HI observations of an Ultra-Compact High-Velocity Cloud

We present H. observations of the compact high-velocity cloud HVC289+33+251 that was discovered by Putman et al. (2002). Observations with the 100-m Effelsberg telescope demonstrate that this cloud is still unresolved by the 9' beam of the Effelsberg telescope. The cloud shows a small line width of Deltav(FWHM) = 4.9 kms(-1) providing an upper limit to the kinetic temperature of the HI gas of T-k = 532 K. The total observed flux indicates an HI mass of M(HI) = 5.66 x 10(4) M-circle dot [d/150 kpc](2). Follow-up HI observations using the Australia Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that are embedded in a common H. envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of theta = 4'.4 makes this HVC the by far most compact HVC known till now. The observed parameters suggest that pressure stabilization by an ambient medium is rather unlikely. At a distance of 150 kpc, the virial mass is by a factor of 5.6 higher than the observed gas mass-consistent with HVC289+33+251 being one of the "missing" dark matter mini halos that were predicted by cosmological LambdaCDM simulations (e.g. Klypin et al. 1999; Moore et al. 1999). Comparable clouds in other groups of galaxies or even around the Milky Way are not detectable with the resolution and sensitivity of present surveys.