Molecular gas, the interstellar medium, and star formation in S0 and SA galaxies

Abstract

We present the results of a survey for CO J = 1 -> 0 emission from early-type disk galaxies--S0's, S0/a's, Sa's--in order to study their cool interstellar media and the global properties of their star formation. Of the 13 nearly normal S0's, we positively detected six and, marginally, a seventh. We also detected five, and possibly a sixth, of the seven S0/a or Sa galaxies in our sample. As have other investigators, we find a wide range of characteristics for the interstellar medium (ISM) in S0 and S0/a galaxies. There is a tendency for peculiar galaxies in our sample to have relatively stronger CO line strengths than do the more normal galaxies of about the same visual luminosity. We find average values for L_co_/M(H I) and the abundance of the far-infrared-emitting dust relative to the interstellar gas to be roughly similar to those derived for late-type spirals. We would expect neither if the interstellar media within the galaxies in our survey have exclusively an external source. Although some S0's have a total gas abundance that is comparable to that found for the spirals, typical fractional gas masses in these galaxies are about an order of magnitude less than those for Sb or Sc spirals. On average, the molecular masses for the early-type disk galaxies in our survey are comparable to those of the atomic gas, although there is wide variation in M(H_2_)/M(H I) with weak evidence for variation in the ratio with Hubble type. We therefore conclude that morphological type is not the sole factor in determining the global molecular-to-atomic gas mass ratio. We repeat our warnings about the interpretation of the far-infrared emission from these galaxies and adopt observations of the visual emission lines as the best tool for studying massive star formation. For the four galaxies in our survey for which such data exist, the rate of star formation is about an order of magnitude less than that in late-type spirals of the same visual luminosity. The efficiency of star formation, however, is similar in this small sample of galaxies. We conclude, therefore, that spiral structure is largely irrelevant to the global rate and efficiency of star formation in galaxies. X-radiation has been detected from some of the objects in our sample. If this emission arises from extended hot gas rather than from discrete sources, we derive a cooling rate for the gas that is comparable to the rate of replenishment due to mass loss from evolved stars. Both rates, however, are considerably below the estimated star formation rate, which indicates that the present-day ISM was not created entirely by material lost by dying stars. For most of the galaxies in our survey, we conclude that the ISM is largely a remnant from earlier epochs of star formation, enriched by evolved stars.