Morphology of the Virgo Cluster: Gas versus Galaxies

Abstract

We draw a quantitative comparison of the distribution of the galaxies and the intra-cluster gas in the Virgo cluster by extending the morphological analysis by Binggeli et al. (1987) to the intra-cluster gas. We use the Virgo Cluster Catalog in combination with data from the ROSAT All-Sky Survey. The galaxy distribution and the gas distribution are relatively similar. In both wavebands the irregular structure of Virgo can be decomposed into three major subclusters centred on M87, M49, and M86. A new statistical method of subcluster decomposition is applied. Radial galaxy and X-ray density profiles of the three subclusters are fitted with beta-models, allowing analytic deprojection. Comparison of these profiles reveals, that there is no general luminosity segregation. The gas density profile is steeper than the galaxy density profile in the inner part, while this trend is reversed in the outer part. A comparison among the subclusters shows the poorer the subcluster the steeper its radial profile, i.e. the more compact it is, both in the optical and the X-rays. The subcluster profiles for different Hubble types confirm the well-known result that the distribution of the late-type galaxies is more extended than the early types. Differential and integrated mass density profiles of the different components in the M87 and M49 subclusters are presented. The total, gravitating mass is inferred from the distribution of the X-ray gas. In the M87 subcluster the gas mass is about three times the mass in galaxies, while it accounts only for 8% to 14% of the total mass at 0.4 and 1 Mpc, respectively. In the M49 subcluster there is more mass in the galaxies than in the gas and the gas-to-total mass fraction is only 1%, which is unusually low for a cluster. The mass-to-light ratios show relatively constant values around 500Msol/Lsol.