Abell 754: A Non–Head-on Collision of Subclusters

Abstract

We have analyzed spatially resolved spectra of the A754 cluster of galaxies obtained with ASCA. Through earlier observations with HEAO-1, Einstein, and ROSAT as well as optical studies, A754 has been established as the prototype system for a merger in progress. The combination of spectral and spatial resolution over a broad energy band provided by ASCA has set unprecedented constraints on the hydrodynamical effects of a cluster merger. We find significant gas temperature variations over the cluster face, indicating shock heating of the atmosphere during the merger. The hottest region, > 12 keV (90% confidence), is located in the region of the Northwest Galaxy clump though the entire region along the cluster axis appears to be hotter than the mean cluster temperature (∼9 keV). The cool, ≤5 keV, gas originally found with the HEAO1-A2 experiment, resides in the exterior of the cluster atmosphere and in plume of gas we identify with a stripped cool atmosphere of the infalling subcluster. We have also attempted to reconstruct an iron abundance map of this merging system. Though poorly constrained, no significant deviations of abundance from the mean value are apparent in the individual regions.A754 is the only cluster so far which shows the significant temperature pattern expected in a subcluster merger, in both the ROSAT (Henry & Briel 1995) and ASCA data, providing the first possibility to compare it with theoretical predictions. The cluster does not feature a hot peak accompanied by two hot lobes perpendicular to the cluster axis, predicted by hydrodynamic simulations of a head-on merger. The observed temperature and surface brightness maps suggest that the two colliding subunits have missed each other by about 1 Mpc, and are now moving perpendicular to the cluster axis in the image plane (as, e.g., in the simulations by Evrard et al. 1996).

Comparisons of Cluster Mass Determinations by X-Ray Observations and Gravitational Lensing

“…The above conclusions are strongly supported by the spatially resolved spectra for some clusters of galaxies obtained with ASCA, Ginga, and ROSAT, which show the complex temperature patterns over the cluster faces (see Henry & Briel 1995;Markevitch 1996;Henriksen & Markevitch 1996;Henriksen & White 1996). These significant temperature varia- tions cannot be described by a simple analytic profile like a ␤-model, and nonisothermality in the hot X-ray-emitting gas of clusters of galaxies is apparently required.…”

“…Indeed, the existence of substructures and the recent detection of the complex two-dimensional temperature patterns in clusters of galaxies (see, e.g., Henry & Briel 1995;Markevitch 1996;Henriksen & Markevitch 1996;Henriksen & White 1996) strongly suggest that clusters of galaxies may not be the well-virialized dynamical systems as previously believed and that the uncertainty in cluster mass determinations assuming hydrostatic equilibrium for the X-ray gas may be quite large (Balland & Blanchard 1996). Therefore, it is desirable that another independent cluster mass estimate is made to test the accuracy of the X-ray cluster mass determinations and, furthermore, to reexamine whether there is a baryon overdensity in clusters of galaxies.…”

Detection of a Temperature Structure in the Coma Cluster of Galaxies with [ITAL]ASCA[/ITAL]

“…In Abell 2163, a sharp drop of the temperature is detected at 1400 kpc from the center, and it suggests that the electron and ion temperatures are not reaching equilibrium in the outer region of the cluster (Markevitch et al 1994. A systematic drop of the temperature in the outer region of the cluster is seen in other systems , but Abell 754 exhibits an asymmetric temperature structure (Henry & Briel 1995;Henriksen & Markevitch 1996). The variation of temperature in the Coma Cluster occurs in a complicated way.…”

On the Origin of Radio Halos in Galaxy Clusters

“…These clusters have similar structure in their X-ray temperature distributions where relatively cool gas exists within the central few hundred kiloparsecs, and hotter gas, consistent with shock heating, is located at larger radii (e.g., Henry & Briel 1995;Henriksen & Markevitch 1996;Markevitch, Sarazin, & Vikhlinin 1999;Markevitch et al 2001;Henriksen et al 2000;Sarazin 2001). The temperature maps of these systems imply mergers that have not disrupted the cores, and detailed hydrodynamical models confirm that the mergers in these systems are off-axis and must be in the very earliest stages (e.g., Roettiger, Stone, & Mushotzky 1998;Flores, Quintana, & Way 2000;Roettiger & Flores 2000).…”