---
author:
- 'V. Charmandaris'
- 'O. Laurent'
- 'E. Le Floc’h'
- 'I. F. Mirabel'
- 'M. Sauvage'
- 'S. Madden'
- 'P. Gallais'
- 'L. Vigroux'
- 'C. J. Cesarsky'
date: 'Received 1 March 2002 / Accepted 30 May 2002'
title: ' Mid-infrared observations of the ultraluminous galaxies , , and [^1]'
---

Introduction
============

It is currently widely accepted that the majority of the most luminous galaxies (L$_{bol}>10^{11}$L$_{\sun}$) in the local universe (z $<0.3$) are luminous in the infrared, and include the ultraluminous infrared galaxies (ULIRGs, L$_{\rm IR}>10^{12}$L$_{\sun}$) which emit the bulk of their energy at infrared wavelengths [@Houck1984; @Soifer1989; @Sanders1996 and references therein]. In those systems most of the infrared emission seems to originate from their dusty nuclear regions. Even though one of the principal heating mechanisms for the lowest luminosity ($\lesssim 10^{11}$ L$_{\sun}$) infrared galaxies is the stellar radiation field of young massive stars, it is still unclear if the star formation is also the dominant heating source for ULIRGs or whether one needs to invoke an active galactic nucleus (AGN) and its strong radiation field as the central engine responsible for the heating of the dust [see @Joseph99; @Sanders99]. The presence of large quantities 