Newcomers Meet the Intracluster Medium in the Coma Cluster

A main topic at this meeting is how galaxies are affected when they enter for the first time the cluster environment from the outskirts. Most of the times we are forced to infer the environmental effects indirectly, relying on systematic variations of galaxy properties with environment, but there aren’t many examples of direct observations able to unveil ongoing transformations taking place, and the corresponding mechanism producing it. We present a case in which it is possible to identify the cluster environment, and in particular the intracluster medium and the recent infall history of galaxies onto the cluster, as the cause for a recent, abrupt change in the evolutionary history of galaxies. 1. Post-starburst galaxies and substructure in Coma This study is based on a photometric and spectroscopic survey of galaxies in the Coma cluster (Mobasher et al. 2001), which is distinctive from other surveys in three ways: for the large galaxy magnitude range covered (almost 7 mag, down to MB ∼ −14), for the large area surveyed (two regions, towards the cluster center and to the SouthWest, ∼ 1 × 1.5 Mpc each), and for being a simply magnitude limited sample, with no morphological selection criteria adopted for spectroscopy. In this survey we have found that a significant fraction (∼ 10 %) of the cluster dwarf galaxy population atMV > −18.5 has post-starburst/post-starforming spectra (Poggianti et al. 2004, hereafter P04). This type of spectrum (“k+a”, or “E+A”) indicates a galaxy with no current star formation activity which was forming stars at a vigorous rate in the recent past (last 1.5 Gyr). In the B − R color-magnitude diagram, a group of blue and a group of red k+a’s can be easily distinguished in Coma. The average EW(Hδ) of the blue group is significantly stronger than that of the red group. The blue, strong k+a’s most likely correspond to “young” k+a’s (observed soon after the termination of star formation, < 300 Myr) and the red, weaker k+a’s are “old” ones (observed at a later stage of the evolution, 0.5-1.5 Gyr). A suggestive clue about the possible physical mechanism responsible for the k+a spectra comes from the recent X-ray mosaic observations of Coma obtained with XMMNewton. Coma has two central dominant galaxies, NGC 4874 (a cD galaxy) and NGC4889 (a very bright elliptical), and another cD galaxy, NGC4839, that dominates a substructure South-West of the center (Fig. 1). Neumann et al. (2003) have identified and dis-