HI Kinematics Along The Minor Axis of M82

TL;DR

This study uses new VLA HI observations to analyze the cold gas kinematics along M82's minor axis, revealing unexpected velocity profiles that challenge simple outflow models and suggest complex interactions with the ambient medium.

Contribution

It provides detailed HI kinematic data along M82's minor axis, showing velocity decreases inconsistent with simple models, and constrains the nature of the outflow and ambient medium interactions.

Findings

HI detected up to 10 kpc along the minor axis

Velocity decreases from 120 km/s to 50 km/s, not explained by gravity alone

HI may not escape the galaxy's halo, with low outflow rate

Abstract

M82 is one of the best studied starburst galaxies in the local universe, and is consequently a benchmark for studying star formation feedback at both low and high redshift. We present new VLA HI observations that reveal the cold gas kinematics along the minor axis in unprecedented detail. This includes the detection of HI up to 10 kpc along the minor axis toward the South and beyond 5 kpc to the North. A surprising aspect of these observations is that the line-of-sight HI velocity decreases substantially from about 120 km/s to 50 km/s from 1.5 to 10 kpc off the midplane. The velocity profile is not consistent with the HI gas cooling from the hot wind. We demonstrate that the velocity decrease is substantially greater than the deceleration expected from gravitational forces alone. If the HI consists of a continuous population of cold clouds, some additional drag force must be present, and the magnitude of the drag force places a joint constraint on the ratio of the ambient medium to the typical cloud size and density. We also show that the HI kinematics are inconsistent with a simple conical outflow centered on the nucleus, but instead require the more widespread launch of the HI over the ~1 kpc extent of the starburst region. Regardless of the launch mechanism for the HI gas, the observed velocity decrease along the minor axis is sufficiently great that the HI may not escape the halo of M82. We estimate the HI outflow rate is much less than 1 M$_{\odot}$ per year at 10 kpc off the midplane.