Jet induced star formation in centrally dominant galaxies?

Using U-I CCD color maps of two centrally dominant cluster galaxies, we find unusual color structures which may be due to star formation which has been induced by their radio sources. These objects, located in the clusters A1795 and A2597, have blue central colors to radii of 20 kpc, spatially extended emission-line structures, and powerful radio sources. They reside at the centers of cooling flows with mass-accretion rates which are estimated to be approximately greater than 300 solar mass/yr. The regions of bluest local color are superposed on or along their radio-source structures. Our observations suggest that the radio sources associated with these objects may be inducing massive star formation in their central 20 kpc. The star formation may be the result of the radio plasma interacting with the warm emission-line gas and dense, x-ray emitting filaments similar to those recently discovered in two other clusters with the ROSAT Observatory. Since radio jets are likely to be transient, this may help to explain the scatter in the correlations between color and mass-accretion rate, although other factors may also contribute. Alternatively, scattered radiation from a hidden active nucleus or recent mergers may be responsible for the color structure. The color and radio properties of these objects are qualitatively similar but smaller in luminosity and spatial extent to those found in high redshift radio galaxies. Our observations of galaxies at z approximately = 0.06-0.1 show that processes similar to 'the alignment effect' found in high redshift radio galaxies occur at more recent epochs

The mass-L_x relation for moderate luminosity X-ray clusters

We present measurements of the masses of a sample of 25 moderate X-ray luminosity clusters of galaxies from the 160 square degree ROSAT survey. The masses were obtained from a weak lensing analysis of deep F814W images obtained using the Advanced Camera for Surveys (ACS). We present an accurate empirical correction for the effect of charge transfer (in)efficiency on the shapes of faint galaxies. A significant lensing signal is detected around most of the clusters. The lensing mass correlates tightly with the cluster richness. We measured the intrinsic scatter in the scaling relation between M_2500 and L_X and find the best fit power law slope and normalisation to be alpha=0.68+-0.07 and M_X=(1.2+-0.12)10^14M_sun (for L_X=2x10^44 erg/s). These results agree well with a number of recent studies, but the normalisation is lower compared to the study of Rykoff et al. (2008b). One explanation for this difference may be the fact that (sub)structures projected along the line-of-sight boost both the galaxy counts and the lensing mass. Such superpositions lead to an increased mass at a given L_X when clusters are binned by richness.Comment: accepted for publication in the Astrophysical Journal; 15 pages, 11 figure

The Insignificance of Global Reheating in the Abell 1068 Cluster: X-Ray Analysis

We report on a Chandra observation of the massive, medium redshift (z=0.1386) cooling flow cluster Abell 1068. We detect a clear temperature gradient in the X-ray emitting gas from kT ~ 5 keV in the outer part of the cluster down to roughly 2 keV in the core, and a striking increase in the metallicity of the gas toward the cluster center. The total spectrum from the cluster can be fit by a cooling flow model with a total mass deposition rate of 150 solar masses/yr. Within the core (r < 30 kpc), the mass depositon rate of 40 solar masses/yr is comparable to estimates for the star formation rate from optical data. We find an apparent correlation between the cD galaxy's optical isophotes and enhanced metallicity isocontours in the central ~100 kpc of the cluster. We show that the approximate doubling of the metallicity associated with the cD can be plausibly explained by supernova explosions associated with the cD's ambient stellar population and the recent starburst. Finally, we calculate the amount of heating due to thermal conduction and show that this process is unlikely to offset cooling in Abell 1068.Comment: Accepted for publication in ApJ, 26 pages, 12 b+w figures, 3 color figure

Evidence for AGN Feedback in Galaxy Clusters and Groups

The current generation of flagship X-ray missions, Chandra and XMM-Newton, has changed our understanding of the so-called "cool core" galaxy clusters and groups. Instead of the initial idea that the thermal gas is cooling and flowing toward the center, the new picture envisages a complex dynamical evolution of the intra-cluster medium (ICM) regulated by the radiative cooling and the nongravitational heating from the active galactic nucleus (AGN). Understanding the physics of the hot gas and its interplay with the relativistic plasma ejected by the AGN is key for understanding the growth and evolution of galaxies and their central black holes, the history of star formation, and the formation of large-scale structures. It has thus become clear that the feedback from the central black hole must be taken into account in any model of galaxy evolution. In this paper, we draw a qualitative picture of the current knowledge of the effects of the AGN feedback on the ICM by summarizing the recent results in this field.Comment: Accepted for publication in Advances in Astronomy, 30 pages, 6 figures. Tutorial Review to appear in the Special Issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

Chandra Observation of the Radio Source / X-ray Gas Interaction in the Cooling Flow Cluster Abell 2052

We present a Chandra observation of Abell 2052, a cooling flow cluster with a central cD that hosts the complex radio source 3C 317. The data reveal ``holes'' in the X-ray emission that are coincident with the radio lobes. The holes are surrounded by bright ``shells'' of X-ray emission. The data are consistent with the radio source displacing and compressing, and at the same time being confined by, the X-ray gas. The compression of the X-ray shells appears to have been relatively gentle and, at most, slightly transonic. The pressure in the X-ray gas (the shells and surrounding cooler gas) is approximately an order of magnitude higher than the minimum pressure derived for the radio source, suggesting that an additional source of pressure is needed to support the radio plasma. The compression of the X-ray shells has speeded up the cooling of the shells, and optical emission line filaments are found coincident with the brightest regions of the shells.Comment: accepted for publication in ApJ Letters; for high-resolution color figures, see http://www.astro.virginia.edu/~elb6n/abell2052.htm

Abell 1201: a Minor merger at second core passage

We present an analysis of the structures and dynamics of the merging cluster Abell~1201, which has two sloshing cold fronts around a cooling core, and an offset gas core approximately 500kpc northwest of the center. New Chandra and XMM-Newton data reveal a region of enhanced brightness east of the offset core, with breaks in surface brightness along its boundary to the north and east. This is interpreted as a tail of gas stripped from the offset core. Gas in the offset core and the tail is distinguished from other gas at the same distance from the cluster center chiefly by having higher density, hence lower entropy. In addition, the offset core shows marginally lower temperature and metallicity than the surrounding area. The metallicity in the cool core is high and there is an abrupt drop in metallicity across the southern cold front. We interpret the observed properties of the system, including the placement of the cold fronts, the offset core and its tail in terms of a simple merger scenario. The offset core is the remnant of a merging subcluster, which first passed pericenter southeast of the center of the primary cluster and is now close to its second pericenter passage, moving at ~1000 km/s. Sloshing excited by the merger gave rise to the two cold fronts and the disposition of the cold fronts reveals that we view the merger from close to the plane of the orbit of the offset core.Comment: accepted by Ap

A 15-Kiloparsec X-Ray Disk in the Elliptical Galaxy NGC 1700

We present Chandra observations of the young elliptical galaxy NGC 1700. The X-ray isophotes are highly flattened between semimajor axes of 30 and 80 arcseconds, reaching a maximum ellipticity of approximately 0.65 at 60 arcsec (15 kpc). The surface brightness profile in the spectrally soft, flattened region is shallower than that of the starlight, indicating that the emission comes from hot gas rather than stellar sources. The flattening is so extreme that the gas cannot be in hydrostatic equilibrium in any plausible potential. A likely alternative is that the gas has significant rotational support. A simple model, representing isothermal gas distributed about a particular angular momentum, can reproduce the X-ray morphology while staying consistent with stellar kinematics. The specific angular momentum of the gas matches that of the stars in the most isophotally distorted outer part of the galaxy, and its cooling time matches the time since the last major merger. We infer that the gas was acquired in that merger, which involved a pre-existing elliptical galaxy with a hot ISM. The hot gas carried the angular momentum of the encounter, and has since gradually settled into a rotationally flattened, cooling disk.Comment: 11 pages, 2 figures, AASTeX 5.0. Accepted for publication in The Astrophysical Journa