We report the detection of diffuse starlight in an extragalactic HI cloud in the nearby Leo I galaxy group .
We detect the source , dubbed BST1047+1156 , in both broadband optical and GALEX ultraviolet light .
Spanning \sim 2 kpc in radius , it has a peak surface brightness of \mu _ { B } =28.8 mag arcsec ^ { -2 } , making it the lowest surface brightness object ever detected via integrated light .
Although the object is extremely gas-rich , with a gas fraction of f _ { g } = 0.99 , its peak HI column density is well below levels where star formation is typically observed in galaxies .
Nonetheless , BST1047+1156 shows evidence for young stellar populations : along with the detected UV emission , the object is extremely blue , with B - V = 0.14 \pm 0.09 .
The object sports two tidal tails and is found embedded within diffuse gas connecting the spiral galaxy M96 to the group ’ s extended HI Leo Ring .
The nature of BST1047+1156 is unclear .
It could be a disrupting tidal dwarf , recently spawned from star formation triggered in the Leo I Group ’ s tidal debris .
Alternatively , the object may have been a pre-existing galaxy — the most extreme example of a gas-rich field LSB known to date — which had a recent burst of star formation triggered by encounters in the group environment .