Spatially resolved kinematics of the multi-phase interstellar medium in the inner disk of M82

TL;DR

This study maps the kinematics of different gas phases in M82's inner disk, revealing counter-rotating and inflowing gas components, and highlights observational challenges in detecting neutral outflows in inclined galaxies.

Contribution

It provides spatially resolved measurements of NaI D and CO kinematics in M82, enabling direct comparison across gas phases and revealing complex gas motions.

Findings

Detection of counter-rotating NaI component.

Evidence of entrained CO gas in the southern plume.

Challenges in detecting neutral outflows at low resolution.

Abstract

We present spatially resolved kinematics of the interstellar NaI D 5890,5896 doublet absorption and 12CO(1-0) emission across the inner ~2x1 kpc of the disk of M82. These data were obtained with the DensePak IFU on the WIYN telescope and the Caltech Owens Valley Radio Observatory (OVRO) millimetre array. By measuring the NaI and CO (and Halpha kinematics from a previous study) at the same spatial resolution, and employing the same line fitting method, we have been able to make meaningful comparisons between the ionized, neutral and molecular gas phases. We detect a component of the NaI line throughout the inner disk with velocities that are forbidden by the known galactic rotation. We interpret this as originating in counter-rotating or perhaps inflowing material. In the southern plume, we find clear evidence of entrained CO gas with kinematics consistent with that of Halpha. On the northern side, the CO kinematics appear to trace more static clouds in the inner halo that could be pre-existing or tidal in origin. We find no evidence that NaI absorption is kinematically associated with the outflow. We conclude that a combination of lack of velocity resolution and confusion of due to the high inclination of the system is acting to prevent detection. Thus, in the search for neutral outflows from galaxies, the signature high velocity components may easily be missed in observations at low spectral resolution and/or sensitivity, and particularly so in highly inclined systems.