BST1047+1156 : an extremely diffuse and gas-rich object in the Leo I group

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>We report the detection of diffuse starlight in an extragalactic H ɪ cloud in the nearby Leo I galaxy group. We detect the source, BST1047+1156, in both broadband optical and the <em>Galaxy Evolution Explorer</em> (<em>GALEX</em>) ultraviolet (UV) light. Spanning ~2 kpc in radius, it has a peak surface brightness of <em>μ_B</em> = 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 <em>f_g</em> = 0.99, its peak H ɪ 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 <em>B</em> − <em>V</em> = 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 ɪ 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.</p></div>