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 ou t to the distance of the Virgo Cluster is important for the determinat ion of the Hubble: constant. The Virgo Cluster is thereby often used a s an important stepping stone. The first Hubble Space Telescope measur ement of the distance of a Virgo galaxy (M100) using Cepheid variables , by Freedman et al., provided a value of H-0 = 80 +/- 17 km s(-1). Th is measurement was preceded by a ground-based study of the Virgo spira l NGC 4571 by Pierce et al., which formally provided H-0 = 87 +/- 7 km s(-1). These determinations rely on the accuracy with which the posit ion of this observed spiral galaxy can be located with respect to the Virgo Cluster's center. This uncertainty introduces a major error in t he determination of the Hubble constant, together with the uncertainty in the adopted Virgo infall velocity of the Local Group. Here we prop ose the use of spiral galaxies that show clear signs of being stripped of their interstellar medium by the intracluster gas of the Virgo Clu ster as targets for Cepheid distance measurements. We show that the st ripping process and the knowledge of the intracluster gas distribution from ROSAT X-ray observations allow us to locate these galaxies with an at least 3 times higher precision with respect to M87 than in the c ase of other spirals like M100. The X-ray observations further imply t hat M87 is well centered within the densest part of the intracluster g as halo of the Virgo Cluster. This seems to imply that M87 is approxim ately at rest in the central core of the cluster. There remains the pr oblem, however, that there is a velocity difference between M87 and th e average velocity of the Virgo galaxies in the larger halo region aro und M87. This may be explained by the disturbance introduced by the ga laxy M86, which is falling into the cluster with high velocity from be hind. 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 account for t he velocity offset of M87 with respect to the velocity average of abou t 100-150 km s(-1). The combination of this information could enable u s to locate the two stripped spiral galaxies quite precisely:within th e Virgo Cluster as a whole and could greatly improve the determination of the Virgo Cluster's distance.