ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240 ^*

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: lutz@mpe.mpg.de;sturm@mpe.mpg.de;genzel@mpe.mpg.de
² Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: spoon@astro.rug.nl
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: amoor@eso.org
⁴ School of Physics and Astronomy and Wise Observatory, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel e-mail: netzer@wise1.tau.ac.il;amiel@wise1.tau.ac.il

Corresponding author: D. Lutz, lutz@mpe.mpg.de

Received: 23 April 2003
Accepted: 22 July 2003

Abstract

We present Infrared Space Observatory mid- and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in [ultra]luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic “PAH” features. A strong high excitation [O IV] line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hydrogen. We argue that this emission is mainly due to shocks in its turbulent central gas component and its starburst superwind. The total shock cooling in infrared emission lines accounts for ~0.6% of the bolometric luminosity, mainly through rotational H₂ emission and the [O I] 63m line. We analyse several ways of estimating the luminosities of the starburst and the AGN in NGC 6240 and suggest that the contributions to its bolometric luminosity are most likely in the range 50–75% starburst and 25–50% AGN.