arXiv subjects

W. Forman

Publications and source records attributed to W. Forman.

117 records · Page 7Linked to original sources

arXiv1998

The rich cluster of galaxies ABCG 85. III. Analyzing the ABCG 85/87/89 complex

We present a combined X-ray and optical analysis of the ABCG 85/87/89 complex of clusters of galaxies, based on the ROSAT PSPC image, optical photometric catalogues (Slezak et al. 1998), and an optical redshift catalogue (Durret et al. 1998). From this combined data set, we find striking alignments at all scales at PA$\simeq$160°. At small scales, the cD galaxy in ABCG 85 and the brightest galaxies in the cluster are aligned along this PA. At a larger scale, X-ray emission defines a comparable PA south-southeast of ABCG 85 towards ABCG 87, with a patchy X-ray structure very different from the regular shape of the optical galaxy distribution in ABCG 87. The galaxy velocities in the ABCG 87 region show the existence of subgroups, which all have an X-ray counterpart, and seem to be falling onto ABCG 85 along a filament almost perpendicular to the plane of the sky. To the west of ABCG 85, ABCG 89 appears as a significant galaxy density enhancement, but is barely detected at X-ray wavelengths. The galaxy velocities reveal that in fact this is not a cluster but two groups with very different velocities superimposed along the line of sight. These two groups appear to be located in intersecting sheets on opposite sides of a large bubble. These data and their interpretation reinforce the cosmological scenario in which matter, including galaxies, groups and gas, falls onto the cluster along a filament.

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arXiv1998

Evidence for Merging in the Centaurus Cluster

We present a two dimensional map of the gas temperature distribution in the Centaurus cluster, based on ASCA observations derived using a novel approach to account for the energy dependent point spread function. Along with a cool region, centered on NGC4696, asymmetric temperature variations of moderate amplitude are detected. The hottest region roughly coincides with the position of the second brightest galaxy NGC4709, known to be the dominant galaxy of one of the subgroups in the Centaurus cluster. ROSAT images show faint surface brightness emission also centered on this galaxy. The imaging and spectral results suggest that a subcluster, centered on NGC4709 is merging with the main cluster centered on NGC4696, in agreement with the earlier suggestion by Lucey et al. (1986) that these two systems are located at the same distance despite their different line-of-sight velocities.

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arXiv1998

The 6.4 keV Fluorescent Iron Line from Cluster Cooling Flows

For the inner region of the cooling flow (e.g. within the radius of $\sim$50--100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as $\sim 0.01$. Assuming that the cooling time in the inner region is few times shorter than the life time of the cluster, the Thomson depth of the accumulated cold gas can be higher accordingly (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4 keV iron fluorescent line, with an equivalent width proportional to $τ_T$. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than few $10^{23} cm^{-2}$. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and it's distribution) can be measured.

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arXiv1998

1E0657-56: A Contender for the Hottest Known Cluster of Galaxies

We identify the extended Einstein IPC X-ray source, 1E0657-56, with a previously unknown cluster of galaxies at a redshift of $z=0.296$. Optical CCD images show the presence of a gravitational arc in this cluster and galaxy spectra yield a cluster velocity dispersion of $1213^{+352}_{-191}$ km s$^{-1}$. X-ray data obtained with the ROSAT HRI and ASCA indicate that 1E0657-56 is a highly luminous cluster in which a merger of subclusters may be occurring. The temperature of the hot gas in 1E0657-56 is $\rm{kT}=17.4 \pm 2.5 keV$, which makes it an unusually hot cluster, with important cosmological implications.

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arXiv1997

Temperature Structure in Abell 1367

We study the temperature structure of the rich cluster of galaxies Abell 1367 using two independent methods of correcting for the ASCA PSF. The results of the two methods are in excellent agreement and give solid evidence of a strong and localized shock in the intracluster medium (ICM). Our analysis suggests that we are observing the merger of two subclusters. We find that the lower luminosity subcluster, located to the northwest, has a higher temperature of 4.2 +/- 0.3 keV, compared to the more luminous southeast subcluster, whose temperature is 3.2 +/- 0.1 keV. An analysis of the ROSAT surface brightness profiles of both subclusters is also presented. The data agree well with predictions from numerical simulations of mergers between subcluster sized masses, in particular, with early stages of the merger, prior to the first core passage.

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arXiv1997

An investigation of cooling flows and general cluster properties from an X-ray image deprojection analysis of 207 clusters of galaxies

We present an X-ray image deprojection analysis of Einstein Observatory imaging data on 207, clusters of galaxies. The resulting radial profiles for luminosity, temperature, and electron density variations are determined from the cluster surface-brightness profiles according to gravitational potential constraints from average Tx and Vopt observations. This enables us to determine cooling-flow and other cluster properties, such as baryon fractions, S-Z microwave decrements, and Thomson depths. We have compiled a catalogue of the detected cooling flows, and investigated their effects on general cluster properties. Self-consistent correlations between the cluster X-ray luminosity, temperature, and optical velocity-dispersion, are determined accounting for errors in both dimensions of the data. These fits indicate that the temperatures of clusters are isothermal and that they have spectral beta-values consistent with unity. We find that the X-ray Lx, Tx, and optical Vopt relations depend significantly on the cooling flow mass-deposition rate, through differences in the density profiles. Clusters of similar mass-deposition rate exhibit self-similar density profiles, with larger cooling flows showing higher central densities. This leads to scatter in the Lx related correlations within the Lx, Tx and Vopt plane. The segregation in density leads to dispersion in `half-light radii' and baryon fractions. The baryon fraction in the cores of cooling flow clusters appears to be higher, but all clusters appear to rise to a concordant value of greater than 10 percent at 1 Mpc. Thus, clusters are inconsistent with primordial nucleosynthesis baryon fraction, for a flat Universe, of 6 percent.

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arXiv1996

Another Collision for the Coma Cluster

We describe a wavelet transform analysis of the ROSAT PSPC images of the Coma cluster. On small scales, ~< 1', the wavelet analysis shows substructure dominated by two extended sources surrounding the two brightest cluster galaxies NGC 4874 and NGC 4889. On slightly larger scales, ~2', the wavelet analysis reveals a filament of X-ray emission originating near the cluster center, curving to the south and east for ~25' in the direction of the galaxy NGC 4911, and ending near the galaxy NGC 4921. These results extend earlier ROSAT observations and further indicate the complex nature of the cluster core. We consider two possible explanations for the production of the filamentary feature as arising from interactions of the main cluster with a merging group. The feature could arise from either ram pressure stripped gas or a dark matter perturbation of tidally stripped material.

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arXiv1996

A Comparison of the ISM in Ellipticals to the Hot Gas in a Dense Group from Combined ROSAT and ASCA Observations

We analyze diffuse X-ray emission from NGC4649, NGC5044 and NGC5846, combining data from two X-ray observatories, ROSAT and ASCA. With ASCA, we perform a detailed analysis of the X-ray emission which properly accounts for the ASCA PSF and also include a 3-dimensional source model. All three sources exhibit cooling flows in their centers. From the derived abundances of Si, S and Fe, we conclude that NGC5044 (a dense group of galaxies) was able to retain significant amounts of gas during an early galaxy wind phase. Fe abundance in NGC5846 (normal E galaxy) corresponds to current SNIa rate of ~0.2 of Tammann rate, while low Fe mass in NGC4649 (normal E in the Virgo cluster) could be an environmental effect.

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arXiv1995

Detection of the Angular Correlation of Faint X-ray Sources

We have analyzed a set of deep ROSAT observations with a total sky coverage of 40 square degrees to search for clustering of faint X-ray sources. Using the resulting catalog of discrete X-ray sources, we detect, for the first time in X-rays, a positive correlation on angular scales of 0.5'-10'. When corrected for a bias due to limited spatial resolution which amplifies the correlation, the observed angular correlation function agrees well with that expected from the spatial correlation of optically selected quasars, provided that they comprise an appreciable fraction (>= ~50%) of detected X-ray sources.

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