Far Infrared Spectroscopy of Star-forming Galaxies: Expectations for the Herschel Space Observatory

ISO has enabled far-infrared spectroscopy of a variety of galaxies. Using the [CII] (158μm) and [OI](63μm) lines, we can characterize the physical conditions in the star-forming ISM. These observations also form the basis of our expectations for what the Herschel Space Observatory will observe for high redshift galaxies. While [CII] is suppressed in ULIRGs and normal galaxies with high dust temperatures, it is stronger than expected in metal poor galaxies by factors of a few. Young galaxies at high redshifts might be expected to be both metal poor and actively star-forming, leading to contrary expectations for the [CII] line strength. The best prediction for [CII] detection is derived by using the observed proportionality between [CII] and mid-IR emission from PAHs. Using the observed [CII]/7 μm ratio and number counts from ISO deep surveys we predict that HSO will be able to detect 100 sources/square-degree in the [CII] line.