Gemini Spectroscopy and HST Imaging of the Stellar Cluster Population in Region B of M82

TL;DR

This study combines Gemini spectroscopy and HST imaging to analyze the ages, velocities, and extinctions of young massive clusters in M82's region B, revealing their disk location and recent formation history.

Contribution

It provides new spectroscopic and photometric data on M82's cluster population, challenging previous notions of region B's star formation history and cluster properties.

Findings

Clusters are 70-200 Myr old with velocities 230-350 km/s.

Region B's clusters are on regular disk orbits.

Low extinction patches make the region optically bright.

Abstract

We present new spectroscopic observations of the stellar cluster population of region B in the prototype starburst galaxy M82 obtained with the Gillett Gemini-North 8.1-metre telescope. By coupling the spectroscopy with UBVI photometry acquired with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST), we derive ages, extinctions and radial velocities for seven young massive clusters (YMCs) in region B. We find the clusters to have ages between 70 and 200 Myr and velocities in the range 230 to 350 km/s, while extinctions Av vary between ~1-2.5 mag. We also find evidence of differential extinction across the faces of some clusters which hinders the photometric determination of ages and extinctions in these cases. The cluster radial velocities indicate that the clusters are located at different depths within the disk, and are on regular disk orbits. Our results overall contradict the findings of previous studies, where region B was thought to be a bound region populated by intermediate-age clusters that formed in an independent, offset starburst episode that commenced 600 Myr-1 Gyr ago. Our findings instead suggest that region B is optically bright because of low extinction patches, and this allows us to view the cluster population of the inner M82 disk, which probably formed as a result of the last encounter with M81. This study forms part of a series of papers aimed at studying the cluster population of M82 using deep optical spectroscopy and multi-band photometry.