Molecular gas in blue compact dwarf galaxies

Blue compact dwarf galaxies (BCDGs) are currently undergoing strong bursts of star formation. Nevertheless, only a few of them have been clearly detected in CO, which is thought to trace the 'fuel' of star formation, Hz. In this paper we present a deep search for CO J = 1 --> 0 and J = 2 --> 1 emission lines in a sample of eight BCDGs and two companions. Only two of them (Haro 2 and UM 465) are detected. For the other galaxies we have obtained more stringent upper limits on the CO luminosity than in previously published values. We could not confirm the previously reported 'detection' of CO for the galaxies UM 456 and UM 462. We analyse a possible relation between metallicity, CO luminosity and absolute blue magnitude of the galaxies. We use previously determined relations between X = N(H-2)/I-CO and the metallicity to derive molecular cloud masses or upper limits for them. With these 'global' X-CO values we find that, for the galaxies that we detect in CO, the molecular gas mass is similar to the HI mass, whereas for the non-detections, the upper limits on the molecular gas masses are significantly lower than on the HI mass. Using an LVG (large velocity gradient) model we show that X-CO depends not only on metallicity but also on other physical parameters, such as volume density and kinetic temperature, which raises the question about the validity of 'global' X-CO factors.