Star Formation in Dwarf Galaxies of the Nearby Centaurus A Group
  We present Halpha narrow-band imaging of 17 dwarf irregular galaxies (dIs) in
the nearby Centaurus A Group. Although all large galaxies of the group have a
current or recent enhanced star formation episode, the dIs have normal star
formation rates and do not contain a larger fraction of dwarf starbursts than
other nearby groups. Relative distances between dIs and larger galaxies of the
group can be computed in 3D since most of them have now fairly accurately known
distances. We find that the dI star formation rates do not depend on local
environment, and in particular they do not show any correlation with the
distance of the dI to the nearest large galaxy of the group. There is a clear
morphology-density relation in the Centaurus A Group, similarly to the Sculptor
and Local Groups, in the sense that dEs/dSphs tend to be at small distances
from the more massive galaxies of the group, while dIs are on average at larger
distances. We find four transition dwarfs in the Group, dwarfs that show
characteristics of both dE/dSphs and dIs, and which contain cold gas but no
current star formation. Interestingly the transition dwarfs have an average
distance to the more massive galaxies which is intermediate between those of
the dEs/dSphs and dIs, and which is quite large: 0.54 +- 0.31 Mpc. This large
distance poses some difficulty for the most popular scenarios proposed for
transforming a dI into a dE/dSph (ram-pressure with tidal stripping or galaxy
harassment). If the observed transition dwarfs are indeed missing links between
dIs and dE/dSphs, their relative isolation makes it less likely to have been
produced by these mechanisms. We propose that an inhomogeneous IGM containing
higher density clumps would be able to ram-pressure stripped the dIs at such
large distances.
