The evolution and star formation of dwarf galaxies in the Fornax Cluster

We present the results of a spectroscopic survey of 675 bright (16.5 < b(J) < 18) galaxies in a 6 degrees field centred on the Fornax cluster with the FLAIR-II spectrograph on the UK Schmidt Telescope. Three galaxy samples we re observed: compact galaxies to search for new blue compact dwarfs, candid ate M 32-like compact dwarf ellipticals, and a subset of the brightest know n cluster members in order to study the cluster dynamics. We measured redsh ifts for 516 galaxies, of which 108 were members of the Fornax Cluster. Def ining dwarf galaxies to be those with b(J) greater than or equal to 15 (M-B greater than or equal to - 16.5), there are a total of 62 dwarf cluster ga laxies in our sample. Nine of these are new cluster members previously misi dentified as background galaxies. The cluster dynamics show that the dwarf galaxies are still falling into the cluster whereas the giants are virializ ed. We classified the observed galaxies as late-type if we detected H alpha emi ssion at an equivalent width greater than 1 Angstrom. The spectra were obta ined through fixed apertures, so they reflect activity in the galaxy cores, but this does not significantly bias the classifications of the compact dw arfs in our sample. The new classifications reveal a higher rate of star fo rmation among the dwarf galaxies than suggested by morphological classifica tion: 35 per cent have significant H alpha emission indicative of star form ations but only 19 per cent were morphologically classified as late-types. The star-forming dwarf galaxies span the full range of physical sizes and w e find no evidence in our data for a distinct class of star-forming blue co mpact dwarf (BCD) galaxy. The distribution of scale sizes is consistent wit h evolutionary processes which transform late-type dwarfs to early-type dwa rfs. The fraction of dwarfs with active star formation drops rapidly toward s the cluster centre: this is the usual density-morphology relation confirm ed here for dwarf galaxies. The star-forming dwarfs are concentrated in the outer regions of the cluster, the most extreme in an infalling subcluster. We estimate gas depletion time-scales for five dwarfs with detected Hi emi ssion: these are long (of order 10(10) yr), indicating that an active gas r emoval process must be involved if they are transformed into gas-poor dwarf s as they fall further into the cluster. Finally, in agreement with our pre vious results, we find no compact dwarf elliptical (M 32-like) galaxies in the Fornax Cluster.