Nuclear star formation in NGC 6240

¹ ESO/ST-ECF, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Institute of Astronomy, ETH Hönggerberg, 8093 Zürich, Switzerland
³ University of Wisconsin, Department of Astronomy, 475 N. Charter St., Madison WI 53706, USA e-mail: jsg@astro.wisc.edu
⁴ University of Sheffield, Department of Physics and Astronomy, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: R.deGrijs@sheffield.ac.uk

Corresponding author: A. Pasquali, pasquali@phys.ethz.ch

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  (B1) and 10 (A1) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44  yr^-1. In the case of continuous star formation, a star-formation rate has been derived for A1 as high as 270  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  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.