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IR/sub-mm spectroscopy offers unique opportunities to probe the physical conditions (n[atoms], P, T, extinction, ionization state) in the various components of the ISM, because:

Table 1 summarizes IR tracers of the various ISM components. Clearly, IR spectroscopy is essential for studies of galaxy activity, though it requires a continuous coverage of the IR spectrum, possible only from space. While ISO allowed to invertigate spectroscopically nearby IR active galaxies, future missions (SIRTF, NGST, FIRST) will make possible similar studies for galaxies at any redshifts.

Table 1.Relevant components and line tracers of the ISM

Component Temperature Density Tracers and IR lines

Cold gas 10-100 K 1-1000 cm-3 H2, CO, PAH's
Diffuse HI 100-1000 K 1 cm-3 HI 21cm, [CII], [OI]
HII regions 1000-10000 K 3-300 cm-3 H$ \alpha$, [OII], [OIII]

5.1. The cold molecular gas

Looking at the mm/sub-mm spectral lines is the usual way to study the cold molecular gas, which typically includes the largest mass fraction of the ISM. The lines come from rotational and vibrational transitions of diatomic and polyatomic molecules.

The very many molecules observable allow to accurately sample the various regimes of $ \rho$, T and elemental abundance. Unfortunately, the most abundant molecule (H2) is not easily observed directly. It is seen in absorption in UV, or in the NIR roto-vibrational transitions at 2.121 and 2.247 µm. Only with mid-IR spectroscopy by ISO it was possible to observe the fundamental rotational lines at 17 µm (S[1]), 28.2 µm (S[0]), and 12.3 µm (S[2]) in NGC6946, Arp220, Circinus, NGC3256, NGC4038 / 39). These observations indicate very cool gas to be present with very high column densities (the transition probabilities of the lines are very low).

Because of the difficulty of a direct measure, the amount of molecular gas (H2) is often inferred from easier measurement of CO emission lines, assumed an H2/CO conversion. CO rotational transitions allow excellent probes of cold ISM in galaxies: the CO brightness temperature ($ \propto$ line intensity) is almost independent on z at z = 1 to 5, due to the additional (1 + z)2 factor with respect to the usual scaling with the luminosity distance (Scoville et al. 1996). CO line measurements have been performed for all IRAS sources in the Bright Galaxy Sample, the majority have been detected with single-dish telescopes. In the most luminous objects the molecular mass is 0.2 - 5 1010 M$\scriptstyle \odot$, i.e. 1 to 20 times the content of Milky Way. Typically 50% or more of this mass is found within the inner kpc from the nucleus, the molecular mass substantially contributing to the total dynamical mass (> 50% of Mdyn). Unfortunately, detecting CO emission by high-z galaxies has proven to be difficult (see below).

5.2. The cold neutral gas

The diffuse neutral ISM is commonly traced by the HI 21 cm line from ground-based observations. HI cooling, which is essential to achieve temperatures and densities needed to trigger SF, depends mainly on emission by the 158µm [CII] line, the 21 cm line and the 63µm [OI] line.

The 158 µm [CII] line is a major coolant for the diffuse neutral gas and a fundamental cooling channel for the photo-dissociation regions (PDR's), the dense phase interfacing cold molecular clouds with the HII or HI lower-density gas. Carbon is the most abundant element with ionization potential (11.3 eV) below the H limit (13.6 eV): CII atoms are then present in massive amounts in neutral atomic clouds. The two levels in the ground state of CII responsible for the $ \lambda$ = 158 µm transition correspond to a relatively low critical density ncrit $ \simeq$ 300 cm-3 [the density at which collisional excitation balances radiative de-excitation]: CII is excited by electrons and protons and cools down by emitting a FIR photon. The CII line intensity is also weakly dependent on T, hence a good measure for P. The [OI]145µm and 63µm lines are also coolants, though less efficient.

5.3. The ionized component of the ISM

Again, a number of lines from atomic species, covering an extremely wide range of ionization conditions, are observable in the far-IR. Their observations allow extensive analyses of the physical state of the gas. This, coupled with the modest sensitivity to dust extinction, provides the ideal tool to probe even the most compact, extinguished sites, e.g. in the inner galactic nuclei.

For a detailed physical investigation, line ratios sensitive to either gas temperature T or density n are used. To estimate electron density n one can use the strong dependence of the fine-structure line intensities for doublets of the same ion on n: one example are the [OIII] lines at 5007 Å, 52 µm and 88 µm. Similarly one can estimate T and the shape of the ionizing continuum.

Particularly relevant to test the spectral shape of the ionizing continuum are the fine-structure lines from photo-ionized gas, which allow to discriminate spectra of stellar and quasar origin. Low-ionization transitions typically strong in starbursts are [OIII]52 and 88, [SiII]34, [NeII]12.8, [NeIII]15.6, [SIII]18.7 and 33.4, while higher ionization lines in AGNs are [OIV]25.9 and [NeV]24. Table 2 reports a few of the most important IR ionic lines.

Table 2. The most important IR fine-structure lines. (a) Line intensity compared with the observed [CII]158µm for the prototypical starburst M82, when available, or predicted by Spinoglio & Malkan (1992) from a model reproducing the physical conditions in M82.

Species Excitation $ \lambda$ ncrit F/F[CII](a)
potential (µm) cm-3

OI - 63.18 5 105 1.4
OI - 145.5 5 105 0.06
FeII 7.87 25.99 2 106
SiII 8.15 34.81 3 105 2.6
CII 11.26 157.7 3 102 1
NII 14.53 121.9 3 102 0.37
NII 14.53 203.5 5 101 0.11
ArII 15.76 6.99 2 105 0.11
NeII 21.56 12.81 5 105 2.1
SIII 23.33 18.71 2 104 0.68
SIII 23.33 33.48 2 103 1.1
ArIII 27.63 8.99 3 105 0.23
NIII 29.60 57.32 3 103 0.31
OIII 35.12 51.82 5 102 0.74
OIII 35.12 88.36 4 103 0.66
NeIII 40.96 15.55 3 105 0.16
OIV 54.93 25.87 104 -

One important application of IR spectroscopy was by Genzel et al. (1998), to investigate the nature of the primary energy source in IR luminous galaxies (see Sect. 6.8).

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