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Star cluster versus field star formation in the nucleus of the prototype starburst galaxy M 82

¹ Department of Physics & Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: [pha04sb;R.deGrijs]@sheffield.ac.uk
² Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de la Palma, Canary Islands, Spain
³ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
⁴ Instituto de Astrofísica de Andalucía (CSIC), Camino Bajo de Huétor 50, Granada 18008, Spain e-mail: mcs@iaa.es

Received: 26 February 2008
Accepted: 9 April 2008

Abstract

We analyse high-resolutionHubble SpaceTelescope/Advanced Camera for Surveys imaging of the nuclearstarburst region of M 82, obtained as part of the Hubble Heritagemosaic made of this galaxy, in four filters (Johnson-Cousinsequivalent, andI broad bands, and an Hα narrow-bandfilter), as well as subsequently acquiredU-band images. We find acomplex system of ~150 star clusters in the inner few 100 pc ofthe galaxy. We do not find any conclusive evidence of acluster-formation epoch associated with the most recent starburstevent, believed to have occurred about 4–6 Myr ago. This apparentevidence of decoupling between cluster and field-star formation isconsistent with the view that star cluster formation requires specialconditions. However, we strongly caution, and provide compellingevidence, that the “standard” simple stellar population analysismethod we have used significantly underestimates the trueuncertainties in the derived ages due to stochasticity in the stellarinitial mass function and the corresponding sampling effects.