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-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging of the nuclear starburst region of M 82, obtained as part of the Hubble Heritage mosaic made of this galaxy, in four filters (Johnson-Cousins equivalent , and I broad bands, and an Hα 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.