Discovery of a 0.6 Mpc Diffuse HI Structure Associated with Stephan's Quintet

Abstract

Stephan's Quintet (SQ) is unique among compact groups of galaxies because of a very rare event currently occurring to it: a high velocity (~1000 km/s) intruder is colliding into a debris field in the intragroup medium (IGM) ^1. The debris has been produced by previous interactions among other members of the group, although the exact details and timing remains to be understood. Here we report the discovery of a very large diffuse atomic hydrogen (HI) structure (size ~ 0.6 Mpc) revealing new clues for the early history of the group (> 1 Gyr ago). This is the largest of such structures ever found in the vicinity of a group. Thanks to the size of the Five-hundred-meter Aperture Spherical Telescope (FAST), its exceptionally sensitive and large-format 19-beam receiver ^2, our observations achieved the HI column density sensitivity limit of 1 sigma = 4.2 ×10^(16) cm^(-2) per channel (20 km/s) when smoothed to 4’ angular resolution. This is one of the deepest extragalactic HI observations ever made.