The Distance and 3D-Structure of Virgo

A clear understanding of the 3D-structure of the nearby Virgo cluster and the knowledge of a precise cluster distance are key issues for many astronomical fields. However, these properties remain highly unknown. Concerning the cluster distance this is exemplified by the controversial results from two Hubble Space Telescope (HST) key projects. Employing Cepheids and Supernovae Ia as distance indicators the mean cluster distance is measured between 16 Mpc (eg. Graham et al. 1998) and 25 Mpc (eg. Saha et al. 1997), respectively. The reason for the apparent discrepancy lies in the nature of spiral galaxies, which are the typical target galaxies for these HST studies. Spirals reside in the outskirts of a cluster (the morphology-density relation for giant galaxies, Dressler 1980). Thus the observed distance range likely reflects the line-of-sigh depth of the large spiral halo of the Virgo cluster. This shows why spirals are only of limited use to nail down the gravitational centre of the Virgo cluster.

From the scientific point of view the best choice for a study of the distance and 3D-structure of the Virgo cluster are the low-surface brightness dwarf elliptical (dE) galaxies. Firstly, their existence is well confined to the highest galaxy densities in the cluster (the morphology-density relation for dwarf galaxies, Binggeli et al. 1987). Secondly, the dwarf ellipticals are the only galaxies which exist in large numbers in the Virgo cluster core (Binggeli et al. 1985). A well selected sample of dEs offers a homogeneous set of objects required for a detailed cluster core study. Our research team is using one of the most modern ground based telescopes in the world, the Very Large Telescope (VLT) of the European Southern Observatory to measure accurate distances to a large number of Virgo dE galaxies by means of the Surface Brightness Fluctuations method.