EDP Sciences
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Volume 415, Number 1, February III 2004
Page(s) 103 - 116
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034183

A&A 415, 103-116 (2004)
DOI: 10.1051/0004-6361:20034183

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 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

(Received 12 August 2003 / Accepted 6 November 2003)

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  $7\, \times\, 10^6~M_{\odot}$ (B1) and 10 $^9~M_{\odot}$ (A1) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44  $M_{\odot}$ yr -1. In the case of continuous star formation, a star-formation rate has been derived for A1 as high as 270  $M_{\odot}$ 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  $M_{\odot}$ 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.

Key words: galaxies: evolution -- galaxies: individual: NGC 6240 -- galaxies: interactions -- galaxies: starburst -- galaxies: star clusters

Offprint request: A. Pasquali, pasquali@phys.ethz.ch

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