5.3. Interpreting the PDR Lines
What could be behind this behavior? At the quiescent end, the relative lack of ionizing photons in the stellar spectrum could naturally explain the low values of [CII]/FIR. UV-poor heating would on one hand lower CII abundance, and on the other hand yield less energetic photo-electrons, so a less energetic CII excitation would be expected. At the starburst end, the extreme excitation conditions could generate lower photo-electric efficiency as proposed by Malhotra et al. (1997), and as detailed in the previous section.
While that picture is physically reasonable, there is evidence however favoring a different interpretation at least at the high excitation end of the sequence, namely that the relative drop in [CII] is due to a decrease in the concentration of the grains crucial for the photo-electric effect. It is well known (e.g. Hollenbach & Tielens 1997) that the smallest grains such as the carriers of AFE are responsible for the bulk of photo-electric yield in a PDR. Depletion of these carriers would decrease the coupling between radiation field and gas, and lead to weaker fine-structure line emission compared to total dust re-radiation. The evidence in question is that unlike [CII]/FIR, the ratio of [CII] to AFE flux does not decrease in normal galaxies with increasing activity (Helou et al. 2000). This suggests that the emissions from [CII] and AFE originate in the same regions of the ISM, so their ratio is dictated by the physics of the photo-electric effect and remains constant. On the other hand, [OI] and FIR originate from distinctly warmer and denser regions, so that the changing ratio [CII]/FIR, just like [CII]/[OI], reflects the systematic shift in density distribution of the ISM as galaxies approach star-burst conditions.
At the high end of activity, the empirical evidence for decreased AFE carriers is quite clear (Helou, Ryter & Soifer 1991; Genzel et al. 1998), and the evidence is strong that they are destroyed by the intense UV radiation from massive stars. There is less empirical evidence however for a systematic lack of Aromatic grains at the low end of the excitation sequence. Perhaps the most interesting hints come from the detailed investigation of the mid-infrared emission by Césarsky et al. (1999) in M31. It is still a matter of debate whether AFE carriers condense out on larger grains and are re-extracted by exposure to UV radiation (Boulanger et al. 1990), or whether the AFE carriers emerge only as a result of the first photo-processing by UV light of newly formed amorphous carbon grains (Pagani et al. 2000).
Most of the interpretation of [CII] and [OI] line fluxes is done in the context of PDR theories and models, which provide the most powerful tools available. It should be recalled however that this context assumes implicitly that all observed fluxes in line and continuum originate in PDRs. The broadest definition of PDR, usually supported in model calculations, includes any non-ionized region heated by stellar photons. Thus defined, PDRs do provide all [OI] emission from galaxies, but not all [CII] or dust continuum. Indeed, both of the latter can have contributions from HII regions of all descriptions, from the compact to the diffuse, extended low density variety. Additional flux of either [CII] or continuum will throw off the ratios based on which PDR models derive density and heating intensity. One can mitigate the problem by estimating HII regions contributions based on the [NII] line for instance, and adjust the [CII] flux accordingly (Malhotra et al. 2000. See also Section 7.1).