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R. Srinivasan, J. J. Mohr (Dept. of Astronomy, University of Illinois)
We present an analysis of the Chandra ACIS-I observation of Abell 1644. Previous studies of this cluster have indicated that A1644 is composed of two bright X-ray subclusters connected by a bridge of emission, suggesting that the two are in the process of merging. However, preliminary temperature maps of this cluster do not reveal the presence of any significantly hotter gas in the bridge, indicating that the two subclusters are likely in the early stages of merging.
The cores of the two subclusters have temperatures of about 1 and 2 keV. Analysis of earlier X-ray observations of the brighter of the two subclusters indicates the presence of a moderate radiative cooling instability. The intracluster gas outside the cores has a temperature of about 3 keV, even in the faint bridge.
Interestingly, we see a bow shaped surface brightness discontinuity associated with a bright knot of emission, which is located at a projected distance of ~30kpc from the emission peak of the main subcluster. There is a sharp temperature discontinuity across this feature of about 2 keV; the high surface brightness (high density) knot has the lower temperature, suggesting a much smaller pressure discontinuity. We are modeling this "contact discontinuity" or "shock feature", which appears to be similar to those found in other clusters, including Abell 3667 and Abell 2142. In general terms, the ACIS-I data suggest that a low mass subcluster has recently merged and penetrated to the core of the main subcluster, perhaps disrupting or moderating the central, radiative cooling instability.
The author(s) of this abstract have provided an email address for comments about the abstract: rsriniva@astro.uiuc.edu