Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

3.1.1. Molecular Spectroscopy

ISO has opened the realm of external galaxies for molecular infrared spectroscopy. Pure H₂ rotational emission lines have been detected in a wide range of galaxies, from normal to ultraluminous (e.g. Rigopoulou et al 1996a, 2000, Kunze et al 1996, 1999, Lutz et al 1999, Moorwood et al 1996, Sturm et al 1996, Valentijn et al 1996, Valentijn & van der Werf 1999a). The lowest rotational lines [28 µm S(0) and 17 µm S(1)] originate in warm (90-200 K; Valentijn & van der Werf 1999a, b) gas that constitutes up to 20% of the total mass of the (cold) molecular ISM in these galaxies (Kunze et al 1999, Rigopoulou et al 2000). High rotation lines [S(5), S(7)] and rovibrational lines come from a small amount of much hotter gas ( 1000 K). It is likely that the H₂ emission arises from a combination of shocks, X-ray illuminated clouds (in AGNs), and photodissociation regions. OH, CH, and H₂O lines are seen in emission or absorption (or a combination of the two) in a number of gas-rich starburst, Seyfert, and ultraluminous galaxies (Colbert et al 1999, Bradford et al 1999, Fischer et al 1997, 1999, Spinoglio et al 1999, Skinner et al 1997, Kegel et al 1999). The observed OH emission can generally be accounted for by infrared pumping through rotational and rovibrational transitions in sources with a warm infrared background source. The ISO SWS/LWS spectroscopy solves the long-standing puzzle of the pumping mechanism for intense radio mega-maser OH emission in luminous infrared galaxies (Baan 1993). In the mega-maser galaxies Arp 220, IRAS 20100-4156, and 3Zw35, absorption is seen in cross-ladder rotational transitions (Figure 4); this strongly supports the idea of rotational pumping models for the masers (Skinner et al 1997, Kegel et al 1999). The OH masers must therefore be located in front of the far-IR continuum source on kpc scales. The OH mega-masers originate in the extended starburst region, not in the circumnuclear environment (Skinner et al 1997). The far-IR molecular line spectra of the ultraluminous galaxies Arp 220 (Figure 4) and Mrk 231 are remarkably similar to those of the galactic center molecular cloud complex/star-forming region SgrB2 (Figure 4, Cox et al 1999, Fischer et al 1997, 1999), which indicates that the properties of the entire molecular ISM in Arp 220 (scale 1 kpc) are comparable to those of this dense galactic cloud [N(H₂) 10²⁴ cm^-2, n(H₂) ~ 10⁴ cm^-3].

Figure 4. Comparison of the LWS spectrum of Arp 220 and the Galactic star-forming region/molecular cloud SgrB2 (Fischer et al 1999, Cox et al 1999). The spectrum of SgrB2 has been divided by a factor of 115 and shifted to the redshift of Arp 220. The main molecular absorption bands are identified, along with the [OI] and [CII] fine structure lines.