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

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 for0.6% 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.

[1]  M. Wolfire,et al.  The [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited , 2003, astro-ph/0305520.

[2]  W. Vacca,et al.  A mid-infrared spectroscopic survey of starburst galaxies: Excitation and abundances , 2003, astro-ph/0301201.

[3]  M. Sauvage,et al.  ISOCAM view of the starburst galaxies M 82, NGC 253 and NGC 1808 , 2002, astro-ph/0212109.

[4]  S. Komossa,et al.  Discovery of a Binary Active Galactic Nucleus in the Ultraluminous Infrared Galaxy NGC 6240 Using Chandra , 2002, astro-ph/0212099.

[5]  E. Oliva,et al.  Mid-Infrared line diagnostics of active galaxies - A spectroscopic AGN survey with ISO-SWS , 2002, astro-ph/0207381.

[6]  M. Tecza,et al.  Ultraluminous Infrared Galaxies: QSOs in Formation? , 2002, astro-ph/0207405.

[7]  R. Genzel,et al.  An iso-sws survey of molecular hydrogen in starburst and seyfert galaxies , 2002, astro-ph/0206135.

[8]  A. Tielens,et al.  Ice features in the mid-IR spectra of galactic nuclei , 2002, astro-ph/0202163.

[9]  D. Lemke,et al.  Infrared to millimetre photometry of ultra-luminous ir galaxies: new evidence favouring a 3-stage dust model , 2001, astro-ph/0110213.

[10]  R. Beswick,et al.  A MERLIN neutral hydrogen absorption study of the luminous infrared merger NGC 6240 , 2001 .

[11]  S. Malhotra,et al.  Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium , 2001, astro-ph/0106485.

[12]  M. Tecza,et al.  Ultraluminous Infrared Mergers: Elliptical Galaxies in Formation? , 2001, astro-ph/0106032.

[13]  M. Sauvage,et al.  Mid-Infrared observations of NGC 1068 with the Infrared Space Observatory , 2001 .

[14]  M. Gerin,et al.  ISOCAM-CVF 5-12 Micron Spectroscopy of Ultraluminous Infrared Galaxies , 2001, astro-ph/0101187.

[15]  A. Sternberg,et al.  Near-Infrared Integral Field Spectroscopy and Mid-Infrared Spectroscopy of the Starburst Galaxy M82 , 2001, astro-ph/0101153.

[16]  M. Sauvage,et al.  Mid-Infrared observations of NGC 1068 with ISOCAM , 2001, astro-ph/0101038.

[17]  G. Helou,et al.  The Infrared Spectral Energy Distribution of Normal Star-forming Galaxies: Calibration at Far-Infrared and Submillimeter Wavelengths , 2000, astro-ph/0011014.

[18]  A. Sternberg,et al.  Massive Star Formation and Evolution in Starburst Galaxies: Mid-infrared Spectroscopy with the ISO Short Wavelength Spectrometer , 2000, astro-ph/0003334.

[19]  T. Nakagawa,et al.  RXTE observation of NGC 6240: a search for the obscured active nucleus , 2000, astro-ph/0003030.

[20]  N. Thatte,et al.  Stellar Dynamics and the Implications on the Merger Evolution in NGC 6240 , 2000, astro-ph/0001424.

[21]  R. Genzel,et al.  00 03 33 4 v 1 2 2 M ar 2 00 0 Massive star formation and evolution in starburst galaxies : mid-infrared spectroscopy with ISO-SWS 1 , 2000 .

[22]  D. Rigopoulou,et al.  A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution , 1999, astro-ph/9908300.

[23]  M. Tecza,et al.  Gasdynamics in the Luminous Merger NGC 6240 , 1999, astro-ph/9905031.

[24]  C. Surace,et al.  The Universe as Seen by ISO , 1999 .

[25]  E. Egami.  An AGN model for NGC 6240 , 1999 .

[26]  G. Zamorani,et al.  A Three-dimensional Diagnostic Diagram for Seyfert 2 Galaxies: Probing X-Ray Absorption and Compton Thickness , 1998, astro-ph/9811074.

[27]  M. A. Malkan,et al.  ISO LWS Spectroscopy of M82: A Unified Evolutionary Model , 1998, astro-ph/9810188.

[28]  A. Sternberg,et al.  The Ratio of Ortho- to Para-H2 in Photodissociation Regions , 1998, astro-ph/9812049.

[29]  P. Cox,et al.  Infrared Space Observatory Measurements of a [C II] 158 Micron Line Deficitin Ultraluminous Infrared Galaxies , 1998 .

[30]  R. Genzel,et al.  The Nature and Evolution of Ultraluminous Infrared Galaxies:A Mid-Infrared Spectroscopic Survey , 1998, astro-ph/9806270.

[31]  I. Simbotin,et al.  Quadrupole Transition Probabilities for the Excited Rovibrational States of H2 , 1998 .

[32]  A. Comastri,et al.  ASCA spectroscopy of the luminous infrared galaxy NGC 6240: X-ray emission from a starburst and a buried active nucleus , 1998, astro-ph/9802336.

[33]  K. Nandra,et al.  ASCA Observations of Seyfert 1 Galaxies. III. The Evidence for Absorption and Emission Due to Photoionized Gas , 1997, astro-ph/9708046.

[34]  P. Lira,et al.  The central regions of the Galaxy and galaxies , 1998 .

[35]  D. Kunze,et al.  What Powers Ultraluminous IRAS Galaxies? , 1997, astro-ph/9711255.

[36]  F. Macchetto,et al.  The Ionization Structure of the Narrow-Line Region of NGC 1068 and Its Relationship to the Extended Radio Emission , 1997 .

[37]  Martin P. Ward,et al.  Excitation Mechanism of Molecular Hydrogen Emission in NGC 6240 , 1997 .

[38]  R. Doyon,et al.  Spectroscopy of Luminous Infrared Galaxies at 2 Microns. II. Data for Galaxies with 11.2 ≲ log (LIR/L☉) ≲ 11.9 , 1997 .

[39]  Simon J. E. Radford,et al.  The Molecular Interstellar Medium in Ultraluminous Infrared Galaxies , 1996, astro-ph/9610166.

[40]  A. Tielens,et al.  Grain Shattering in Shocks: The Interstellar Grain Size Distribution , 1996 .

[41]  D. Sanders,et al.  LUMINOUS INFRARED GALAXIES , 1996 .

[42]  A. Tielens,et al.  X-Ray--irradiated Molecular Gas. I. Physical Processes and General Results , 1996 .

[43]  A. Poglitsch,et al.  Atomic Oxygen in Molecular Clouds? High-Resolution Spectroscopy of the [O I] 63 Micron Line toward DR 21 , 1996 .

[44]  M. Kaufman,et al.  Far-Infrared Water Emissions from Magnetohydrodynamic Shock Waves , 1996 .

[45]  L. Spinoglio,et al.  Multiwavelength energy distributions and bolometric luminosities of the 12 micron galaxy sample , 1995, astro-ph/9506139.

[46]  R. Doyon,et al.  Stellar velocity dispersion in ARP 220 and NGC 6240: Elliptical galaxies in formation , 1994 .

[47]  M. S. Oey,et al.  Atlas of quasar energy distributions , 1994 .

[48]  D. Lester,et al.  Stellar kinematics in the nucleus of NGC 6240: A massive galaxy revealed , 1994 .

[49]  R. Genzel,et al.  Near-infrared line imaging of NGC 6240 : collision shock and nuclear starburst , 1993 .

[50]  A. Poglitsch,et al.  The 158 micron forbidden C II line - A measure of global star formation activity in galaxies , 1991 .

[51]  George K. Miley,et al.  On the nature and implications of starburst-driven galactic superwinds , 1990 .

[52]  B. Draine,et al.  On the H2 line emission from NGC 6240 and other starburst galaxies , 1990 .

[53]  D. Hollenbach,et al.  Molecule Formation and Infrared Emission in Fast Interstellar Shocks. III. Results for J Shocks in Molecular Clouds , 1989 .

[54]  C. Telesco,et al.  The infrared structure and the origin of the starburst disk in NGC 1068 , 1988 .

[55]  M. Pérault,et al.  Small grains and IRAS colors , 1988 .

[56]  J. Graham,et al.  Infrared spectroscopy of the supernova remnant IC 443 , 1987 .

[57]  R. Wade,et al.  Detection of molecular hydrogen in two merging galaxies , 1984, Nature.

[58]  A. Dalgarno,et al.  The Quadrupole Vibration-Rotation Transition Probabilities of Molecular Hydrogen , 1977 .