The HI environment of the M101 group

TL;DR

This study provides a detailed, deep HI map of the M101 galaxy group, revealing new HI sources, tidal features, and complex kinematics indicative of recent interactions and dynamic activity within the group.

Contribution

It presents the first wide, deep HI mapping of the M101 group, discovering new HI sources and tidal structures, and analyzing their kinematics and optical counterparts.

Findings

Discovery of a ~100 kpc HI tidal plume with peculiar kinematics.

Identification of two new HI sources, including a low surface brightness dwarf galaxy.

Evidence of recent interaction between M101 and NGC 5474.

Abstract

We present a wide (8.5x6.7 degree, 1050x825 kpc), deep (sigma(N_HI)=10^(16.8-17.5) cm^-2) neutral hydrogen (HI) map of the M101 galaxy group. We identify two new HI sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless HI cloud, possibly primordial in origin. Our data show that M101's extended HI envelope (Huchtmeier & Witzel 1979) takes the form of a ~100 kpc long tidal loop or plume of HI extending to the southwest of the galaxy. The plume has an HI mass ~ 10^8 Msun and a peak column density of N_HI=5x10^17 cm^-2, and while it rotates with the main body of M101, it shows kinematic peculiarities suggestive of a warp or flaring out of the rotation plane of the galaxy. We also find two new HI clouds near the plume with masses ~ 10^7 Msun, similar to HI clouds seen in the M81/M82 group, and likely also tidal in nature. Comparing to deep optical imaging of the M101 group, neither the plume nor the clouds have any extended optical counterparts down to a limiting surface brightness of mu_B = 29.5. We also trace HI at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex HI structure in the M101 group, coupled with the optical morphology of M101 and its companions, suggests that the group is in a dynamically active state that is likely common for galaxies in group environments.