A high-resolution mosaic of the neutral hydrogen in the M81 triplet

TL;DR

This study provides a detailed high-resolution map of neutral hydrogen in the M81 triplet, revealing complex velocity features, low-density gas reservoirs, and insights into star formation thresholds and tidal dynamics.

Contribution

It offers the first high-resolution, wide-area HI mosaic of the M81 triplet, highlighting small-scale features and comparing with GBT data to understand gas distribution and dynamics.

Findings

Detection of a substantial low-density gas reservoir near M82.

Identification of kpc-sized low-mass HI clouds with possible dark matter.

High velocity dispersions explained by tidal effects, winds, or outflows.

Abstract

We present a 3x3 degrees, 105-pointing, high-resolution neutral hydrogen (HI) mosaic of the M81 galaxy triplet (including the galaxies M81, M82 and NGC 3077, as well as dwarf galaxy NGC 2976) obtained with the Very Large Array (VLA) C and D arrays. This uniformly covers the entire area and velocity range of the triplet with a resolution of ~20'' or ~420 pc. The data reveal many small-scale anomalous velocity features highlighting the complexity of the interacting M81 triplet. We compare our data with Green Bank Telescope (GBT) observations of the same area. This provides evidence for a substantial reservoir of low-column density gas in the northern part of the triplet, probably associated with M82. Such a reservoir is not found in the southern part. We report a number of kpc-sized low-mass HI clouds with HI masses of a few times 10^6 Msun. Their dynamical masses are much larger than their baryonic masses, which could indicate the presence of dark matter if the clouds are rotationally supported. However, due to their spatial and kinematical association with HI tidal features, it is more likely that the velocity widths indicate tidal effects or streaming motions. We do not find any clouds not associated with tidal features down to an HI mass limit of a few times 10^4 Msun. We compare the HI column densities with resolved stellar density maps and find a star formation threshold around 3-6 10^20 cm-2$. We find that extreme velocity dispersions can be explained by a superposition of multiple components along the line of sight near M81 as well as winds or outflows around M82. The velocity dispersions found are high enough that these processes could explain the linewidths of Damped-Lyman-alpha absorbers observed at high redshift.