Symposium Proceedings

Clusters of Galaxies

Chandra Observation of the Cooling Flow Cluster A262

Loren Anderson, Elizabeth L. Blanton (BU), Tracy E. Clarke (NRL), Craig L. Sarazin (UVa)

We present a detailed study of the cooling flow cluster A262, including the projected and deprojected profiles of temperature and abundance as well as profiles of pressure, density, gas mass, total mass, and entropy. We find that the temperture drops by a factor of 3 from 2.7 to 0.9 keV in the center and that the abundance climbs steeply towards the center before falling in the very inner regions. We also present image analysis highlighting complex structure in the inner regions including a tunnel to the west and several interesting features surrounding the cluster center. We compare new low frequency (330 MHz) radio observations with the structure in the inner regions.

The Chandra View of RXJ1716

Douglas Burke (SAO)

We present the results of a 50 ks Chandra observation of the $z=0.809$ galaxy cluster RX J$1716.9+6708$. Previous X-ray and optical observations have shown that this system is not relaxed and that the large-scale structure in which the cluster is embedded exhibits the filamentary nature predicted by models of structure formation. The high spatial resolution of Chandra reveals that the core of the cluster X-ray emission shows signs of recent merger activity and is aligned with the NE-SW filament seen in the galaxy and mass distributions. On a larger scale, we detect-- at a projected distance of 1 Mpc NE of the core-- X-ray emission from a group of galaxies located in the NE-SW filament. It appears that we are observing RX J$1716.9+6708$ during its formation epoch.

Chandra Observation of the Merging Cluster Abell 2065

Marios Chatzikos, Craig Sarazin (Univ. of Virginia)

We present analysis of a 41 ks observation of the merging cluster Abell 2065 with the ACIS-I detector. Previous observations with ROSAT and ASCA provided evidence for an ongoing merger, but also suggested that there were two surviving cooling cores, which were associated with the two cD galaxies in the center of the cluster. The Chandra observation reveals only one X-ray surface brightness peak, which is associated with the more luminous, southern cD galaxy. The gas related with that peak is cool and displaced from the position of the cD. The data suggest that this cool material has formed a cold front. On the other hand, in the higher spatial resolution Chandra image, the second feature to the north is not associated with the second cD, rather it appears to be a trail of gas behind the main cD. We argue that only one of the two cooling cores has survived the merger, which suggests that the cluster is undergoing an unequal merger. The data indicate that a shock front is propagating through the cluster's ICM, which we use to constrain the kinematics of the system.

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Chandra Observations of Abell 222/Abell 223

David Davis, Mark Henriksen (UMBC)

The formation of galaxy clusters and their subsequent evolution provide insight into not only how such large gravitationally bound objects are formed but also into cosmology. Simulations of large scale structure show that clusters are formed hierarchically and are formed from the aggregation of smaller structures. Identifying clusters in the early stages of a merger will provide observational constraints for the early stages of N-body/hydrodynamical simulations of merging clusters. Here we present the morphology and temperature for the double cluster Abell 222, Abell 223 derived from a 50 ksec Chandra ACIS-I observation.

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Chandra Observation of the Cluster Environment of a WAT Radio Source in Abell 1446

E. M. Douglass, Elizabeth L. Blanton (Boston Univ.), Tracy E. Clarke (NRL), Craig L. Sarazin (Univ. of Virginia), Michael W. Wise (MIT)

Wide-angle tail (WAT) radio sources are often found in the center of galaxy clusters where large scale flow of the ICM may bend the lobes into their characteristic C-shape. We present Chandra observations of the low redshift (z=0.1035), WAT containing galaxy cluster, Abell 1446. An infall of gas from the north appears to be bending one of the WAT radio lobes associated with the cluster's central elliptical galaxy. Further, a cavity within the X-ray gas appears coincident with the lobe. A temperature profile shows that the cluster is isothermal (kT= 4.1 keV) in a series of annuli reaching a radius of $\sim$350 kpc. There is no evidence of a cooling flow. In addition to images, profiles of temperature, abundance, surface brightness, pressure, density, mass, and a detailed temperature map will be presented.

Analysis of X-MAS Images of a Simulated Cool Core Cluster

Alessandro Gardini, Alastair Sanderson (Univeristy of Illinois at Urbana-Champaign)

We show the results of the data analysis of simulated X-ray observations of a cool core cluster, in order to assess the robustness and reliability of results of analogous observations by Chandra. We generate simulated X-ray images using the X-MAS package of a cluster model in hydrostatic equilibrium resembling A2390. We consider different distances and exposure times, comparing surface brightness and temperature profiles to the results of deprojection analysis applied to the simulated observations. The effect of presence of bubbles of hot plasma in the cluster atmosphere is also taken in account.

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Finding Galaxy Groups in the Fields of Lensed Quasars

Catherine Grant, Mark Bautz (MIT)

Gravitationally lensed quasars can be used as an invaluable cosmological tool, however lens models often require a strong external shear in addition to the intrinsic eccentricity of the lensing galaxy to produce the observed asymmetry in the lensed images. The shear may be from a galaxy cluster or group associated with the lensing galaxy or otherwise along the line-of-sight to the quasar. We are searching for X-ray emission from groups in the fields of strong gravitational lenses using Chandra's resolving power to separate the bright quasar images from the much fainter diffuse emission. We present Chandra images and luminosities or luminosity limits for optically detected lensing groups associated with multiply imaged gravitationally lensed quasars. We also will discuss our search for extended X-ray emission in the fields of lensed quasars that are not yet known to contain a galaxy group.

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Limitations on Precision Cosmology Using Mass Measurements of Galaxy Clusters

Eric Hallman, Patrick Motl, Jack Burns (University of Colorado, Boulder), Michael Norman (Univ. of California, San Diego)

We critically analyze the role of clusters of galaxies as probes for precision cosmology. Using synthetic observations of numerically simulated clusters viewed through their X-ray emission and thermal Sunyaev-Zeldovich effect (SZE), we reduce the observations to attain measurements of the cluster gas mass. We utilize both parametric models such as the isothermal cluster model and non-parametric models that involve the geometric deprojection of the cluster emission assuming spherical symmetry. We are thus able to quantify the possible sources of uncertainty and systematic bias associated with the common simplifying assumptions used in reducing real cluster observations including isothermality and hydrostatic equilibrium.

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X-ray Observations of Galaxies in an Off-Center Region in Coma

Ann Hornschemeier (NASA GSFC), Bahram Mobasher (STScI), David Alexander (IoA Cambridge), Franz Bauer (Columbia), Mark Bautz (KIASR, MIT), Derek Hammer (JHU), Bianca Poggianti (Padova)

We have performed a pilot Chandra ACIS-I survey of a region in an off-center region of the Coma cluster to explore the X-ray properties and Luminosity Function of normal galaxies. We present results on 13 Chandra-detected galaxies with optical photometric matches, 9 of which are ascertained to be Coma members. We have taken the well-measured optical galaxy luminosity function in this region of the Coma cluster and translated it to an X-ray luminosity function. We use this XLF to find that the galaxies in the outskirts of Coma have very low X-ray-to-optical flux ratios. This confirms the Finoguenov et al. (2003) Coma result that the X-ray emission per unit optical flux per galaxy is suppressed in clusters of galaxies, but extends this work to a specific off-center environment of the Coma cluster. We also report on our work with the XMM Optical Monitor to classify galaxies in Coma based on their ultraviolet properties.

X-ray Sources in Groups of Galaxies

Tesla Jeltema, Breanna Binder, John Mulchaey (OCIW),

We have undertaken an archival Chandra study to investigate the previously unknown AGN content of groups of galaxies. Our sample consists of 23 nearby groups with relatively long Chandra observations. Detected X-ray sources are matched to galaxies with known redshifts in NED to confirm group membership. While we generally find a couple of matches per group, we do not find a population of luminous AGN ($>10^{41}$ ergs s$^{-1}$) outside of the group center. However, we do detect central X-ray point sources in most of the groups, and in several cases we detect diffuse emission from group galaxies.

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Chandra Observation of Abell 13: Understanding X-ray-Radio Interaction

Adrienne M. Juett, Craig L. Sarazin (Univ. of Virginia), Tracy Clarke (NRL), Yutaka Fujita (Nat. Astron. Obs. of Japan), Heinz Andernach (Univ. of Guanajuato), Matthias Ehle (XMM-Newton SOC), Joshua Kempner (Bowdoin College), Alan Roy (MPIfR), Larry Rudnick (Univ. of Minnesota)

We present results from the recent Chandra observation of Abell 13. This galaxy cluster also contains an unusual noncentral radio source, commonly referred to as a radio relic. We present a comparison of the properties of the hot X-ray gas derived from Chandra with those of the radio relic from VLA data, to study the interaction of the X-ray gas with the radio emitting electrons. Initial results suggest that the radio relic is associated with cooler gas in the cluster. We also present results on structure and temperature distributions of the gas in the cluster using the Chandra data, which suggest that a recent galaxy or group merger has occurred in the cluster.

Cluster Samples with the Long Wavelength Array

Namir Kassim, Tracy Clarke, Emil Polisensky, Carl Gross, Aaron Cohen, Wendy Lane, Joseph Lazio (Naval Research Lab)

The Long Wavelength Array (LWA) will be a powerful new instrument that will open a new window of high resolution and sensitivity on the poorly explored region between 20 and 80 MHz. We will describe how the LWA will be an excellent instrument for studying clusters of galaxies and addressing key questions in Cosmology. The assembly of large-scale structures in the Universe is thought to proceed in a hierarchical manner, with smaller structures merging to form larger structures. At least a fraction of the most energetic mergers in the present-day Universe are marked by cluster halos and relics, which are characterized by diffuse, steep-spectrum radio emission not associated with any galaxy in the merging cluster. The current census of cluster halos and relics is considered incomplete due to the low sensitivity of existing long-wavelength instruments. The LWA will have sufficient sensitivity at long wavelengths to detect less energetic and more distant mergers, and it is estimated that there may be thousands of them. When compared with X-ray observations, a complete census of diffuse emission in merging clusters can be used to trace the formation of structure, measure the input of energy into the intracluster medium, and constrain the dark matter potentials that govern mergers. Additionally, the census would define a non-merging cluster sample which would provide the undisturbed systems necessary to study the dark energy equation of state through determination of the baryonic mass fraction in massive clusters. Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research.

Temperature and Metallicity in the Central Region of Cooling Flow Clusters of Galaxies

Naomi Kawano, Kengo Kawashima, Yasushi Fukazawa (Hiroshima Univ.)

Most cooling flow clusters show the temperature decline toward the center. The temperature in the central region reaches about a half of the outer region within 100$-$200 kpc, and widely scatters between galaxy clusters contrary to the prediction cooling flow model. On the other hand, the scale of the central cool region is consistent with the cooling radius, implying that the radiative cooling is surely occur and some heating mechanism to reduce it is necessary. Among the prevailing heating model, the hierarchical evolution with mergers provides better consistency with our results, and then seems to play an important role for heating in the intracluster medium (ICM).

The iron abundance increases in the central 100 kpc region, which is almost the same scale as the optical body of the cD galaxy. The mass of iron within the abundance-excess region is as much as that contained by stars in the elliptical galaxies. These results indicate that the cD galaxy can be a major source of metal supply into the ICM. If so, the Type Ia supernova and stellar mass loss are important processes to transport metals. Calculation of the amount of iron supply due to these process over the Hubble time shows that they are enough to provide the iron in the ICM. Thus, it appears certain that the cD galaxy can largely contribute to the metal in the ICM.

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Fake Source Experiment for Chandra ACIS Observations

Eunhyeuk Kim (Harvard-Smithsonian Center for Astrophysics) Paul Martini (The Ohio State Univ.), Minsun Kim, Dong-Woo Kim (Harvard-Smithsonian Center for Astrophysics)

High spatial resolution Chandra X-ray observations have resolved many point sources in both galaxies and clusters of galaxies. The observed spatial distribution of these points sources suffer from incompleteness due to source location, background variations, and brightness. The source counts may also suffer from inaccurate count measurements, particularly in high background regions such as clusters or diffuse emission from bright, nearby galaxies.

To investigate variations in completeness, we perform a large number of fake source experiments for Chandra ACIS observations using custom software called XFAKE. The detection probability is found to decrease with increasing off-axis angle(OAA), an angle measured from on-axis to the position of a source. For example, the detection probability for point sources with net counts $> 10$ and OAA $< 5$ arcmin is found to be greater than $95\%$, with a slight dependence on background variation.

We describe the simulation software XFAKE along with an accompanying software package XPROCES and discuss the detection probability and the count recovery rate for different background environments in detail.

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Synchronism of Events in Space Conditions

Dmitri Kirko (Dept. Plasma Phys., MEPI, Moscow, Russia)

The observed universe structure assume of some ordering of spatial and time characteristics of astronomical objects. At the supposition of presence of interaction between galaxy clusters of the universe in the present time range, it is expedient to allow the manifestation of information processes. We assume the existence of physical information interaction in the universe in the present time range or property as synchronism of events. The observed universe is compared with some event space of non-Euclidean (nonlinear) structure. Within that space information signals can move at a speed exceeding the speed of light. At each random moment of time transition from event space to Minkowski's space is established. Some laboratory and astrophysical experiments show the possibility on registration the images of stars (galaxies) within a time lag. In this work the correlation between synchronism of events, and the theory of relativity and the superstrings theory are assumed.

Re-energizing a Cluster Cooling Core: the ICM/Radio Plasma Interaction in 3C 388

Ralph Kraft (SAO), Jaione Azcona (Observatiore de Paris-Meudon-Nancy and SAO), William Forman (SAO), Martin Hardcastle (Univ. of Hertfordshire), Christine Jones, Stephen Murray (SAO)

We present results from analyis of a 35 ks Chandra/ACIS-I observation of the hot ICM around the nearby (z=0.0908) FR II radio galaxy 3C 388. This radio galaxy resides in a cluster environment with a gas temperature of $\sim$3.5 keV. We detect cavities in the ICM coincident with the radio lobes and find no evidence for surface brightness discontinuities in the gas or hot rims around the lobes indicative of shocks. The lobes are therefore evolving at or below the sound speed of the ICM. The work done on the gas by the inflation of the lobes is $\sim$3$\times$10$^{59}$ ergs, or $\sim$0.87 keV per particle out to the radius of the lobes. The radiative timescale for gas at the center of the cluster at the current temperature is a few Gyrs. The gas in the core was probably cooler and denser before the outburst, so the cooling time was considerably shorter in the past. Therefore, we suggest that gas cooling is being quenched by a radio galaxy outburst. The bubble enthalpy, $\sim$1.2$\times$10$^{60}$ ergs, is sufficient to completely re-energize the cooling core if it is converted to thermal gas energy in or near the core. The mechanical power of the outburst is roughly two orders of magnitude larger than either the X-ray emission from the active nucleus or the radio emission from the lobes. The equipartition pressure of the radio lobes is more than an order of magnitude lower than that of the ambient medium, indicating that the pressure of the lobes is dominated by something other than the relativistic electrons radiating at GHz frequencies unless the lobe is far from equipartition.

The Large AGN Population in Clusters of Galaxies

Paul Martini (Ohio State Univ.) Eunhyeuk Kim (Harvard-Smithsonian Center for Astrophysics), Dan Kelson, John Mulchaey (OCIW),

Chandra has revealed significant point source overdensities in the fields of many clusters of galaxies. We have spectroscopically observed eight clusters of galaxies at $z < 0.3$ and identified 40 cluster members with broad-band X-ray luminosities between $L_X = 8 \times
10^{40}$ and $4 \times 10^{43}$ erg/s and host galaxies more luminous than $M_R < -20$ mag. These cluster members make a sufficient contribution to the point source surface density to explain the higher densities observed in cluster fields. We compare the X-ray and visible-wavelength properties of these galaxies to observational datasets of galaxies with various X-ray emission mechanisms and conclude that most of these sources are AGN, although the X-ray emission from as many as a quarter of the least luminous sources may be due to other mechanisms, most notably LMXBs. This conclusion is in spite of the observation that only three of these 40 galaxies have obvious AGN signatures in our visible-wavelength spectra. We ascribe this significant difference between visible and X-ray identification to both obscuration of the visible-wavelength spectral signatures and their dilution by host galaxy emission.

Chandra Observations of Distant Galaxy Clusters

Ben Maughan, Christine Jones, Bill Forman (SAO), Harald Ebeling (IfA, Hawaii), Leon Van Speybroeck (SAO)

We present the results of an analysis of 67 clusters of galaxies in the redshift range 0.1-0.9 (median=0.4). We give an overview of the sample, which includes a diverse array of galaxy clusters and discuss some of the interesting individual systems. The statistical properties of the sample are then investigated with an emphasis the scaling relations between cluster properties.

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X-ray Properties of a Mass-Selected Group Catalog

P. Mazzotta (SAO), R. Bower, M. Balogh, T. Ponman, T. Theuns, A. Edge, V. Eke, H. Bohringer, C. Collins, M. Colless, S. Maddox, J. Peacock

The observed X-ray luminosities of groups are inconsistent with a model in which the intragroup medium is shock-heated during the collapse. It is thought that a combination of pre-heating, gas cooling and energy injection removes low entropy gas, reducing the system's X-ray luminosity. However, the extent of this process is uncertain because the previous selection of group catalogs has been based on X-ray emission. To address this issue we have constructed a complete, mass-selected catalog of 18 groups from the 2dFGRS that we proposed for observation with both Chandra and XMM-Newton. To date twelve of these groups have been observed and here we present some preliminary results.

Centroid Offsets in Simulated Clusters of Galaxies

Patrick Motl (Louisiana State Univ.), Michael Norman (Univ. of California, San Diego), Jack Burns, Eric Hallman (Univ. of Colorado)

Clusters of Galaxies are frequently used as cosmological probes and have been studied observationally through a variety of means; including the thermal Sunyaev-Zeldovich effect, through their X-ray emission as well as by weak gravitational lensing. These techniques sample, respectively, the projected thermal pressure of the ICM, the projected X-ray emissivity and the total surface density of the cluster material. In as much as clusters are dynamic systems, it is not clear that all three of these signals will have coincident maxima for a given cluster. Here, we investigate the frequency of centroid offsets (between thermal SZE, X-ray and total mass images) in a large catalog of simulated clusters of galaxies. We also investigate the role that centroid offsets may play in assessing the kinematic state of observed clusters as well as their interaction history.


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Chandra and ROSAT Observations of the Hot X-ray Emitting Gas in NGC 5044 Group

Jelly Grace Nonesa (Hiroshima Univ., Univ. of Southern Mindanao), Yasushi Fukazawa, Takashi Ohsugi (Hiroshima Univ.)

We present a combined Chandra and ROSAT observations of the hot intragroup gas in one of the X-ray brightest galaxy groups, NGC 5044. A soft linear feature to the southeast has been observed in the X-ray image, indicating that the group has been slightly disturbed. Such feature is dramatically lying within the position of the prominent X-ray cavity, suggesting the existence of an active galactic nucleus (AGN). However, the absence of powerful radio emissions filling the cavities indicate a weak AGN. While within the inner central regions, the total mass profiles exhibit a bending feature, typical for groups containing a cD galaxy. Such feature have been observed in many clusters but the nature of such an interesting feature is not yet fully understood up to present. To investigate the origin of such feature, we performed various X-ray analyses using spatially resolved Chandra data. To constrain the mass distribution beyond the central regions, we simultaneously analyzed the Chandra data with the ROSAT data. With our joint analysis, we are able to constrain the dark matter distribution as well as many other important properties of this nearby group.

Dark Matter Profile in Low-Redshift Galaxy Groups

Jelly Grace Nonesa, Yasushi Fukazawa, Takashi Ohsugi (Hiroshima Univ.)

We combine spatially resolved Chandra data with the ROSAT data to constrain the dark matter distribution in five low-redshift galaxy groups. Within the inner central regions, the total mass profiles exhibit a bending feature, typical for groups containing a cD galaxy. However, the nature of such an interesting feature is not yet fully understood up to present. Following previous works of mostly rich galaxy clusters, we made a detailed X-ray analysis on five nearby groups to investigate the dark matter distribution and the possible origin of such a feature. In the baryonic mass, the stars are dominant within the inner central regions. As such, we infer that this bending feature may be partly caused by the scale difference between stars and dark matter, not by dark matter alone. Consequently, this feature also signifies the likely interface between the cD galaxy and the surrounding group of galaxies. For these groups, the interface radius is found to be in the range 7-15 kpc from the peak of the X-ray emission. The total mass internal to each interface radius is found to be around 10$^{10}$-10$^{11}$ $M_{\odot}$. Beyond this radius, the total mass profiles are apparently becoming DM-dominated. An NFW fit to the DM mass profiles yield results within reasonable scatter expected for CDM halos. While a power-law fit to the DM profiles give slopes comparable to that of rich galaxy clusters but is significantly larger than that of LSBs and self-interacting dark matter halos.

The Dynamical State of Abell 1689

En-Hsin Peng, John Arabadjis, Eric Miller, Marshall Bautz (MIT)

We present projected temperature, electron density, plasma pressure, and entropy maps of the galaxy cluster Abell 1689 derived from Chandra ACIS observations using three different techniques. The morphology of the pressure and entropy maps suggests that the overall dynamical state of the cluster is consistent with hydrostatic equilibrium, although we confirm the presence of a hot substructure about 100 kpc (projected) from the cluster center. We provide an estimate its effect on the measurement of the mass profile.

Constraining the Dark Matter Self-Interaction Cross-Section with Numerical Simulations of E1 0657-56

Scott Randall, Maxim Markevitch (Harvard-Smithsonian Center for Astrophysics)

We present results for constraining the self-interaction cross-section of dark matter by comparing X-ray and weak-lensing observations of the galaxy cluster 1E 0657-56 (the so-called bullet cluster) with results from N-body/hydrodynamical simulations. This cluster shows a high-velocity merger in the plane of the sky with a prominent bow shock that gives a subcluster velocity of roughly 4800 km s-1. A comparison of the X-ray image and weak-lensing mass map shows that the subcluster gas core lags the dark matter clump, which is coincident with the subcluster galaxies, indicating that the dark matter is not fluid-like. These observations allow for three independent methods for estimating the self-interaction cross-section of dark matter. Analytic estimates based on these methods have previously been determined. This work seeks to put tighter constraints on these results using a N-body/hydrodynamical code that is capable of simultaneously tracking gas particles and collisional dark matter particles. In this poster we present results from simulations with self-interacting dark matter and galaxies only.

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The Megasecond Chandra Observation of the Perseus Cluster

Jeremy Sanders, Andrew Fabian (Univ. of Cambridge), Gregory Taylor (Univ. of New Mexico), Steven Allen (Stanford Univ.), Carolin Crawford, Roderick Johnstone, Kazushi Iwasawa (Univ. of Cambridge)

We discuss the latest results from the Chandra megasecond observation of the Perseus cluster. This very deep observation of the X-ray brightest cluster in the sky reveals a wealth of structure in its core. We examine, by the use of extremely detailed temperature and pressure maps, the interaction of the radio source with the cluster. We show the evidence for the existence of further ancient radio bubbles in the cluster, highlighting the activity of this central source in the past. In addition we present results indicating the presence of X-ray cool gas in the central regions.

Constraining the WHIM Mass Using XMM-Newton and Chandra Observations of 1ES1028+511

Katrien Steenbrugge, Martin Elvis, Fabrizio Nicastro (Harvard-Smithsonian Center for Astrophysics)

We report the discovery of redshifted Warm Hot Intergalactic Medium gas along the line of sight toward the medium redshift blazar 1ES1028+511. The spectra show absorption from one high column density, moderately ionized filament. The measured column density from this absorber and previous detected WHIM gas toward Mrk 421, allows us to better constrain the cosmological mass density. The calculated mass density compare well with the theoretical predicted value as well as the the number of missing baryons in the local universe, i.e. z $<$ 2.

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Radio Galaxy Heating of Cooling Flow Clusters: Problems with Pure Hydrodynamic Models

John Vernaleo, Christopher Reynolds (Department of Astronomy, Univ. of Maryland)

Rich relaxed, clusters of galaxies cool, preferentially in the inner regions. Observationally however, there are strong limits to the amount of gas that could have cooled. Some form of heating (or some way to suppress cooling) is needed to reconcile these two observational facts. Energetic AGN are often viewed as a way to provide this heating. Most work indicates that they can provide enough energy to offset cooling. We have done a set of high resolution, three dimensional simulations that show that in the case of purely hydrodynamical AGN jets, although there is enough energy present, it is not spatially deposited in a way that can prevent catastrophic cooling of the cluster. We conclude that either some other physics (e.g., plasma transport processes or cosmic ray heating) is relevant for thermalizing the AGN energy output, or the role of AGN heating has been overestimated.

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