2004A&A...426L...9B

We present HI observations of the compact high-velocity cloud HVC289+33+251 that was discovered by Putman et al. (2002AJ....123..873P). 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 Δv_FWHM=4.9km/s providing an upper limit to the kinetic temperature of the HI gas of T_k≤532K. The total observed flux indicates an HI mass of M(HI)=5.66x10⁴M_☉[d/150kpc]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 HI envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of θ=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 150kpc, 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., 1999ApJ...522...82K; Moore et al., 1999ApJ...524L..19M). Comparable clouds in other groups of galaxies or even around the Milky Way are not detectable with the resolution and sensitivity of present surveys.