Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group

We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology, they appear to be low surface brightness dwarf spheroidals, yet they are gas rich (M_{H i}/L_B > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed colour magnitude diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs, we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas rich because their star formation rates have been very low (≲10^-3 M _⊙ yr^-1) throughout. In such systems, star formation appears to have been sporadic and local, though one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that many of its stars were formed in a 'miniburst' 300-500 Myr ago. The question of why there are no similar dwarf galaxies in the Local Group remains open.