Astron. Astrophys. 324, 41-50 (1997)

1. Introduction

Despite numerous studies of starburst galaxies, many questions remain about the starburst phenomenon, its triggering and evolution. Gravitational interactions and mergers seem to play a major role in triggering starbursts, but violently interacting galaxies are not necessarily the seat of starbursts (Bushouse 1986) and most starbursts seem to be isolated (Contini 1996, Coziol et al. 1996b). Numerical simulations have shown that the bar plays a major role in this process, by efficiently funneling molecular clouds toward the inner few kiloparsecs of galaxies (Noguchi 1988, Friedli & Benz 1993). This is confirmed by radio continuum observations (e.g. Puxley et al. 1988). However, the link between bar and far infrared luminosity (which traces young massive stars) in starburst galaxies remains controversial (Hawarden et al. 1996).

Molecular clouds obviously play a crucial role in the process of star formation, and their properties have been extensively studied, via millimeter observations of the CO molecule. A strong far infrared (FIR) luminosity has generally been a successful criterion for detection of the molecule in external galaxies, confirming the link between the two indicators of star formation. A CO-N (H₂) conversion factor has been proposed (Strong et al. 1988) and validated by observations of normal as well as starburst galaxies (e.g. Sage et al. 1990); further studies have shown that it depends on metallicity (Wilson 1995, Arimoto et al. 1996). But its validity has recently been questioned (e.g. Nakai & Kuno 1995).

It has recently been emphasized that CO only traces low-density molecular gas and several searches for dense (n(H₂)   cm^-3) molecular clouds in normal and starburst galaxies, generally selected to be strong CO emitters, via detection of HCN, CS, , and other molecules, have been initiated (e.g. Mauersberger et al. 1989, Nguyen-Q-Rieu et al. 1992, Helfer & Blitz 1993, Aalto et al. 1995). It turns out that bulges of normal as well as starburst galaxies contain large quantities of dense gas, and that a threshold in the surface density of dense gas does not seem to be required for violent star formation (Helfer & Blitz 1993).

In view of these mixed results, we have adopted a global view on starbursts in galaxies, based on multi-wavelength observations of a large and homogeneous sample of barred starburst galaxies. Such an approach should enable us to establish quantitative relationships between the properties of starbursts (age, star formation rate and initial mass function), the neutral (atomic and molecular) gas content and the morphology of the host galaxies. It has in fact already given rise to new and original results (Contini et al. 1995, Contini 1996, Contini et al. 1997, Coziol et al. 1996a).

In this paper, we combine millimetric observations of the dense molecular gas with optical images and long-slit spectroscopy of a few examples of very young starbursts galaxies, namely Wolf-Rayet galaxies, in order to investigate the properties of the dense gas of these galaxies in relation to their optical properties. The images provide morphological and photometric information on the central regions and the bar, and the spectra are used to determine the starburst ages and star formation rates as well as an independent estimate of the column density of H₂. A preliminary report on this research project has been published by Contini et al. (1996).

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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