Measuring the temperature of the intrasupercluster medium

Diffuse gas may exist in superclusters over scales of several Mpc because likely only a small fraction of the primordial gas was converted into bound objects during structure formation. We present the results of our investigation on the intrasupercluster medium (ISCM) within the Shapley supercluster using multi-X-ray-colour images obtained from the ROSAT all sky survey (RASS) data archive and the digitised galaxy catalogue scanned by the MAMA machine at the Observatory of Paris. An upper limit on large-scale (over 100 square degree) diffuse thermal emission from the Shapley supercluster is set after most contaminations are removed from the RASS data. X-ray counts contributed from resolved sources, including extended and point sources, are all rejected; the extragalactic X-ray background is estimated from RASS data of the Lockman Hole region and subtracted from the supercluster data. It is followed by an X-ray colour analysis (hardness ratios of RASS bands) of the residual emission above 1 keV (to avoid diffuse Galactic contaminations) to estimate the temperature range of the intrasupercluster gas. After correcting the Galactic ISM absorption across this large region, we estimate that the X-ray luminosity of the ISCM within the 6 degree radius region centered at the Shapley core can be comparable to that of the total contribution from rich clusters in the same region, and the derived temperature is lower than that of average intracluster medium. With combined results of the rather low temperature ISCM and a morphology analysis of galaxy groups/clusters detected in the MAMA catalogue, the dynamical status of the Shapley cluster is discussed.