5.2. Fine-Structure Lines
Far-infrared fine structure lines, [CII] 157.7 µm and [OI] 63.2 µm in particular, have long been used for estimating density and radiation intensity in photo-dissociation regions (PDR) (e.g. Hollenbach & Tielens 1997), which are the interfaces between HII regions and molecular clouds. These dense (n ~ 10-105 cm-3 or more), warm ( T ~ 100-300 K), neutral media are preferentially cooled by [CII] because carbon is abundant, easy to ionize (IP = 11.26 eV), and easy to excite ( E/k ~ 90 K). At T 200 K and n 105 cm-3, [OI] takes over as the main coolant, with its higher excitation threshold ( E/k ~ 224 K). In PDRs, both transitions are excited predominantly by electrons which have been extracted from dust particles by ultraviolet photons usually assumed to have 6eV. This well known and studied photo-electric effect (Field, Goldsmith & Habing 1969; Hollenbach & Tielens 1997) is estimated to have a yield 10-2 in an ISM illuminated by starlight. Traditionally, various line ratios and line-to-continuum ratios are used to constrain the main parameters of PDR regions, by comparison to calculations from models of slab PDRs. In such models, the emission is integrated along a line of sight sampling the PDR, starting at the HII region front, through the molecular cloud, and on towards the molecular cloud core, to the point where emission becomes negligible.
Just as [CII] and [OI] are important coolants of the neutral ISM, so are [NII] 121.9 µm, [OIII] 88.4 µm and 51.8 µm, and [NIII] 57.3 µm significant for HII regions. While ISO collected substantial data on these lines, they will not be discussed in any detail in this review because little has been published yet on this topic. It should be kept in mind however that [CII] can arise in HII regions as well as PDRs, and that complicates the interpretation of line ratios. Similarly, the continuum emission from dust will arise from a variety of media, complicating the interpretation of line-to-continuum ratios (Section 5.3).
ISO-LWS (Clegg et al. 1996) has provided a wealth of data, whose interpretation is creating controversy and challenging theoretical models. Using the Normal Galaxy Key Project sample, Malhotra et al. (1997) showed that while two thirds of normal galaxies have L(CII) / L(FIR) in the range 2-7 x 10-3, this ratio decreases on average as the 60-to-100 µm or the L(FIR) / L(B) ratios increase, both indicating more active star formation (Figure 9). The same CII deficiency is also observed in ultra-luminous infrared galaxies such as Arp 220 (Luhman et al. 1998). Malhotra et al. linked this decrease to elevated heating intensities, which ionize grains and thereby reduce the photo-electric yield. They discussed other possible causes, such as self-absorption of [CII], heating by non-ionizing stars, or the influence of an AGN. Optical depth effects were dismissed because no deficiency trend is observed in [OI] even though it is expected to have greater optical depth than [CII]. The possibility of non-ionizing stars was dismissed since it required the most unlikely scenario that such stars would dominate the heating systematically in the most actively star forming galaxies, including objects such as Arp 220 (Fischer et al. 1999). Finally, since the normal galaxy sample was selected to avoid AGN, the latter are unlikely to be the sole reason behind the [CII] deficiency trend. Fischer et al. (1999) and Malhotra et al. (1999) give updated discussions of this topic, while Lord et al. (1999) and Unger et al. (1999) discuss PDR properties in NGC 4945 and Cen A.
Figure 9. The CII deficiency in active star forming galaxies from Malhotra et al. (1997). The ratio of [CII] to FIR luminosity drops as the star formation activity and interstellar heating intensify, as measured either by R(60, 100) (left-hand side panel), or by the infrared-to-visible light ratio (right-hand side panel). See also Malhotra et al. 1999.
The relation of the [CII] line luminosity to the total star formation rate in a galaxy has been debated just as vigorously as other infrared observables (Stacey 1991). One of the outstanding questions there has been the importance of low density HII regions as a source of [CII] emission. Stacey et al. (1999) present detailed maps of M 83 in several fine-structure lines, and address this question directly, estimating that 27% of the total emission may well originate in that diffuse component. This same contribution by diffuse PDRs is invoked by Pierini et al. (1999) in explaining the high [CII]-to-CO ratios in quiescent Virgo cluster galaxies. Using the same Virgo galaxies data, Leech et al. (1999) report a trend of decreasing [CII]/FIR ratios as galaxies become less active in star formation. Combining the results of Leech et al. with those of Malhotra et al. leads to a picture where [CII]/FIR rises by an order of magnitude as galaxies move away from complete quiescence, reaches a broad maximum for normal galaxies actively forming stars, then decreases again by more than an order of magnitude as galaxies begin to approach the extreme properties of starbursts.