Anomalous Evolution of the Dwarf Galaxy HIPASS J1321–31

We present Hubble Space Telescope/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This unusual galaxy lies in the direction of the Centaurus A group of galaxies and has a color-magnitude diagram with a distinctive red plume of luminous stars. This feature could arise from (1) a red giant branch if the galaxy were much nearer than previously recognized, (2) a peculiar asymptotic giant branch, or (3) an ~1 Gyr old population of intermediate-mass red supergiants, which we find to be the most likely explanation. However, the lack of equally luminous blue stars requires that the star formation has dropped substantially since these stars were formed. Evidently HIPASS J1321-31 experienced an episode of enhanced star formation rather recently in its star formation history followed by a period of relative quiescence that has led to the evolution of the main-sequence stars into the red supergiant branch. The stellar populations in HIPASS J1321-31 reflect a star formation history that is uncommon in star-forming dwarf galaxies. This is the first time such a star formation history has been noted, although the literature contains a small number of other dwarf galaxies with similar color-magnitude diagrams. Therefore, HIPASS J1321-31 and these other galaxies represent a different path of dwarf galaxy evolution that has not been well explored and an important probe into how dwarf galaxies evolve.