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

Abstract

We present spatially resolved kinematics of the interstellar Na i D λλ5890,5896 doublet absorption and CO (1→0) emission across the inner ∼2× 1 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 Na i and CO (and Hα 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 Na i line throughout the inner disk with velocities that are forbidden by the known galactic rotation. We interpret this as originating in counterrotating or perhaps inflowing material. In the southern plume, we find clear evidence of entrained CO gas with kinematics consistent with that of Hα. 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 Na i 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.