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 dataset 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 poststarburst 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 in the range v R ∈ (−160, 220) km s −1 (wrt the galaxy centre) and line of sight Na I D interstellar absorption line velocities v Na I D R ∈ (−75, 200) km s −1 , in many cases entirely decoupled from the clusters. As the disk cluster radial velocities 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 WR 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.

CS, HC3N, and CH3CCH multi-line analyses toward starburst galaxies

“…The stellar clusters in the center of M 82 are estimated to be the result of starbursts from at least ∼ 10 − 15 Myr ago, as seen at optical wavelengths by Konstantopoulos et al (2009). This value indicates that we are seeing a more evolved starburst (or post-starburst) in M 82, older than the NGC 253 and IC 342 ones (we are not aware of a published stellar population age in the center of Maffei 2).…”

Testing Density Wave Theory With Resolved Stellar Populations Around Spiral Arms in M81

“…According to Yun's simulation (Yun 1999), M81 underwent the closest passage about 220 and 280 Myr ago by its companion galaxies M82 and NGC 3077, respectively. In addition, many observational studies (e.g., Yun et al 1994;Chandar et al 2001;Pérez-González et al 2006;de Mello et al 2008;Konstantopoulos et al 2009;Santiago-Cortés et al 2010;Hoversten et al 2011;Lim et al 2013) have found evidence for tidally induced SF in both M81 and its companion galaxies: ages of compact star clusters and star-forming regions range ∼100-500 Myr. Although we also detected enhanced SF at ages between 200 and 350 Myr in all 20 regions, the associated systematic errors are significant since the stars sensitive to this SF peak are near the 50% completeness limit of the data, and therefore this result would require additional observations to confirm with confidence.…”

A large12C/13C isotopic ratio in M 82 and NGC 253

“…However, the ongoing nuclear starbursts started ∼15 Myr ago, with nuclear star clusters dated between 7 and 15 Myr (Konstantopoulos et al 2009). This dating agrees with the two short-duration bursts of star formation in the nuclear region of M 82, with peaks at 5 and 10 Myr (Förster Schreiber et al 2003).…”

“…However, this fast evolution might also result in an overproduction of 18 O , which is not observed. Moreover, if the timescale of the main burst in the nucleus of M 82 is less than 15 Myr (Förster Schreiber et al 2003;Konstantopoulos et al 2009), then most of the 12 C enriched material would be in the hot outflow with a contribution to the molecular gas. The timescale is rather short to have the 12 C enriched material well mixed over the ∼300 pc of our beam size, in both M 82 and NGC 253.…”