CII] Emission and Star Formation in Late-Type Galaxies

Abstract

A linear correlation between the ratio of the[CII( $$^{\text{2}}$$ P $$_{3/2}$$ → $$^{\text{2}}$$ P $$_{1/2}$$ )] line intensity to the [ $$^{{\text{12}}}$$ CO(J:1 →0)] line emission, I $$_{{\text{CII}}}$$ /I $$_{{\text{CO}}}$$ and the $${\text{H}}_\alpha$$ equivalent width (EW) is found, over the range 2–71 A in $${\text{H}}_\alpha$$ EW, for a sample of 21late-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, $${\text{H}}_\alpha$$ EW data are available in the literature. As a result we infer I $$_{{\text{CII}}}$$ /I $$_{{\text{CO}}}$$ 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 $$_{{\text{CII}}}$$ /I $$_{{\text{FIR}}}$$ , is found to decrease from ∼0.5% to ∼0.1% with decreasing SFR which we propose is due to a `[CII]-quiet' component of I $$_{{\text{FIR}}}$$ from dust heated by the general interstellar radiation field (ISRF). The more `quiescent' galaxies in the sample have values of I $$_{{\text{CII}}}$$ /I $$_{{\text{CO}}}$$ 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.