# A 5deg x 5deg deep HI survey of the M81 group

## TL;DR

This deep HI survey of the M81 group maps the distribution and kinematics of neutral hydrogen over a large area, revealing tidal features, clouds, and a flat rotation curve indicating large-scale ordered motion dominated by M81.

## Contribution

First detailed large-scale HI mapping of the M81 group at 1 kpc resolution, revealing tidal structures and rotation dynamics beyond the galaxy disks.

## Key findings

- Detected HI clouds likely tidally stripped from past interactions.
- Observed a flat rotation curve from 20 to 80 kpc, indicating large-scale ordered motion.
- Identified tidal features connecting M81, M82, and NGC 2976.

## Abstract

A 25 $\rm deg^2$ region, including the M81 complex (M81, M82, NGC 3077), NGC 2976 and IC2574, was mapped during ~3000 hours with the DRAO synthesis telescope. With a physical resolution of ~1 kpc, these observations allow us to probe a large region down to column density levels of ~$\rm 1\times10^{18}\, cm^{-2}$ over 16 $\rm km\, s^{-1}$, mapping the extent of the HI arm connecting the system and NGC 2976, and resolving the HI clouds adjacent to the arm. The observations also reveal a few clouds located between the system and IC 2574, probably tidally stripped from a past interaction between the two systems. Given the regular velocity distribution in the HI envelope of the system, we attempt and derive an idealised large-scale rotation curve of the system. We observe a flat trend for the rotation velocity of the overall system from 20 kpc out to 80 kpc, well beyond the outskirts of the M81 disk, although with asymmetries like a wiggle at the vicinity of M82. This supports the assumption that intergalactic gas and galaxies in the system participate to a large-scale ordered rotation motion which is dominated by M81. Also, our HI analysis of the group further supports the hypothesis that the galaxies forming the system moved closer from afar, in agreement with numerical simulations.