[C II] emission and star formation in late-type galaxies

We demonstrate the existence of a reasonably tight linear correlation betwe en the ratio of the [C II (P-2(3/2)-P-2(1/2))] (lambda 158 mu m) line-struc ture cooling line intensity to the [(CO)-C-12 (J = 1 --> 0)] (lambda 2.6 mm ) line emission, I-CII/I-CO, and the Her (lambda 6563 Angstrom) equivalent width (EW), over the range 2-71 Angstrom in H alpha EW, for a sample of 21. late-type galaxies. The latter is composed of an optically selected sample of 12 normal Virgo cluster spiral galaxies with [C-II] detections obtained by us with LWS on board ISO, supplemented by nine galaxies with higher sta r formation rates (SFRs), for which [C-II] data and, in particular, H alpha EW data are available in the literature. We infer I-CII/I-CO to be a relia ble tracer of the current normalized, global SFR for non-starburst spiral g alaxies. Moreover, we fmd that the ratio of the [C-II] line to the total fa r-infrared (FIR) continuum intensity, I-CII/I-FIR, decreases from similar t o 0.5 to similar to 0.1 per cent with decreasing SFR, which we propose is d ue to a '[C-II]-quiet' component of I-FIR from dust heated by the general i nterstellar radiation field. The more 'quiescent' galaxies in the sample ha ve I-CII/I-CO different from those observed in 'compact' Galactic interstel lar regions. Therefore their [C-II] emission is interpreted to be dominated by diffuse regions of the interstellar medium. For normal 'star-forming' g alaxies the diffuse component of the [C-II] emission is estimated to accoun t for at least 50 per cent of the total.