BST1047+1156: An extremely diffuse and gas-rich object in the Leo I Group

TL;DR

BST1047+1156 is an extremely diffuse, gas-rich object in the Leo I Group, with very low surface brightness and evidence of recent star formation, possibly formed through tidal interactions or as a pre-existing galaxy.

Contribution

This paper reports the discovery and detailed characterization of BST1047+1156, the lowest surface brightness object detected via integrated light, and discusses its possible origins in a galaxy group environment.

Findings

Detected in optical and UV, with very low surface brightness.

Highly gas-rich with a gas fraction of 0.99.

Shows signs of recent star formation despite low HI density.

Abstract

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 ~ 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.