M ar 1 99 7 Stripped Spiral Galaxies as Promising Targets for the Determination of the Cepheid Distance to the Virgo Cluster

The measurement of precise galaxy distances by Cepheid observations out to the distance of the Virgo cluster is important for the determination of the Hubble constant. The Virgo cluster is thereby often used as an important stepping stone. The first HST measurement of the distance of a Virgo galaxy (M100) using Cepheid variables provided a value for H 0 = 80(±17) km s −1 (Freedman et al. 1994). This measurement was preceeded by a ground based study of the Virgo spiral NGC4571 (Pierce et al. 1994) formally providing H 0 = 87 ± 7 km s −1. These determinations rely on the accuracy with which the position of this observed spiral galaxy can be located with respect to the Virgo cluster center. This uncertainty introduces a major error in the determination of the Hubble constant, together with the uncertainty in the adopted Virgo infall velocity of the Local Group. Here we propose the use of spiral galaxies which show clear signs of being stripped off their interstellar medium by the intracluster gas of the Virgo cluster as targets for the Cepheid distance measurements. We show that the stripping process and the knowledge of the intracluster gas distribution from ROSAT X-ray observations allow us to locate these galaxies with an at least three times higher precision with respect to M87 than in the case of other spirals like M100. The X-ray observations further imply that M87 is well centered within the densest part of the intracluster gas halo of the Virgo cluster. This seems to imply that M87 is approximately at rest in the central core of the cluster. There remains the problem, however, that there is a velocity difference between M87 and the average velocity of the Virgo galaxies in the larger halo region around M87. This may be explained by the disturbance introduced by the galaxy M86, which is falling into the cluster with high velocity from behind. The X-ray and optical observations imply that M86 is associated with a group of galaxies and a larger dark matter halo with a mass of up to about 10% of the mass of the M87 halo, which would – 2 – account for the velocity offset of M87 with respect to the velocity average of about 100 to 150 km s −1. The combination of these informations could enable us to locate the two stripped spiral galaxies quite precisely within the …