Far-infrared fine structure lines, especially [CII] 157.7µm and [OI] 63.2 µm, have long been used for estimating density and radiation intensity in photo-dissociation regions (PDR) (e.g. Hollenbach & Tielens 1997). ISO has provided for this topic a wealth of data, whose interpretation is creating controversy and challenging theoretical models. Malhotra et al. (1997) showed that while two thirds of normal galaxies have (CII) / (FIR) in the range 2-7 x 10-3, this ratio decreases on average as the 60-to-100 or the (FIR) / (B) ratios increase, both indicating more active star formation (Figure 3). They linked this decrease to elevated heating intensities, which ionize grains and thereby reduce the photo-electric yield.
Figure 3. The CII deficiency in active star forming galaxies from Malhotra et al. (1997).
The same CII deficiency is also observed in ultra-luminous infrared galaxies (Fischer et al. 1999; Luhman et al. 1998), who favor optical depth effects as the origin of the effect. This is hard to reconcile however with a similar deficiency effect occurring for [NII] 121.9 µm as well, but not for [OI] 63.2µm. Detailed discussion is found in Malhotra et al. 1999, Fischer et al. 1999, or Luhman et al. 1999.