HI, H-2 AND STAR-FORMATION IN SPIRAL GALAXIES IN THE REGION OF THE COMA SUPERCLUSTER

We present a study of the relationships between the gas content and st ar formation activity in a sample of 101 disc galaxies located in the region of the Coma supercluster. We use as star formation indicators t he optical color indices, the Her equivalent width, the radio continuu m and far-infrared emissions, while the gas content of the galaxies is measured by the H I line emission at 21 cm for atomic hydrogen and in dicated by the CO(1-0) line at 2.6 mm for molecular hydrogen. The CO(1 -0) data come from previous observations using the IRAM 30 m, the Onsa la 20 m, and the NRAO 12 m telescopes. We have found that molecular ga s constitutes only one fifth of the gas content of the sample galaxies which are not strongly H I-deficient. Late-type galaxies (Sc) are sli ghtly richer in both atomic and molecular gas than early-type ones (Sa ), but the molecular fraction does not vary significantly between Sa a nd Sc. The average total gas surface density, including upper limits b ut excluding H I-deficient objects, amounts to 10((6.88+/-0.04)) M. kp c(-2). In this sample, the H I-deficient galaxies are similar in their CO emission to the H I-normal galaxies belonging to the periphery of the cluster and globally gas-deficient. It is thus likely that the mis sing H I has not been converted into H-2, but rather removed from the galaxy. The above conclusions do not depend on the quantity chosen for the normalization of the observed quantities, optical area or H-band luminosity. On the other hand, using one or the other scaling gives a fairly different picture of the relationships between the gas contents and the star formation indicators. The H-2 surface density is well co rrelated with the far-infrared and radio continuum surface brightness, but shows no trend with either the color indices U-B and B-TV or the Ha equivalent width. On the other hand, the H I surface density is wel l correlated with these optical star formation tracers, but neither wi th the radiocontinuum and far-infrared surface brightnesses, nor with the H-2 surface density. The various star formation tracers show a bet ter correlation with the H-2 content when it is normalized to L(H). Th ey are however still correlated with the H I and total gas contents, e xcept for the radio continuum emission. This discrepancy with what is found when scaling with the optical area suggests that the lack of cor relation of the H-2 surface density with the optical star formation tr acers cannot be entirely attributed to the uncertainties on the comput ation of Hz masses, or to extinction effects in the optical data.