Chandra's First Decade of Discovery

Clusters of Galaxies

Chandra observations of clusters detected with the South Pole Telescope

Karl Andersson, MIT
The SPT collaboration

We present the first results from the Chandra X-ray followup of the Sunyaev Zel'dovich (SZ) survey currently conducted with the South Pole Telescope (SPT). The sample includes multiple objects not previously identified as clusters of galaxies and these represent the first clusters discovered using the Sunyaev Zel'dovich effect. Temperatures, luminosities and gas mass estimates are presented for the eight clusters followed up by Chandra to date. The clusters are a subset of a larger sample from a 200 sq deg area survey observed with SPT during the 2007 and 2008 observing seasons, selected using the SZ signal. We relate the derived X-ray mass proxies to the observed SZ flux.

Studying Clusters of Galaxies with Chandra and Sunyaev-Zel'dovich Effect Measurements from the South Pole Telescope

Bradford Benson (University of Chicago), and the SPT Team

The combination of measurements from the South Pole Telescope (SPT) and Chandra will provide a unique data set to understand the thermal history and formation of clusters of galaxies. The SPT is a 10-meter millimeter wavelength telescope designed to do a Sunyaev Zel'dovich (SZ) survey for clusters of galaxies in three frequency bands at 95, 150, and 220 GHz. Relative to previous SZ experiments, SPT has an unprecedented sensitivity that is allowing it to discover a new sample of high redshift clusters. Already, Chandra has made measurements of several SPT discovered clusters. In addition, SPT's multiple frequencies and relatively soft spatial filtering helps it to study gas in the outskirts of clusters. The combination of SPT and Chandra cluster measurements will be a unique data set to probe the thermodynamics of intra-cluster gas, in principle out to a cluster's virial radius, and will do this for a SZ selected cluster sample that will extend to higher redshifts than previous samples. Work is underway performing a joint analysis of Chandra and SPT measurements for a set of massive well-known X-ray luminous clusters. As the SPT catalog expands, we hope to extend this analysis to more clusters, and to learn about the accretion and thermal history of intra-cluster gas out to the earliest epochs of cluster formation in the universe.

Physically Motivated Models of Galaxy Clusters Based on a Generalized NFW Profile and an Application to Chandra Observations

Gul Bulbul, University of Alabama in Huntsville
Nicole Hasler (U.A.Huntsville), Max Bonamente (U.A.Huntsville), Marshall Joy (MSFC), Jeff Kolodziejczak (MSFC), John Carlstrom (University of Chicago), Daisuke Nagai (Yale University),Tony Mroczkowski (Univ. of Pennsylvania), Daniel Marrone (Univ. of Chicago)

Precise measurements of galaxy cluster masses are crucial for providing a foundation to investigate theoretically predicted physical processes which should be occurring in the intra-cluster medium(ICM), such as the helium sedimentation. We present physically motivated temperature and density models of galaxy clusters based on a generalized NFW dark matter and a polytropic equation of state density profile in order to profile the masses of galaxy clusters. Our temperature and density models have been derived analytically, and contain only 6 free parameters, resulting in less degeneracy when compared to other models of ICM properties. We present a comparison of these models with the Chandra spectral and imaging data of three clusters Abell 1835, Abell 2204 and CL J1226.9+3332 and show that our new, analytic models are in good agreement with the data and previous work in the area. This work is the first step towards the analysis of a large sample in which we will test Peng & Nagai (2009) helium sedimentation processes.

The Cluster Environment of Wide Angle Tail Radio Sources

Edmund Douglass, Boston University
Elizabeth L. Blanton (BU) T. E. Clarke (NRL) Craig L. Sarazin (UVa)

Due to their frequent association with galaxy clusters and their connection to intracluster medium ram pressure, wide angle tail (WAT) radio sources have proven to be reliable tracers of high-density environments at a range of redshifts and may be used as probes of the cluster gas environment. In an effort to determine whether the X-ray properties of WAT clusters define them as a population, we have examined a sample of 14 WAT clusters observed with the Chandra X-ray Observatory, which are publicly available in the archive. Many of these clusters are found to display both merger signatures and evidence of cool gas coincident with the WAT host galaxy, suggesting a possible relation between the formation of WATs and the presence of perturbed cool cores. To examine if there is a distinction between the X-ray properties of WAT clusters and WAT-less clusters we compare the results with those of an identical analysis of an archival sample of cool core and non-cool core clusters in which WAT radio sources are not present. Temperature, abundance, pressure, density, and mass profiles, as well as two-dimensional maps of temperature are presented.

3C28 in Abell 115- A Radio Source With a Twist: Tracing Gas Vortices in a Merging Subcluster Core

William Forman, SAO-CFA
E. Churazov (MPA, IKI), S. Giacantucci (CFA), C. Jones (CFA), L. David (CFA), M. Markevitch (CFA), A. Vikhlinin (CFA), S. Randall (CFA), S. Heinz (CFA), M. Murgia (CFA), R. Kraft (CFA)

Abell 115 is one of the “bimodal” clusters, first identified from Einstein Observatory X-ray images. The X-ray image is dominated by emission from two subclusters, separated by about 900 kpc, that are in the process of merging. The northern subcluster (A115-N) contains a bright central galaxy that hosts the radio source 3C28. 3C28 has a remarkable morphology. Although there is no evidence of an active nucleus, there are two prominent jets connected to a pair of radio lobes, each of which exhibits a radio tail. A115-N shows a classic cold front, the remarkable phenomenon first studied from Chandra cluster observations. We exploit the Chandra data and the cold front phenomenon to study the gas motions in and around A115-N that hosts 3C28. We estimate the velocity of A115-N with respect to the ambient intracluster medium. This motion of A115-N through the cluster induces counter-rotating vortices in the subcluster gas that give rise to the unique morphology of 3C28 with its two radio tails pointing in the direction of motion of A115-N. Thus, the radio emitting plasma acts as a dye in a fluid tracing the vortices in the X-ray emitting gas, resembling text book pictures of fluid motions. This makes A115 an ideal object for detailed studies of plasma dynamics in clusters, including effects of thermal conduction and viscosity.

X-ray and Optical Observations of the Merging Cluster Abell S1063.

Percy Gomez, Gemini Observatory
L.E. Valkonen (Sussex), K. Romer (Sussex), K. Sabirli (Sussex), E. Lloyd-Davies (Sussex)

We present the first in depth analysis of the massive cluster Abell S1063. This is one of the hottest X-ray clusters discovered to date and is undergoing a major merging event. It provides an important comparison system for the well known “bullet cluster”, as it has a similar redshift, X-ray temperature and luminosity. The average temperature of the hot intracluster medium has been measured for the first time, using Chandra ACIS-I, and found to be > 12 keV. Optical spectroscopy, from GMOS-S, has provided a mean cluster recessional velocity of 103800+300-330 km s-1 and a velocity dispersion of 1840+230-150 km s-1 (1 σ errors). Both the large velocity dispersion and high X-ray temperature suggest either a very massive cluster (M (<r200) > 4.0 x 1015 Msun) or a merger system. The merger model is supported by a small offset between the galaxy density and the peak of the X-ray emission, by the presence of offset and twisted X-ray isophotes, and by a non-Gaussian galaxy velocity distribution. We also report that the velocity distribution is consistent with the velocity distributions found in N-body simulations of head-on cluster mergers (e.g. 1:4 mass ratio). Moreover, we find that the best fit model for the distribution of the cluster gas consists of two, offset, isothermal beta model components. Therefore, we propose that a recent merger event close to the plane of the sky is responsible for the observed properties of the cluster. In addition, optical imaging, from SuSI2 on the NTT and GMOS-S at Gemini, have also uncovered the presence of several gravitational arcs that can be used to further constrain the mass of the cluster.

Probing Evolution of Hot Gas and Galaxy Distributions in Galaxy Clusters

Naohisa Inada, RIKEN
Madoka Kawaharada (RIKEN), Saori Konami (RIKEN), Poshak Gandhi (RIKEN), Toru Misawa (RIKEN), Kazuaki Ota (RIKEN), Kazuo Makishima (RIKEN, Univ. of Tokyo), Rohta Takahashi (RIKEN)

How have spatial distributions of components in galaxy clusters (dark matter, hot gas, metals and galaxies) evolved over the history of the universe? It is commonly known that cluster hot gas distribution is more extended than that of dark matter (beta in the beta model is less than unity for hot gas distribution), which means that the hot gas has higher specific energy than dark matter. In a study of 12 clusters with XMM-Newton (Kawaharada et al. 2009, ApJ, 691, 971), we have found that galaxy luminosity distributions are more centrally-concentrated than corresponding metal distributions in their central ~100 kpc regions. One simple interpretation of the result is that galaxies have gradually fallen to the cluster center and merged into a single central galaxy while providing metals to surrounding hot gas, and that kinetic energy lost from galaxies (by interactions of interstellar media and intergalactic hot gas) has expanded the cluster hot gas distribution. If the scenario is correct, the relative distribution of galaxies to hot gas should change/evolve with redshifts. To see this phenomenon, we have started a project to compare distributions of galaxies and hot gas in galaxy clusters at different redshifts. We are utilizing optical data from the Sloan Digital Sky Survey, while those in X-rays from XMM-Newton. Some preliminary results will be presented in the Symposium.

Core Gas Sloshing in the Double Cluster Abell 1644

Ryan Johnson, Dartmouth College / CfA
Maxim Markevitch (CfA), Gary Wegner (Dartmouth College), Christine Jones (CfA), Bill Forman(CfA)

Chandra's unparalleled angular resolution has allowed us to probe deep into the cores of many galaxy clusters, revealing edges in their X-ray surface brightness distributions. These edges may be the result of one cluster halo passing through another (as in A3667 and the “bullet” cluster), or AGN blown bubbles in the ICM (as in M87 and Hydra A), or by the core gas sloshing about the cluster potential minimum in response to the gravitational perturbation by the passage of another cluster (as in Ophiuchus and A1644). Here we present a canonical example of this latter process in the double cluster Abell 1644, where not only is the core gas sloshing visible, but the perturbing sub-cluster is as well. We show how observations of core gas sloshing, used in concert with simulations, place constraints on core reheating and the merger history of the cluster.

A Tidal Disruption Flare from a Rich Galaxy Cluster

Peter Maksym, Northwestern University
Melville Ulmer (Northwestern University), Michael Eracleous (Pennsylvania State University)

Galaxy clusters are natural targets for studies of rare, luminous transients and we have undertaken an X-ray survey of rich galaxy clusters for tidal disruption flares. We introduce the first results our study and identify a candidate tidal disruption event that exhibits characteristics consistent with theoretical predictions and known examples of these rare events. Its peak observed luminosity is between 0.6 x 1043 and 1.3 x 1043 ergs s-1 although its unobserved peak could be as high as ≈1045 ergs s-1. Its spectrum is supersoft, consistent with a blackbody of kT=0.12 keV.

Chandra measurements of non-thermal-like X-ray emission from massive, merging, radio-halo clusters

Evan Million, Stanford, SLAC
Steve Allen (Stanford, SLAC)

We report the discovery of spatially-extended, non-thermal-like emission components in Chandra X-ray spectra for five of a sample of seven massive, merging galaxy clusters with powerful radio halos. The emission components can be fitted by power-law models with mean photon indices in the range 1.5 < Γ < 2.0. A control sample of regular, dynamically relaxed clusters, without radio halos but with comparable mean thermal temperatures and luminosities, shows no compelling evidence for similar components. Detailed X-ray spectral mapping reveals the complex thermodynamic states of the radio halo clusters. Our deepest observations, of the Bullet Cluster 1E0657-56, demonstrate a spatial correlation between the strongest power-law X-ray emission, highest thermal pressure, and brightest 1.34GHz radio halo emission in this cluster. We confirm the presence of a shock front in the 1E0657-56 and report the discovery of a clear, large-scale shock front in Abell 2219. We explore possible origins for the power-law X-ray components. One explanation of particular interest is that the power-law signatures may be due to complex temperature and/or metallicity structure in clusters particularly in the presence of metallicity gradients. In this case, an important distinguishing characteristic between the radio halo clusters and control sample of predominantly cool-core clusters is the relatively low central X-ray surface brightness of the former. Our results highlight the importance of further deep X-ray and radio mapping, coupled with new hard X-ray, γ-ray and TeV observations, for improving our understanding of the non-thermal particle populations in these systems.

Effects of the Non-Equipartition of Electrons and Ions in the Outskirts of Relaxed Galaxy Clusters

Ka-Wah Wong, University of Virginia
Craig L. Sarazin (University of Virginia)

We have studied the effects of electron-ion non-equipartition in the outer regions of relaxed clusters for a wide range of masses in the ΛCDM cosmology using one-dimensional hydrodynamic simulations. Signatures of non-equipartition on X-ray and SZ observables are studied systematically. The effects of the non-adiabatic electron heating efficiency, β, on the degree of non-equipartition are also studied. We have calculated the effect of non-equipartition on the projected temperature and X-ray surface brightness profiles using the MEKAL emission model. The non-equipartition effect can introduce a ~10% bias in the projected temperature at Rvir for a wide range of beta. We also found that the effect of non-equipartition on the projected temperature profiles can be enhanced by increasing metallicity. In the low energy band less than ~1 keV, the non-equipartition model surface brightness can be higher than that of the equipartition model in the cluster outer regions. Future X-ray observations extending to ~Rvir or even close to the shock radius should be able to detect these non-equipartition signatures. For a given cluster, the difference between the SZ temperature decrements for the equipartition and the non-equipartition models, δΔ TSZE, is larger at a higher redshift. For the most massive clusters at z ≈ 2, the differences can be δΔ TSZE ≈ 4-5μK near the shock radius. We also found that for our model in the ΛCDM Universe, the integrated SZ bias, Ynon-eq/Yeq, evolves slightly (at a percentage level) with redshift, which is in contrast to the self-similar model in the Einstein-de Sitter Universe. This may introduce biases in cosmological studies using the fgas technique. We discussed briefly whether the equipartition and non-equipartition models near the shock region can be distinguished by future radio observations with, for example, ALMA.

[PDF of this poster]

Density Profile and X-ray Luminosity function of AGNs in galaxy clusters

Yuxuan Yang, University of Illinois
Joe Mohr, Tim O'Hara

We report a statistical study of contents of active galactic nuclei (AGNs) in galaxy clusters using the data of 68 galaxy clusters from the Chandra archive. The redshifts of the clusters range from 0.2 to 1.3, with median redshift of 0.38. The ensemble averaged surface density distribution of point sources show clear excess within virial radius (r200) while away from the cluster the surface density of point sources agree with that of non-cluster fields. We found that the average deprojected radial density profile of X-ray selected AGNs have similar shape as normal galaxies rather than radio loud AGNs. On average the normalization of the cluster X-ray luminosity functions (XLFs) within r200 are ~324 and 231 times higher than that of the field luminosity function in the 0.5-2 and 2-8 keV bands respectively. This translates to an AGN fraction in clusters is (73 +- 21)% of the field value in the soft band, and (52 +- 18)% in the hard band.

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Cold Fronts and Gas Sloshing in Galaxy Clusters: Making Connections Between Simulation and Observation

John ZuHone, Harvard-Smithsonian Center for Astrophysics
Maxim Markevitch (CfA)

One of the earliest discoveries resulting from observations of clusters of galaxies with Chandra was the existence of “cold fronts” in the hot intracluster medium. These features appear as sharp discontinuities in the X-ray surface brightness where the spectral temperature is measured to be colder on the brighter (and therefore denser) side of the front. The presence of cold fronts in the cores of relaxed galaxy clusters suggests the cool gas is sloshing back and forth in the cluster potential minimum. We present simulations where such sloshing is initiated by gravitational disturbances from passing subclusters. We show how the resulting sloshing cold fronts help to place constraints on the physics of the intracluster medium and how sloshing may serve as an additional form of feedback to offset cooling in cluster cores.