Ripping Apart at the Seams: The Network of Stripped Gas Surrounding M86

We present a new study of the Virgo Cluster galaxies M86, M84, NGC 4338, and NGC 4438 using a mosaic of five separate pointings with XMM-Newton. Our observations allow for robust measurements of the temperature and metallicity structure of each galaxy along with the entire ~ 1 degree region between these galaxies. When combined with multiwavelength observations, the data suggest that all four of these galaxies are undergoing ram pressure stripping by the Intracluster Medium (ICM). The manner in which the stripped gas trailing the galaxies interacts with the ICM, however, is observably distinct. Consistent with previous observations, M86 is observed to have a long tail of ~ 1 keV gas trailing to the north-west for distances of ~ 100-150 kpc. However, a new site of ~ 0.6 keV thermal emission is observed to span to the east of M86 in the direction of the disturbed spiral galaxy NGC 4438. This region is spatially coincident with filaments of H-alpha emission, likely originating in a recent collision between the two galaxies. We also resolve the thermodynamic structure of stripped ~ 0.6 keV gas to the south of M84, suggesting that this galaxy is undergoing both AGN feedback and ram pressure stripping simultaneously. These four sites of stripped X-ray gas demonstrate that the nature of ram pressure stripping can vary significantly from site to site.Comment: 10 pages, 5 figures. Please contact Steven Ehlert ([email protected]) for higher resolution figure

A very deep Chandra observation of Abell 1795: The Cold Front and Cooling Wake

We present a new analysis of very deep Chandra observations of the galaxy cluster Abell 1795. Utilizing nearly 750 ks of net ACIS imaging, we are able to resolve the thermodynamic structure of the Intracluster Medium (ICM) on length scales of ~ 1 kpc near the cool core. We find several previously unresolved structures, including a high pressure feature to the north of the BCG that appears to arise from the bulk motion of Abell 1795's cool core. To the south of the cool core, we find low temperature (~ 3 keV), diffuse ICM gas extending for distances of ~ 50 kpc spatially coincident with previously identified filaments of H-alpha emission. Gas at similar temperatures is also detected in adjacent regions without any H-alpha emission. The X-ray gas coincident with the H-alpha filament has been measured to be cooling spectroscopically at a rate of ~ 1 Solar Masses/ yr, consistent with measurements of the star formation rate in this region as inferred from UV observations, suggesting that the star formation in this filament as inferred by its H$\alpha$ and UV emission can trace its origin to the rapid cooling of dense, X-ray emitting gas. The H-alpha filament is not a unique site of cooler ICM, however, as ICM at similar temperatures and even higher metallicities not cospatial with H$\alpha$ emission is observed just to the west of the H-alpha filament, suggesting that it may have been uplifted by Abell 1795's central active galaxy. Further simulations of cool core sloshing and AGN feedback operating in concert with one another will be necessary to understand how such a dynamic cool core region may have originated and why the H-alpha emission is so localized with respect to the cool X-ray gas despite the evidence for a catastrophic cooling flow.Comment: 14 Pages, 10 Figures, Resubmitted to ApJ after first referee report, Higher Resolution Figures available upon reques

X-ray bright active galactic nuclei in massive galaxy clusters III: New insights into the triggering mechanisms of cluster AGN

We present the results of a new analysis of the X-ray selected Active Galactic Nuclei (AGN) population in the vicinity of 135 of the most massive galaxy clusters in the redshift range of 0.2 < z < 0.9 observed with Chandra. With a sample of more than 11,000 X-ray point sources, we are able to measure, for the first time, evidence for evolution in the cluster AGN population beyond the expected evolution of field AGN. Our analysis shows that overall number density of cluster AGN scales with the cluster mass as $\sim M_{500}^{-1.2}$. There is no evidence for the overall number density of cluster member X-ray AGN depending on the cluster redshift in a manner different than field AGN, nor there is any evidence that the spatial distribution of cluster AGN (given in units of the cluster overdensity radius r_500) strongly depends on the cluster mass or redshift. The $M^{-1.2 \pm 0.7}$ scaling relation we measure is consistent with theoretical predictions of the galaxy merger rate in clusters, which is expected to scale with the cluster velocity dispersion, $\sigma$, as $\sim \sigma^{-3}$ or $\sim M^{-1}$. This consistency suggests that AGN in clusters may be predominantly triggered by galaxy mergers, a result that is further corroborated by visual inspection of Hubble images for 23 spectroscopically confirmed cluster member AGN in our sample. A merger-driven scenario for the triggering of X-ray AGN is not strongly favored by studies of field galaxies, however, suggesting that different mechanisms may be primarily responsible for the triggering of cluster and field X-ray AGN.Comment: 21 Pages, 8 figures, 5 tables. Submitted to MNRAS. Comments are welcome, and please request Steven Ehlert for higher resolution figure

Probing the extreme realm of AGN feedback in the massive galaxy cluster, RX J1532.9+3021

We present a detailed Chandra, XMM-Newton, VLA and HST analysis of one of the strongest cool core clusters known, RX J1532.9+3021 (z=0.3613). Using new, deep 90 ks Chandra observations, we confirm the presence of a western X-ray cavity or bubble, and report on a newly discovered eastern X-ray cavity. The total mechanical power associated with these AGN-driven outflows is (22+/-9)*10^44 erg/s, and is sufficient to offset the cooling, indicating that AGN feedback still provides a viable solution to the cooling flow problem even in the strongest cool core clusters. Based on the distribution of the optical filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map, we suggest that the cluster harbours older outflows along the north to south direction. The jet of the central AGN is therefore either precessing or sloshing-induced motions have caused the outflows to change directions. There are also hints of an X-ray depression to the north aligned with the 325 MHz jet-like structure, which might represent the highest redshift ghost cavity discovered to date. We further find evidence of a cold front (r=65kpc) that coincides with the outermost edge of the western X-ray cavity and the edge of the radio mini-halo. The common location of the cold front with the edge of the radio mini-halo supports the idea that the latter originates from electrons being reaccelerated due to sloshing induced turbulence. Alternatively, its coexistence with the edge of the X-ray cavity may be due to cool gas being dragged out by the outburst. We confirm that the central AGN is highly sub-Eddington and conclude that a >10^10M_Sun or a rapidly spinning black hole is favoured to explain both the radiative-inefficiency of the AGN and the powerful X-ray cavities.Comment: Accepted for publication to ApJ (minor corrections), 16 pages, 16 figures, 5 tables. Full resolution at http://www.stanford.edu/~juliehl/M1532

Azimuthally Resolved X-Ray Spectroscopy to the Edge of the Perseus Cluster

We present the results from extensive, new observations of the Perseus Cluster of galaxies, obtained as a Suzaku Key Project. The 85 pointings analyzed span eight azimuthal directions out to 2 degrees = 2.6 Mpc, to and beyond the virial radius r_200 ~ 1.8 Mpc, offering the most detailed X-ray observation of the intracluster medium (ICM) at large radii in any cluster to date. The azimuthally averaged density profile for r>0.4r_200 is relatively flat, with a best-fit power-law index of 1.69+/-0.13 significantly smaller than expected from numerical simulations. The entropy profile in the outskirts lies systematically below the power-law behavior expected from large-scale structure formation models which include only the heating associated with gravitational collapse. The pressure profile beyond ~0.6r_200 shows an excess with respect to the best-fit model describing the SZ measurements for a sample of clusters observed with Planck. The inconsistency between the expected and measured density, entropy, and pressure profiles can be explained primarily by an overestimation of the density due to inhomogeneous gas distribution in the outskirts; there is no evidence for a bias in the temperature measurements within the virial radius. We find significant differences in thermodynamic properties of the ICM at large radii along the different arms. Along the cluster minor axis, we find a flattening of the entropy profiles outside ~0.6r_200, while along the major axis, the entropy rises all the way to the outskirts. Correspondingly, the inferred gas clumping factor is typically larger along the minor than along the major axis.Comment: submitted to MNRA

X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters II: The Fraction of Galaxies Hosting Active Nuclei

We present a measurement of the fraction of cluster galaxies hosting X-ray bright Active Galactic Nuclei (AGN) as a function of clustercentric distance scaled in units of $r_{500}$. Our analysis employs high quality Chandra X-ray and Subaru optical imaging for 42 massive X-ray selected galaxy cluster fields spanning the redshift range of $0.2 < z < 0.7$. In total, our study involves 176 AGN with bright ($R <23$) optical counterparts above a $0.5-8.0$ keV flux limit of $10^{-14} \rm{erg} \ \rm{cm}^{-2} \ \rm{s}^{-1}$. When excluding central dominant galaxies from the calculation, we measure a cluster-galaxy AGN fraction in the central regions of the clusters that is $\sim 3$ times lower that the field value. This fraction increases with clustercentric distance before becoming consistent with the field at $\sim 2.5 r_{500}$. Our data exhibit similar radial trends to those observed for star formation and optically selected AGN in cluster member galaxies, both of which are also suppressed near cluster centers to a comparable extent. These results strongly support the idea that X-ray AGN activity and strong star formation are linked through their common dependence on available reservoirs of cold gas.Comment: 9 Pages, 4 Figures, accepted for publication in MNRAS, please contact Steven Ehlert ([email protected]) with any querie

Multipartite Entanglement and Frustration

Some features of the global entanglement of a composed quantum system can be quantified in terms of the purity of a balanced bipartition, made up of half of its subsystems. For the given bipartition, purity can always be minimized by taking a suitable (pure) state. When many bipartitions are considered, the requirement that purity be minimal for all bipartitions can engender conflicts and frustration arises. This unearths an interesting link between frustration and multipartite entanglement, defined as the average purity over all (balanced) bipartitions.Comment: 15 pages, 7 figure

Extreme AGN Feedback and Cool Core Destruction in the X-ray Luminous Galaxy Cluster MACS J1931.8-2634

We report on a deep, multiwavelength study of the galaxy cluster MACS J1931.8-2634 using Chandra X-ray, Subaru optical, and VLA 1.4 GHz radio data. This cluster (z=0.352) harbors one of the most X-ray luminous cool cores yet discovered, with an equivalent mass cooling rate within the central 50 kpc is approximately 700 solar masses/yr. Unique features observed in the central core of MACSJ1931.8-2634 hint to a wealth of past activity that has greatly disrupted the original cool core. We observe a spiral of relatively cool, dense, X-ray emitting gas connected to the cool core, as well as highly elongated intracluster light (ICL) surrounding the cD galaxy. Extended radio emission is observed surrounding the central AGN, elongated in the east-west direction, spatially coincident with X-ray cavities. The power input required to inflate these `bubbles' is estimated from both the X-ray and radio emission to reside between 4 and 14e45 erg/s, putting it among the most powerful jets ever observed. This combination of a powerful AGN outburst and bulk motion of the cool core have resulted in two X-ray bright ridges to form to the north and south of the central AGN at a distance of approximately 25 kpc. The northern ridge has spectral characteristics typical of cool cores and is consistent with being a remnant of the cool core after it was disrupted by the AGN and bulk motions. It is also the site of H-alpha filaments and young stars. The X-ray spectroscopic cooling rate associated with this ridge is approximately 165 solar masses/yr, which agrees with the estimate of the star formation rate from broad-band optical imaging (170 solar masses/yr). MACS J1931.8-2634 appears to harbor one of most profoundly disrupted low entropy cores observed in a cluster, and offers new insights into the survivability of cool cores in the context of hierarchical structure formation.Comment: 19 pages, 15 figures, 5 tables. Accepted by MNRAS for publication September 30 201

X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters I: Number Counts and Spatial Distribution

We present an analysis of the X-ray bright point source population in 43 massive clusters of galaxies observed with the Chandra X-ray Observatory. We have constructed a catalog of 4210 rigorously selected X-ray point sources in these fields, which span a survey area of 4.2 square degrees. This catalog reveals a clear excess of sources when compared to deep blank-field surveys, which amounts to roughly 1 additional source per cluster, likely Active Galactic Nuclei (AGN) associated with the clusters. The excess sources are concentrated within the virial radii of the clusters, with the largest excess observed near the cluster centers. The average radial profile of the excess X-ray sources of the cluster are well described by a power law (N(r) ~ r^\beta) with an index of \beta ~ -0.5. An initial analysis using literature results on the mean profile of member galaxies in massive X-ray selected clusters indicates that the fraction of galaxies hosting X-ray AGN rises with increasing clustercentric radius, being approximately 5 to 10 times higher near the virial radius than in the central regions. This trend is qualitatively similar to that observed for star formation in cluster member galaxies.Comment: 18 Pages, 10 Figures, Submitted to MNRAS. Please contact Steven Ehlert ([email protected]) for higher resolution figures. Updated to reflect small changes requested by referee. This version has been accepted into MNRA

HESS-II reconstruction strategy and performance in the low-energy (20-150 GeV) domain

International audienceIn mid-2009 a notable upgrade of the H.E.S.S. telescope system will take place: a new telescope with a 600 m2 mirror area and very-high-resolution camera (0.07°) will be positioned at the centre of the present configuration, with the aim of lowering the threshold and enhance its sensitivity in the 100 GeV to several TeV energy range. HESS-II will permit the investigation of the lower energy gamma-ray spectra in various cosmic accelerators, giving information on the origin of the gamma-rays observed, and will detect AGNs with a redshift greater than 0.2 (being less affected by absorption by Extragalactic Background Light-EBL-in this energy range) and will search for new classes of very high energy gamma-ray emitters (pulsars, microquasars, GRB, and dark matter candidates)