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Register a new userCold fronts and metal anisotropies in the X-ray cool core of the galaxy cluster Zw1742+3306
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(Context) In recent years, our understanding of the cool cores of galaxy clusters has changed. Once thought to be relatively simple places where gas cools and flows toward the centre, now they are believed to be very dynamic places where heating from the central Active Galactic Nucleus (AGN) and cooling, as inferred from active star formation, molecular gas, and Halpha nebulosity, find an uneasy energetic balance. (Aims) We want to characterize the X-ray properties of the nearby cool-core cluster Zw1742+3306, selected because it is bright at X-ray (with a flux greater than 1e-11 erg/s/cm2 in the 0.1-2.4 keV band) and Halpha wavelengths (Halpha luminosity > 1e40 erg/s). (Methods) We used Chandra data to analyze the spatial and spectral properties of the cool core of Zw1742+3306, a galaxy cluster at z=0.0757 that emits in Halpha and presents the brightest central galaxy located in a diffuse X-ray emission with multiple peaks in surface brightness. (Results) We show that the X-ray cool core of the galaxy cluster Zw1742+3306 is thermodynamically very active with evidence of cold fronts and a weak shock in the surface brightness map and of an apparently coherent, elongated structure with metallicity greater than the value measured in the surrounding ambient gas by about 50 per cent. This anisotropic structure is 280 x 90 kpc2 and is aligned with the cold fronts and with the X-ray emission on larger scales. We suggest that all these peculiarities in the X-ray emission of Zw1742+3306 are either a very fine-tuned output of a sloshing gas in the cluster core or the product of a metal-rich outflow from the central AGN.
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We present a new Chandra X-ray observation of the off-axis galaxy group merger RXJ0751.3+5012. The hot atmospheres of the two colliding groups appear highly distorted by the merger. The images reveal arc-like cold fronts around each group core, produced by the motion through the ambient medium, and the first detection of a group merger shock front. We detect a clear density and temperature jump associated with a bow shock of Mach number M=1.9+/-0.4 ahead of the northern group. Using galaxy redshifts and the shock velocity of 1100+/-300 km/s, we estimate that the merger axis is only 10deg from the plane of the sky. From the projected group separation of 90 kpc, this corresponds to a time since closest approach of 0.1 Gyr. The northern group hosts a dense, cool core with a ram pressure stripped tail of gas extending 100 kpc. The sheared sides of this tail appear distorted and broadened by Kelvin-Helmholtz instabilities. We use the presence of this substructure to place an upper limit on the magnetic field strength and, for Spitzer-like viscosity, show that the development of these structures is consistent with the critical perturbation length above which instabilities can grow in the intragroup medium. The northern group core also hosts a galaxy pair, UGC4052, with a surrounding IR and near-UV ring 40 kpc in diameter. The ring may have been produced by tidal stripping of a smaller galaxy by UGC4052 or it may be a collisional ring generated by a close encounter between the two large galaxies.
Cold fronts have been detected both in merging and in cool core clusters, where little or no sign of a merging event is present. A systematic search of sharp surface brightness discontinuities performed on a sample of 62 galaxy clusters observed with XMM-Newton shows that cold fronts are a common feature in galaxy clusters. Indeed most (if not all) of the nearby clusters (z < 0.04) host a cold front. Understanding the origin and the nature of a such frequent phenomenon is clearly important. To gain insight on the nature of cold fronts in cool core clusters we have undertaken a systematic study of all contact discontinuities detected in our sample, measuring surface brightness, temperature and when possible abundance profiles across the fronts. We measure the Mach numbers for the cold fronts finding values which range from 0.2 to 0.9; we also detect a discontinuities in the metal profile of some clusters.
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.
We investigate whether the swirling cold front in the core of the Perseus Cluster of galaxies has affected the outer buoyant bubbles that originated from jets from the Active Galactic Nucleus in the central galaxy NGC1275. The inner bubbles and the Outer Southern bubble lie along a North-South axis through the nucleus, whereas the Outer Northern bubble appears rotated about 45 deg from that axis. Detailed numerical simulations of the interaction indicates that the Outer Northern bubble may have been pushed clockwise accounting for its current location. Given the common occurrence of cold fronts in cool core clusters, we raise the possibility that the lack of many clear outer bubbles in such environments may be due to their disruption by cold fronts.
We present a two-dimensional analysis of the bright nearby galaxy group NGC 5044 using the currently available Chandra and XMM data. In the inner 10 kpc a pair of cavities are evident together with a set of bright X-ray filaments. If the cavities are interpreted as gas displaced by relativistic plasma inflated by an AGN, even in the absence of extended 1.4 GHz emission, this would be consistent with a recent outburst as also indicated by the extent of dust and H_alpha emission. The soft X-ray filaments coincident with H_alpha and dust emission are cooler than the ones which do not correlate with optical and infrared emission. We suggest that dust-aided cooling contributes to form warm (T =10^4 K) gas, emitting H_alpha radiation. At 31 kpc and 67 kpc a pair of cold fronts are present, indicative of sloshing due to a dynamical perturbation caused by accretion of a less massive group, also suggested by the peculiar velocity of the brightest galaxy NGC 5044 with respect to the mean group velocity.
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