Blue compact dwarf galaxies and new velocities in Virgo

We present new spectral observations of 303 galaxies brighter than BJ = 17.6 in the central 30 deg2 of the Virgo cluster field. The galaxies were selected from two overlapping samples designed for a search for blue compact dwarf (BCD) galaxies and for a more general study of Virgo dwarf elliptical (dE) galaxies. Many of these galaxies were designated ‘background’ (i.e., not members of the Virgo cluster) on morphological grounds by Binggeli, Sandage & Tammann in their Virgo Cluster Catalog (VCC). Our principal aim was to measure radial velocities of these galaxies in order to determine which were members of the cluster, looking in particular for any cluster members previously misclassified as background galaxies. We measured reliable velocities for 291 galaxies, of which nine were found to be cluster members. Five of these were listed in the VCC as ‘members’ and four as ‘possible members’. We also confirmed that 10 VCC ‘background’ galaxies were correctly identified, and determined that an additional three VCC ‘possible members’ were background galaxies. These results show that the VCC membership estimates were generally correct. Our sample of candidate BCD galaxies was defined by objective criteria from digitized plate material and therefore allows us to place new limits on the dwarf-galaxy population. We confirm the drop in the BCD luminosity function at an absolute magnitude of MB = − 14 reported by Binggeli, Sandage & Tammann (using their Virgo distance modulus). We also show that if BCDs fade at the end of the current burst of star formation, such ‘dead’BCDs, if they exist, would all have to be fainter than our sample limit of BJ = 17.6. We use a (U — BJ) (BJ — RF) colour-colour diagram plotted from our photographic data to separate the different dwarf-galaxy types effectively and use this colour information to argue that there are no blue star-forming progenitors of dE galaxies in the cluster. Most of the galaxies of compact appearance we selected were background objects which we show to be significantly younger than general field galaxies; this sample also shows considerable clustering in redshift space.