2018ApJ...863L...7M

We report the detection of diffuse starlight in an extragalactic H I cloud in the nearby Leo I galaxy group. We detect the source, BST1047+1156, in both broadband optical and the Galaxy Evolution Explorer (GALEX) ultraviolet (UV) light. Spanning ∼2 kpc in radius, it has a peak surface brightness of µ_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 H I 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 ± 0.09. The object has two tidal tails and is found embedded within diffuse gas connecting the spiral galaxy M96 to the group's extended H I 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 low surface brightness galaxy known to date-which had a recent burst of star formation triggered by encounters in the group environment.