Measurement of the Mass and Stellar Population Distribution in M82 with the LBT

TL;DR

This study uses K-band spectroscopy to analyze M82's stellar and gas kinematics, mass distribution, and stellar populations, revealing a flat rotation curve and the dominance of red supergiants within the central 500 pc.

Contribution

First detailed K-band spectroscopic analysis of M82's stellar and gas kinematics, mass distribution, and stellar populations with new insights into its rotation curve and RSG population.

Findings

Rotation curve is flat from 1-4 kpc, contrasting previous declining models.

Stellar and gas kinematics are consistent, with gas showing slightly higher velocities.

Mass estimate of M82 is approximately 10^10 solar masses.

Abstract

We present a K-band spectroscopic study of the stellar and gas kinematics, mass distribution, and stellar populations of the archetypical starburst galaxy M82. Our results are based on a single spectrum at a position angle of 67.5 degrees through the K-band nucleus. We used the CO stellar absorption band head at 2.29 {\mu}m (CO_2.29) to measure the rotation curve out to nearly 4 kpc radius on both the eastern and western sides of the galaxy. Our data show that the rotation curve is flat from 1 - 4 kpc. This stands in sharp contrast to some previous studies, which have interpreted H I and CO emission-line position-velocity diagrams as evidence for a declining rotation curve. The kinematics of the Br\gamma, H_2, and He I emission lines are consistent with, although characterized by slightly higher velocities than, the stellar kinematics. We derived M82's mass distribution from our stellar kinematic measurements and estimate its total dynamical mass is ~10^10 Msun. We measured the equivalent width of CO_2.29 (W_2.29) as a function of distance from the center of the galaxy to investigate the spatial extent of the red supergiant (RSG) population. The variation in W_2.29 with radius clearly shows that RSGs dominate the light inside 500 pc radius. M82's superwind is likely launched from this region, where we estimate the enclosed mass is <= 2 x 10^9 Msun.