A&A 415, 103–116 (2004) Astronomy DOI: 10.1051/0004-6361:20034183 & c ESO 2004 Astrophysics Nuclear star formation in NGC 6240 A. Pasquali1,2,J.S.Gallagher3, and R. de Grijs4 1 ESO/ST-ECF, Karl-Schwarzschild-Strasse 2, 85748 Garching bei M¨unchen, Germany 2 Institute of Astronomy, ETH H¨onggerberg, 8093 Z¨urich, Switzerland 3 University of Wisconsin, Department of Astronomy, 475 N. Charter St., Madison WI 53706, USA e-mail:
[email protected] 4 University of Sheffield, Department of Physics and Astronomy, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail:
[email protected] Received 12 August 2003 / Accepted 6 November 2003 Abstract. We have made use of archival HST BVIJH photometry to constrain the nature of the three discrete sources, A1, A2 and B1, identified in the double nucleus of NGC 6240. STARBURST99 models have been fitted to the observed colours, under the assumption, first, that these sources can be treated as star clusters (i.e. single, instantaneous episodes of star formation), and subsequently as star-forming regions (i.e. characterised by continuous star formation). For both scenarios, we estimate ages as young as 4 million years, integrated masses ranging between 7 106 M (B1) and 109 M (A1) and a rate of 1 supernova per × 1 year, which, together with the stellar winds, sustains a galactic wind of 44 M yr− . In the case of continuous star formation, 1 a star-formation rate has been derived for A1 as high as 270 M yr− , similar to what is observed for warm Ultraluminous 3 Infrared Galaxies (ULIRGs) with a double nucleus.