Searching for stars in compact high-velocity clouds – I. First results from VLT and 2MASS

We investigate the hypothesis that compact high-velocity clouds (CHVC) are the 'missing' dwarf galaxies of the Local Group, by searching them for populations of resolved stars. To this end we conducted two distinct tests based on optical and near-infrared single-star photometry. The optical and the near-infrared experiments complement one another; the optical data help us to rule out distant populations but they are restricted to the central regions of the gas distributions, whereas the near-infrared photometry allows us to set limits on nearby populations spread over the typical cloud size. First, we discuss deep optical single-star photometry of five CHVCs in the V and I filters, obtained with the FORS instrument at the Very Large Telescope (VLT). We find that their optical colour-magnitude diagrams are indistinguishable from that of a population of Galactic stars, and attribute all of the resolved stars to the Galactic foreground. We present simulations that address the question of how much of a 'normal' dwarf-galaxy-type population we might hide in the data. A Kolmogorov-Smirnov test allows us to set very stringent limits on the absence of a resolved stellar population in CHVCs. Secondly, we also culled near-infrared single-star photometry in the J, H, and K S bands for four of the CHVCs from the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any stellar content in the CHVCs which resembles that of nearby dwarf galaxies either, and are explained with Galactic foreground as well. We interpret our null detections as indicating that the five CHVCs investigated by us do not host an associated stellar content which is similar to that of the known dwarf galaxies of the Local Group. These CHVCs are very likely to be pure hydrogen clouds in which no star formation has taken place over cosmic time.