We present H i 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 \Delta v _ { FWHM } = 4.9 km s ^ { -1 } providing an upper limit to the kinetic temperature of the H i gas of T _ { k } \leq 532 K. The total observed flux indicates an H i mass of M ( H i ) = 5.66 \cdot 10 ^ { 4 } M _ { \odot } \left [ d / 150 { kpc } \right ] ^ { 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 \vartheta = 4 \aas@@fstack { \prime } 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 \Lambda CDM 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 .