New Results on the Ages of Star Clusters in Region B of M82

TL;DR

This study revises the ages of star clusters in M82-B to be significantly younger than previously thought, showing that earlier mass distribution features are likely artifacts and that clusters are unbound.

Contribution

It provides new spectroscopic age estimates for clusters in M82-B, challenging previous photometric results and clarifying the nature of their mass distribution and dynamical state.

Findings

Cluster ages are between 10 and 300 Myr, younger than prior estimates.

The apparent luminosity/mass distribution turn-over is an artifact, not real.

Clusters are not gravitationally bound based on their velocities.

Abstract

The post-starburst region B in M82 and its massive star cluster component have been the focus of multiple studies, with reports that there is a large population of coeval clusters of age ~1 Gyr, which were created with a Gaussian initial mass distribution. This is in disagreement with other studies of young star clusters, which invariably find a featureless power-law mass distribution. Here, we present Gemini-North optical spectra of seven star clusters in M82-B and show that their ages are all between 10 and 300 Myr (a factor of 3-100 younger than previous photometric results) and that their extinctions range between near-zero and 4 mag (Av). Using new HST ACS-HRC U-band observations we age date an additional ~30 clusters whose ages/extinctions agree well with those determined from spectroscopy. Completeness tests show that the reported `turn-over' in the luminosity/mass distributions is most likely an artefact, due to the resolved nature of the clusters. We also show that the radial velocities of the clusters are inconsistent with them belonging to a bound region.