Star cluster versus field star formation in the nucleus of the prototype starburst galaxy M82

TL;DR

This study uses high-resolution HST imaging to analyze star cluster and field star formation in M82's nucleus, finding complex clusters but no clear recent burst, and highlighting uncertainties in age estimation methods.

Contribution

It provides detailed imaging analysis of M82's nuclear star clusters and discusses the limitations of standard age determination techniques.

Findings

Approximately 150 star clusters identified in the inner 100 pc.

No conclusive evidence of a recent cluster formation epoch 4-6 Myr ago.

Standard stellar population analysis underestimates age uncertainties due to stochastic effects.

Abstract

We analyse high-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging of the nuclear starburst region of M82, obtained as part of the Hubble Heritage mosaic made of this galaxy, in four filters (Johnson-Cousins equivalent B, V, and I broad bands, and an Halpha narrow-band filter), as well as subsequently acquired U-band images. We find a complex system of ~150 star clusters in the inner few 100 pc of the galaxy. We do not find any conclusive evidence of a cluster-formation epoch associated with the most recent starburst event, believed to have occurred about 4-6 Myr ago. This apparent evidence of decoupling between cluster and field-star formation is consistent with the view that star cluster formation requires special conditions. However, we strongly caution, and provide compelling evidence, that the `standard' simple stellar population analysis method we have used significantly underestimates the true uncertainties in the derived ages due to stochasticity in the stellar initial mass function and the corresponding sampling effects.