the high power recycled in RXC J1504-0248 over cooling timescales, which is about 1 order of magnitude higher than what occurs in the studied, nearby cooling core clusters. The assumption that cooling is exactly balanced by the AGN heating implies a central black hole mass growth rate of the order of 0.5 M☉ yr-1. This cluster is therefore a prime target for the study of AGN-intracluster medium interaction at very extreme conditions. Further features common to cooling cores found in this cluster are a strong temperature drop toward the center and narrow, low-ionization emission lines in the central cluster galaxy. The cluster is also found to be very massive, with a global X-ray temperature of about 10.5 keV and a total mass of about 1.7 × 1015 h M☉ inside 3 h Mpc.

XMM‐NewtonObservations of A133: A Weak Shock Passing through the Cool Core

We use XMM-Newton observations of the cluster of galaxies A133 to study the X-ray spectrum of the intracluster medium (ICM). We find a cold front to the southeast of the cluster core. From the pressure profile near the cold front, we derive an upper limit to the velocity of the core relative to the rest of the cluster of less than 230 km s-1. Our previous Chandra image of A133 showed a complex, birdlike morphology in the cluster core. On the basis of the XMM-Newton spectra and hardness ratio maps, we argue that the wings of this structure are a weak shock front. The shock was probably formed outside the core of the cluster and may be heating the cluster core. Our Chandra image also showed a "tongue" of relatively cool gas extending from the center of the cD galaxy to the center of the radio relic. The XMM-Newton results are consistent with the idea that the tongue is the gas that has been uplifted by a buoyant radio bubble including the radio relic to the northwest of the core. Alternatively, the tongue might result from a cluster merger. The small velocity of the core suggests that the bubble including the relic has been moved by buoyancy rather than by motions of the core or the ICM. We do not find clear evidence for nonthermal X-ray emission from the radio relic. On the basis of the upper limit on the inverse Compton emission, we derive a lower limit on the magnetic field in the relic of B ≥ 1.5 μG.

The Complex Cooling Core of A2029: Radio and X‐Ray Interactions

We present an analysis of Chandra observations of the central regions of the cooling flow cluster A2029. We find a number of X-ray filaments in the central 40 kpc, some of which appear to be associated with the currently active central radio galaxy. The outer southern lobe of the steep-spectrum radio source appears to be surrounded by a region of cool gas and is at least partially surrounded by a bright X-ray rim similar to that seen around radio sources in the cores of other cooling flow clusters. Spectroscopic fits show that the overall cluster emission is best fitted by either a two-temperature gas (kThigh = 7.47 keV and kTlow = 0.11 keV) or a cooling flow model with gas cooling over the same temperature range. This large range of temperatures (over a factor of 50) is relatively unique to A2029 and may suggest that this system is a very young cooling flow in which the gas has only recently started cooling to low temperatures. The cooling flow model gives a mass deposition rate of = 56 M☉ yr-1. In general, the cluster emission is elongated along a position angle of 22° with an ellipticity of 0.26. The distribution of the X-ray emission in the central region of the cluster is asymmetric, however, with excess emission to the northeast and southeast compared with that to the southwest and northwest, respectively. Fitting and subtracting a smooth elliptical model from the X-ray data reveals a dipolar spiral excess extending in a clockwise direction from the cluster core to radii of ~150 kpc. We estimate a total mass of Mspr ~ 6 × 1012 M☉ in the spiral excess. The most likely origin of the excess is either stripping of gas from a galaxy group or from bare dark matter potential that has fallen into the cluster or sloshing motions in the cluster core induced by a past merger.

Can Supermassive Black Holes Sufficiently Heat Cool Cores of Galaxy Clusters?

The activities of a supermassive black hole or active galactic nucleus in the central galaxy of a cluster of galaxies have been promising candidates for the heating sources of cool cluster cores. We estimate the masses of black holes using known correlations between the mass of a black hole and the velocity dispersion or the luminosity of the host galaxy. We find that the masses are ~108-109 M☉ and the central X-ray luminosities of the host clusters (the strength of the cooling flow) are well below the Eddington luminosities. However, we do not find a correlation between the mass and the central X-ray luminosity of the host cluster. If the heating is stable, this seems to contradict a simple expectation if a supermassive black hole is the main heating source of a cluster core. Moreover, if we assume a canonical energy conversion rate (10%), black holes alone would be unable to sufficiently heat the clusters with strong centrally peaked X-ray emission (massive cooling flows) over the lifetime of cluster cores. These results may indicate that massive cooling flows are a transient phenomenon, perhaps because the black holes are activated periodically. Alternatively, in the massive cooling flow clusters, the energy conversion rate may be larger than 10%, that is, the black holes may be Kerr black holes.

Tsunamis in Galaxy Clusters: Heating of Cool Cores by Acoustic Waves

Using an analytical model and numerical simulations, we show that acoustic waves generated by turbulent motion in intracluster medium effectively heat the central region of a so-called cooling flow cluster. We assume that the turbulence is generated by substructure motion in a cluster or cluster mergers. Our analytical model can reproduce observed density and temperature profiles of a few clusters. We also show that waves can transfer more energy from the outer region of a cluster than thermal conduction alone. Numerical simulations generally support the results of the analytical study.

ChandraObservations of the Disruption of the Cool Core in A133

We present the analysis of a Chandra observation of the galaxy cluster A133, which has a cooling flow core, a central radio source, and a diffuse, filamentary radio source that has been classified as a radio relic. The X-ray image shows that the core has a complex structure. The most prominent feature is a of emission that extends from the central cD galaxy to the northwest and partly overlaps the radio relic. Spectral analysis shows that the emission from the tongue is thermal emission from relatively cool gas at a temperature of ~1.3 keV. One possibility is that this tongue is produced by Kelvin-Helmholtz (KH) instabilities through the interaction between the cold gas around the cD galaxy and hot intracluster medium. We estimate the critical velocity and timescale for the KH instability to be effective for the cold core around the cD galaxy. We find that the KH instability can disrupt the cold core if the relative velocity is 400 km s-1. We compare the results with those of clusters in which sharp, undisrupted cold fronts have been observed; in these clusters, the low-temperature gas in their central regions has a more regular distribution. In contrast to A133, these cluster cores have longer timescales for the disruption of the core by the KH instability when they are normalized to the timescale of the cD galaxy motion. Thus, the other cores are less vulnerable to KH instability. Another possible origin of the tongue is that it is gas that has been uplifted by a buoyant bubble of nonthermal plasma that we identify with the observed radio relic. From the position of the bubble and the radio estimate of the age of the relic source, we estimate a velocity of ~700 km s-1 for the bubble. The structure of the bubble and this velocity are consistent with numerical models for such buoyant bubbles. The energy dissipated by the moving bubble may affect the cooling flow in A133. The combination of the radio and X-ray observations of the radio relic suggest that it is a relic radio lobe formerly energized by the central cD rather than a merger shock-generated cluster radio relic. The lobe may have been displaced from the central cD galaxy by the motion of the cD galaxy or by the buoyancy of the lobe.