H I observations of an Ultra-Compact High-Velocity Cloud

We present H I observations of the compact high-velocity cloud HVC289+33+251 that was discovered by Putman et al. (\cite{putman}). Observations with the 100-m Effelsberg telescope demonstrate that this cloud is still unresolved by the 9 arcmin beam of the Effelsberg telescope. The cloud shows a small line width of Δ v_FWHM = 4.9 km s^-1 providing an upper limit to the kinetic temperature of the H I gas of T_k ≤532 K. The total observed flux indicates an H I mass of M(H I) = 5.66×10⁴ M_⊙ [d/150 kpc]². Follow-up H I observations using the Australia Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that are embedded in a common H I envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of ǎrtheta = 4.4 arcmin 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 ΛCDM simulations (e.g. Klypin et al. \cite{klypin}; Moore et al. \cite{moore}). Comparable clouds in other groups of galaxies or even around the Milky Way are not detectable with the resolution and sensitivity of present surveys.