A Spectroscopic census of the m82 stellar cluster population

Abstract

We present a spectroscopic study of the stellar cluster population of M82, the archetype starburst galaxy, based primarily on new Gemini-North multi-object spectroscopy of 49 star clusters. These observations constitute the largest to date spectroscopic data set of extragalactic young clusters, giving virtually continuous coverage across the galaxy; we use these data to deduce information about the clusters as well as the M82 post-starburst disk and nuclear starburst environments. Spectroscopic age dating places clusters in the nucleus and disk between (7, 15) and (30, 270) Myr, with distribution peaks at 10 and 140 Myr, respectively. We find cluster radial velocities (RVs) in the range v R ∈ (-160, 220)km s -1 (with respect to the galaxy center), and line-of-sight NaI D interstellar absorption line velocities v Na IDR ∈ (-75, 200) km s-1, in many cases entirely decoupled from the clusters. As the disk cluster RVs lie on the flat part of the galaxy rotation curve, we conclude that they comprise a regularly orbiting system. Our observations suggest that the largest part of the population was created as a result of the close encounter with M81 220 Myr ago. Clusters in the nucleus are found in solid body rotation on the bar. The possible detection of Wolf-Rayet features in their spectra indicates that cluster formation continues in the central starburst zone. We also report the potential discovery of two old populous clusters in the halo of M82, aged ≳8 Gyr. Using these measurements and simple dynamical considerations, we derive a toy model for the invisible physical structure of the galaxy, and confirm the existence of two dominant spiral arms. © 2009 The American Astronomical Society. All rights reserved.