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

We present observations of the compact high-velocity cloud HVC289+33+251 that was discovered by Putman et al. ([CITE]). 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 km s-1 providing an upper limit to the kinetic temperature of the gas of K. The total observed flux indicates an mass of M() = . Follow-up observations using the Australia Telescope Compact Array (ATCA) resolve HVC289+33+251 into 5 condensations that are embedded in a common envelope. The HVC shows a faint tail, indicating an ongoing ram-pressure interaction with an ambient low-density medium. A FWHM diameter of ϑ = 44 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]; Moore et al. [CITE]). Comparable clouds in other groups of galaxies or even around the Milky Way are not detectable with the resolution and sensitivity of present surveys.