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.