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

Abstract

We demonstrate the existence of a reasonably tight linear correlation between the ratio of the [CII (^2P_3/2-^2 P_1/2)] (lambda 158 mu m) fine-structure cooling line intensity to the [^12CO (J=1 -> 0)] (lambda 2.6 mm) line emission, I_CII/I_CO, and the H alpha (lambda 6563 A ) equivalent width (EW), over the range 2-71 A 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 [CII] detections obtained by us with LWS on board ISO, supplemented by nine galaxies with higher star formation rates (SFRs), for which [CII] data and, in particular, H alpha EW data are available in the literature. We infer I_CII/I_CO to be a reliable tracer of the current normalized, global SFR for non-starburst spiral galaxies. Moreover, we find that the ratio of the [CII] line to the total far-infrared (FIR) continuum intensity, I_CII/I_FIR, decreases from ~ 0.5 to ~ 0.1 per cent 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. The more `quiescent' galaxies in the sample have I_CII/I_CO different from those observed in `compact' Galactic interstellar regions. Therefore their [CII] emission is interpreted to be dominated by diffuse regions of the interstellar medium. For normal `star-forming' galaxies the diffuse component of the [CII] emission is estimated to account for at least 50 per cent of the total.