The Young Cluster Population of M82 Region B

TL;DR

This study uses Hubble Space Telescope data to determine that the peak formation of star clusters in M82's Region B occurred around 150 million years ago, revising previous estimates and clarifying the galaxy's starburst history.

Contribution

The paper demonstrates the importance of U-band data for accurate age determination of star clusters and revises the cluster formation epoch in M82's Region B.

Findings

Peak cluster formation occurred ~150 Myr ago.

U-band data are crucial for accurate age estimates.

The cluster luminosity function turnover was misinterpreted due to detection biases.

Abstract

We present observations obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope of the "fossil" starburst region B in the nearby starburst galaxy M82. By comparing UBVI photometry with models, we derive ages and extinctions for 35 U-band selected star clusters. We find that the peak epoch of cluster formation occurred ~ 150 Myr ago, in contrast to earlier work that found a peak formation age of 1.1 Gyr. The difference is most likely due to our inclusion of U-band data, which are essential for accurate age determinations of young cluster populations. We further show that the previously reported turnover in the cluster luminosity function is probably due to the neglect of the effect of extended sources on the detection limit. The much younger cluster ages we derive clarifies the evolution of the M82 starburst. The M82-B age distribution now overlaps with the ages of: the nuclear starburst; clusters formed on the opposite side of the disk; and the last encounter with M81, some 220 Myr ago.