VLA H92 Alpha and H53 Alpha Radio Recombination Line Observations of M82

Abstract

We present high angular resolution (0.6'') observations made with the VLA of the radio continuum at 8.3 and 43 GHz as well as H92 Alpha and H53 Alpha radio recombination lines from the nearby (3 Mpc) starburst galaxy M82. In the continuum we report 19 newly identified sources at 8.3 GHz and 5 at 43 GHz that were unknown previously. The spatial distribution of the H92 Alpha line is inhomogeneous; we identify 27 features. The line and continuum emission are modeled using a collection of HII regions at different distances from the nucleus assuming a single-density component and two-density components. The high-density component has a density of 4 X 10^{4} cm^{-3}. However, the bulk of the ionization is in regions with densities which are typically a factor 10 lower. The gas kinematics, using the H92 Alpha line, confirms the presence of steep velocity gradient (26 km s^{-1} arcsec^{-1}) in the nuclear region. As this steep gradient is observed not only on the major axis but also at large distances along a band of PA of 150 degrees, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. Ad-hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated as we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. The H92 Alpha line also reveals the base of the outflow where the injection towards the halo on the Northern side occurs. The kinematical pattern suggests a connection between the gas flowing in the plane of M82 towards the center; this behavior most likely originates due to the presence of a bar and the outflow out of the plane.