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Issue A&A
Volume 391, Number 2, August IV 2002
Page(s) 417 - 428
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20020784



A&A 391, 417-428 (2002)
DOI: 10.1051/0004-6361:20020784

Extended mid-infrared emission from VV 114: Probing the birth of a ULIRG

E. Le Floc'h1, V. Charmandaris2, O. Laurent1, 3, I. F. Mirabel1, 4, P. Gallais1, M. Sauvage1, L. Vigroux1 and C. Cesarsky5

1  CEA/DSM/DAPNIA Service d'Astrophysique, 91191 Gif-sur-Yvette, France
2  Cornell University, Astronomy Department, Ithaca, NY 14853, USA
3  Max Planck Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
4  Instituto de Astronomía y Física del Espacio, cc 67, suc 28. 1428 Buenos Aires, Argentina
5  European Southern Observatory, Karl-Schwarzschild-Str, 85748 Garching bei München, Germany

(Received 4 March 2002 / Accepted 22 May 2002 )

Abstract
We present our 5-16 $\mu$m spectro-imaging observations of VV 114 , an infrared luminous early-stage merger of two galaxies VV 114E and VV 114W, taken with the ISOCAM camera on-board the Infrared Space Observatory. We find that only 40% of the mid-infrared (MIR) flux is associated with a compact nuclear region of VV 114E, while the rest of the emission originates from a rather diffuse component extended over several kpc in the regions between VV 114E and VV 114W. This is in stark contrast with the very compact MIR starbursts usually seen in luminous and ultraluminous infrared galaxies. A secondary peak of MIR emission is associated with an extra-nuclear star forming region of VV 114W which displays the largest H $\alpha$ equivalent width in the whole system. Comparing our data with the distribution of the molecular gas and cold dust, as well as with radio observations, it becomes evident that the conversion of molecular gas into stars can be triggered over large areas at the very first stages of an interaction. This extended star formation along with the extreme nuclear starburst observed in VV 114E can easily lead to the heating of dust grains found in the tidally disturbed disks of the progenitor galaxies and subsequently traced via their MIR emission. The presence of a very strong continuum at the 5-6.5 $\mu$m range in the spectrum of VV 114E indicates that an enshrouded active galactic nucleus (AGN) may contribute to ~40% of its MIR flux. We finally note that the relative variations in the UV to radio spectral properties between VV 114E and VV 114W provide evidence that the extinction-corrected star formation rate of similar objects at high z, such as those detected in optical deep surveys, cannot be accurately derived from their rest-frame UV properties.


Key words: galaxies: active -- galaxies: starburst -- galaxies: ISM -- galaxies: individual: ARP 236 -- galaxies: individual: VV 114 -- infrared: galaxies

Offprint request: E. Le Floc'h, elefloch@cea.fr




© ESO 2002


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