The radial distribution of morphological types of galaxies in X-ray clusters

Abstract

Galaxies have been classified in six clusters which are known to emit X-rays. We find that in all cases the ratio of the surface density of spiral galaxies to that of SO's increases outward. Moreover, there is a correlation between the velocity dispersion of the galaxies in a cluster and the overall proportion of SO's among disk galaxies. These facts are interpreted as support for Gunn and Gott's hypothesis that SO's are produced from spirals by a process in which the interstellar gas in spirals is swept out as these galaxies move through the hot gas presumed to be responsible for the X-ray emission. A further test of this hypothesis is suggested. In addition, new redshift measurements are presented for galaxies in several X-ray clusters. Malina et al. 1976). In any case, in view of Gunn and Gott's suggestion regarding the origin of SO's from spiral galaxies in clusters, we thought it important to investigate whether the effect described by Baade and Spitzer also obtains in the X-ray clusters. Accordingly, we have obtained high-scale plates and have classified the galaxies in six of these objects as a function of radius. Also as part of this work we obtained new redshifts for galaxies in several clusters. We find that the ratio of spiral to SO galaxies increases outward in all of the X-ray clusters for which good data are available. We shall also show that it is possible to classify clusters according to the characteristic behavior of the radial distribution of morphological types. A more surprising result is that there is a good correlation between the velocity dispersion of a cluster and the overall proportion of SO galaxies. Our improved data show that the correlation between the X-ray luminosity of a cluster and its velocity dispersion is poor and cannot be used to distinguish between the two principal theories of the X-ray emission. Overall, our results are consistent with Gunn and Gott's explanation for the origin of SO's from spirals, although we have not thought of a convincing detailed explanation of the correlation between the proportion of SO's and the dispersion. II. OBSERVATIONS AND DATA d) Direct Photography Morphological types of galaxies brighter than ~ 16 have been determined for six clusters possibly associated with Uhuru X-ray sources (Kellogg et al. 1973). Good scale plates obtained with the 60 inch (1.5 m), f/8.75 telescope at Palomar Observatory , covering an area of about 1.5 deg 2 , were used for the clusters Abell 262, 376, 576, and 2666. The remaining two, Abell 426 and 1060, were classified on 48 inch (1.2 m) Schmidt plates. A summary of the 401