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1. Introduction

The Virgo Cluster, being the dominant nearby cluster of galaxies, has long attracted much attention. It is irregular and exhibits significant substructure. Binggeli et al. (1987) divided Virgo into two main clusters: a dominant cluster, Cluster A, which is rich in early type galaxies and contains the giant elliptical NGC 4486 (otherwise known as M 87) and a secondary cluster, Cluster B, dominated by late spirals, in the direction of the giant elliptical NGC 4472 (M 49). Their Clusters A and B correspond to de Vaucouleurs's concentric E tex2html_wrap_inline3353 S and S' Clouds respectively (see e.g. de Vaucouleurs & Corwin 1986). The VPC covers the central region of Cluster A/Clouds E&S. It does not cover the central region of Cluster B/Cloud S', but there may of course still be many objects from this secondary cluster within the VPC survey area.

The equatorial coordinates of the centre of what is generally held to be the Virgo Cluster proper (i.e. of Cluster A or the E/S Clouds) are approximately tex2html_wrap_inline3355 in right ascension (1950.0) and tex2html_wrap_inline3357 in declination (1950.0). The entire complex including substructures, is generally thought of as subtending a radius of about 6tex2html_wrap_inline3359 on the celestial sphere, though its true extent has yet to be established.

Radial velocities of the constituent galaxies vary from the most negative ones known (and the only negative ones known for galaxies outside the neighbourhood of the Local Group) to a maximum of about +3000 km s-1, as is evident from Binggeli et al.'s (1985) Virgo Cluster Catalog (VCC). There appears to be a tex2html_wrap_inline33654000 km s-1-deep velocity void behind the cluster, as the background system is at +7000 km s-1 (see e.g. Drinkwater et al. 1996).

Most attempts to pin down the value of the Hubble constant have long been based on distance measurements for Virgo-Cluster galaxies. There are probably two main reasons for this. First, it is the nearest cluster in which a full complement of morphological types is represented: i.e. giant elliptical galaxies; classical ellipticals, spirals and irregulars; dwarf ellipticals and irregulars; blue compact dwarfs; galaxies in the process of merging and others that defy classification. It has therefore been possible to apply a wider variety of different distance indicators to Virgo galaxies than it has been for other clusters at similar redshifts such as Fornax or Leo. Second, Virgo has the added advantage of being easily observable from observatories in both the Northern and Southern hemispheres.

Virgo's popularity as a pivotal step in the cosmic distance scale has nevertheless not been without its difficulties and controversies. Until recently, the possibility that the spatial extent of the Virgo Cluster may have considerable line-of-sight depth was not taken seriously. Although Pierce & Tully (1988) suggested the "possible presence of superposed foreground and background galaxies'' in their sample of Virgo spirals, it was not until the distance-scale work of Tonry et al. (1990) and Tonry (1991) that any author was prepared to stand by the case for considerable depth. However, these authors did not confront the potentially embarrassing issue of the cluster's elongation ratio in the line of sight; which would have to exceed 5:1 in order to explain the depth in their derived galaxy distributions. Other authors have understandably tended to explain any apparent depth effect in Virgo galaxy distributions in terms of random observational errors and scatter intrinsic to the distance indicators employed.

The first authors not to avoid the elongation issue were Fukugita et al. (1993). These authors suggested that the Virgo Cluster's spiral-galaxy distribution is in fact filamentary in shape, rather than approximately spherically symmetric [as had been assumed before] with spirals lying at distances from 13 Mpc all the way through to 30 Mpc. In Young & Currie (1995) we also found a considerable depth effect, that we could not explain away in terms of errors or intrinsic scatter, this time in the distribution of dwarf-elliptical galaxies in the direction of the Virgo Cluster's core itself. Furthermore, we found preliminary evidence for a foreground concentration of galaxies several Mpc in front of the main cluster, which led us to suggest that the depth effect may be due to the projection of more than one distinct galaxy concentration onto the same sight line. This would avoid the need for elongation ratios of the order of 5:1.

This catalgoue has been compiled primarily with a view to investigating the Virgo Cluster's distance, three-dimensional structure and kinematics; but will also provide the basis for a pencil beam survey in the Virgo direction. We will also be investigating the luminosity functions of Virgo dwarfs, with the triple benefits of: galaxy samples selected according to objective criteria, independently calibrated machine generated magnitude measurements and the consideration of depth effects. These studies will be the subjects of future papers but a preliminary study of the spatial distribution of 64 Virgo dwarf-ellipticals has already been presented in Young & Currie (1995) and a comparison with the existing magnitude scales for Virgo galaxies will be presented by Young et al. (in preparation).


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