We report the discovery , from the H i Parkes All-Sky Survey ( HIPASS ) , of an isolated cloud of neutral hydrogen which we believe to be extragalactic .
The H i mass of the cloud ( HIPASS J1712-64 ) is very low , 1.7 \times 10 ^ { 7 } \mbox { $ { \cal M } _ { \odot } $ } , using an estimated distance of \sim 3.2 Mpc .
Most significantly , we have found no optical companion to this object to very faint limits ( \mu ( B ) \sim 27 mag arcsec ^ { -2 } ) .
HIPASS J1712-64 appears to be a binary system similar to , but much less massive than , HI 1225+01 ( the Virgo H i Cloud ) and has a size of at least 15 kpc .
The mean velocity dispersion , measured with the Australia Telescope Compact Array ( ATCA ) , is only 4 km s ^ { -1 } for the main component and because of the weak or non-existent star-formation , possibly reflects the thermal linewidth ( T < 2000 K ) rather than bulk motion or turbulence .
The peak column density for HIPASS J1712-64 , from the combined Parkes and ATCA data , is only 3.5 \times 10 ^ { 19 } cm ^ { -2 } , which is estimated to be a factor of two below the critical threshold for star formation .
Apart from its significantly higher velocity , the properties of HIPASS J1712-64 are similar to the recently recognised class of Compact High Velocity Clouds .
We therefore consider the evidence for a Local Group or Galactic origin , although a more plausible alternative is that HIPASS J1712-64 was ejected from the interacting Magellanic Cloud/Galaxy system at perigalacticon \sim 2 \times 10 ^ { 8 } yr ago .