[CII] emission and star formation in late-type galaxies

A linear correlation between the ratio of the [CII(P-2(3/2) --> (2) P-1/2)] line intensity to the [(CO)-C-12(J : 1 --> 0)] line emission, I-CII/I-CO and the H-alpha equivalent width (EW) is found, over the range 2-71 Angstrom in H-alpha EW, for a sample of 21 late-type galaxies. The latter is comprised of an optically selected sample of 12 normal Virgo Cluster spiral galaxies with [CII] detections obtained by us with ISOLWS, plus nine late-type galaxies with higher star formation rates (SFRs), for which [CII] data and, especially, H-alpha EW data are available in the literature. As a result we infer I-CII/I-CO to be a reliable tracer of the current mass-normalized global SFR for non-starburst spiral galaxies. Moreover, the ratio of the [CII] line to the total far-infrared (FIR) continuum intensity, I-CII/I-FIR, is found to decrease from similar to0.5% to similar to0.1% with decreasing SFR which we propose is due to a '[CII]-quiet' component of I-FIR from dust heated by the general interstellar radiation field (ISRF). The more 'quiescent' galaxies in the sample have values of I-CII/I-CO different from those observed in 'compact' Galactic interstellar regions. Their [CII]-emission is interpreted to be dominated by diffuse regions of the interstellar medium (ISM). For normal 'star-forming' galaxies the diffuse component of the [CII] emission is estimated to account for at least 50% of the total