Nuclear star formation in NGC 6240

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 (Al) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44 M_⊙ yr^-1. In the case of continuous star formation, a star-formation rate has been derived for Al as high as 270 M_⊙ yr^-1, similar to what is observed for warm Ultraluminous Infrared Galaxies (ULIRGs) with a double nucleus. The A1 source is characterised by a mass density of about 1200 M_⊙ pc^-3 which resembles the CO molecular mass density measured in cold ULIRGs and the stellar density determined in "elliptical core" galaxies. This, together with the recent discovery of a supermassive binary black hole in the double nucleus of NGC 6240, might indicate that the ongoing merger could shape the galaxy into a core elliptical.