8.1. Introduction. Interplay between dust and molecules in galaxies
The main topic of this review is small molecules in the interstellar gas of galaxies. However, the overall molecular material, excluding H2, is distributed with comparable masses, between the gas, dust mantles and in intermediate-size aromatic particles, mostly Polycyclic Aromatic Hydrocarbons (PAHs). The interstellar gas, dust and PAHs belong indeed to the same world, the ecosystem of the interstellar medium, with constant exchanges and interplay. A detailed discussion of dust properties and its molecular compounds in galaxies is outside our scope. However, we have stressed the importance of the exchanges with dust for the gas chemical composition through accretion, surface reactions and desorption. PAHs may play a significant role in the chemistry of small carbon-based molecules such as C2H or C3H2. They cannot be absent from this general panorama of molecules in galaxies, but they will be discussed briefly because their problematics is quite different from small molecules, somewhat midway between molecules and dust.
Dust plays a cornerstone role in the construction of molecular material in galaxies through H2 formation. Through subsequent reactions, the latter is at the origin of most of molecular bonds in the gas phase. We have seen the resulting paramount importance of the various metallicity of galaxies for their molecular abundances in H2, CO and other molecules. Efficient dust desorption is a more direct major factor which shapes many molecular abundances in star-forming media such as starburst galaxies. Dust accretion is a universal process which depletes molecular abundances in the cold dense gas. It is probable that all these processes depend not only on the dust abundance, but also on its actual composition which depends on the various galactic metallicity, environment and evolution. However, we still lack detailed evidence for such effects.
Anyway, dust chemistry is intimately linked to that of the molecular gas, and it remains a central issue for understanding the various galaxies and their evolution. Dust is essential in many key processes in galaxies and the information we can get about their various regions (Section 2). Its abundance, resulting from the complex cycle of formation-growth-erosion-destruction, is thus crucial in various environments of metallicity, density and UV intensity. While most dust processes are still poorly known in the local interstellar medium, it is a real challenge to address them in the extreme conditions of the world of galaxies, such as galaxy formation and merging, or the vicinity of AGN.
The biggest PAHs and their small complexes partially belong to the world of dust, and may indeed encompass the so called `Very Small Grains' (Désert et al. 1990). More generally, there are certainly continuous exchanges between PAHs and carbonaceous dust grains, through accretion of PAHs, and release of PAHs by shattering.in grain-grain collisions. Despite the stability of the aromatic hexagonal cycle, small aromatic molecules such as benzene (C6H6) are hardly detected in the local interstellar medium (Cernicharo et al. 2001), and not at all in external galaxies, while the only widespread small cycle molecule is cyclic c-C3H2. On the other hand, PAHs with ~ 50-100 atoms or more play a major role in the interstellar medium of the Milky Way and most galaxies. They are very different from the bona fide small interstellar molecules with 10 atoms discussed in this review. Their size and number of vibration modes make their physics quite different from the latter. In addition, if their general nature and even size range are well established, not a single specific PAH molecule has ever been identified in the interstellar medium. It is neither known whether observed PAHs contain a significant amount of other atoms than C and H, such as nitrogen. However, this review would not be complete without some mention of PAHs as essential compounds of the interstellar medium of galaxies, so close to regular molecules by their nature and chemical exchanges. We will now briefly consider the state of our knowledge about PAHs in galaxies, especially from the results of ISO and Spitzer.
8.2. Mid-infrared emission of PAHs in galaxies
Mid-infrared spectra of most galaxies, as a variety of Galactic sources including HII regions, post-AGB stars, planetary nebulae, young stellar objects (YSOs), are dominated by a family of emission features at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 µm (Fig. 3), identified as aromatic infrared bands (AIBs) of C-H and C-C vibration modes (Duley & Williams 1981). They are generally attributed to Polycyclic Aromatic Hydrocarbon (PAH) molecules (Léger & Puget 1984, Allamandola et al. 1985, Puget & Léger 1989, Allamandola et al. 1989), although the exact molecular identification of the carriers remains unknown. The intensity of these features suggests that PAHs are the most abundant complex polyatomic molecules in the interstellar medium, accounting perhaps for as much as 10-20% of all carbon in a galaxy like ours. The physics of this infrared emission, induced by fluorescence after UV absorption, has been discussed in many places [in addition to the references above, see e.g. Sellgren (1984), Omont (1986), Peeters et al. (2002, 2004), Rapacioli et al. (2005), Tielens (2005) and references therein]. This emission is generally attributed to small aromatic hydrocarbon species, transiently heated by the absorption of a single far-UV photon, because the band ratios, reflecting the very high temperatures of the emitters in cold environments, are practically independent of the UV intensity. This requires the emitting species to be typically of the order of 10-30 A (~ 50-1000 carbon atoms, Sellgren, 1984, Draine & Li 2006). PAHs are expected to exist in various proportion of the different ionization states: neutral, singly positively (cations) or negatively charged (anions) (see e.g. Bakes et al. 2001a, b; Tielens 2005). In strong UV environments they have a good chance to be positively ionized and more or less dehydrogenated. Indeed, significant variations of the spectral system of AIBs have been observed in photodissociation regions, attributed to changes in the proportion of cations vs neutral PAHs. There is also evidence for colder components in the observed spectra (Rapacioli et al. 2005). The latter probably correspond to larger aromatic particles with several hundred carbon atoms, which could be a part of the commonly called `very small grains' (VSG, Désert et al. 1990). It has been proposed (Rapacioli et al. 2005) that such particles with cold AIBs could be, at least partially, PAHs clusters, and that they could be photoevaporated into free-flying PAHs in strong UV radiation fields.
Figure 3. (reproduced from Figs. 1 & 2 of Haas et al. 2005). ISO mid-infrared observations of the Antennae galaxy pair, which is at an early stage of galaxy collision: PAH aromatic bands and exceptional H2 line emission. (left) Optical three colour image of the Antennae with contours of the continuum-subtracted H2 S(3) line emission superimposed. The H2 emission extends between the two nuclei of NGC 4038/4039. (right) Mid-infrared spectrum of the Antennae, obtained with the ISOCAM-CVF mode. It has been derived from a 2' × 2' region, encompassing both nuclei and the entire overlap region. The prominent emission features are those of polycyclic aromatic hydro-carbons (PAHs) and Ne lines. Three H2 lines are marked. While the H2 S(5) and H2 S(2) lines are blended with [ArII] = 6.99 µm and [NeII] = 12.8 µm, respectively, the H2 S(3) line can unambiguously be detected above the continuum (dotted).
Beyond serving as simple PAH indicators, the observation of mid-infrared (MIR) AIBs in galaxies can be used as redshift indicators, and as tracers of elemental and chemical evolution, and of environmental conditions. After early detections of AIBs in galaxies from the ground (Gillett et al. 1975, and e.g. Aitken & Roche 1985 and references therein), comprehensive surveys of MIR emission of local galaxies are among the most important programmes of the space observatories ISO (Kessler et al. 1996) and Spitzer (Werner et al. 1984 (see e.g. Fig. 3). The demonstration of the universality of almost pure AIB spectra in the MIR range for `normal' star-forming disk galaxies in the local universe is one of the major results from ISO (Mattila et al. 1999, Laurent et al. 2000, Lu et al. 2003 and references therein). In particular, the atlas of MIR spectra of 45 disk galaxies by Lu et al. (2003) shows that most of them are completely dominated by AIBs and strikingly similar to each other and to the predominant type of MIR pattern in the interstellar medium of our own Galaxy. The combined luminosity of the AIBs in the region 5.8-11.3 µm is typically ~ 10-20% of the FIR luminosity in such spiral galaxies. However, there is a trend to decrease this ratio LAIB / LFIR in IR starburst galaxies, LIRGs and ULIRGs. This is interpretated as the result of the destruction of the PAHs by the hard UV radiation of HII regions, maybe complemented by shocks or reabsorption of mir-IR radiation. Such a trend is still enhanced in the central regions of AGN where the AIBs disappear with respect to the strong emission of the hot dust of the AGN torus, so that it is a key element for discriminating AGN-dominated from starburst-dominated galaxies (Laurent et al. 2000). In nearby Seyferts, spatially resolved mid-infrared spectroscopy with ISO suggested that the absence or suppression of AIB emission is due to the fact that the dust is predominantly heated by processes related to the central AGN (e.g. in NGC 1068: Le Floc'h et al. 2001, Circinus: Moorwood, 1999, NGC 4151: Sturm et al. 1999, Mrk 279: Santos-Lleo et al. 2001), with the additional possibility that the PAHs are destroyed by the AGN X-rays (Voit 1992).
Lu et al. (2003) have also confirmed that the weak NIR excess continuum, which has a color temperature of ~ 103 K (Sellgren 1984), is well correlated with AIB emission, confirming that they are produced by similar mechanisms and similar (or the same) material. But the precise origin of the NIR excess is still unknown.
The space observatory Spitzer has already produced a wealth of results about AIBs in local galaxies. Dedicated large programmes, such as the Legacy Project SINGS (Kennicutt et al. 2003, Smith et al. 2007), IRS-GTO (Houck et al. 2005), MIPS-GTO (Gordon et al. 2006), are studying MIR AIBs in a large sample of galaxies (Draine et al. 2007, often with good angular and spectral resolution, allowing their use for the diagnostic of the interstellar medium. For instance, it is confirmed that their emission has an extension comparable to the diffuse interstellar medium (see e.g. Engelbracht et al. 2006 for M 82), and that, in starburst regions, the bulk of AIB emission is not associated with spectacular superclusters of massive star formation, but with the diffuse medium (van der Werf & Snijders 2006). The complex behaviour of PAH emission in the context of AGN is also further documented (e.g. Smith et al. 2007, Dale et al. 2007, Lutz et al. 2007). Another important result is the demonstration of the spectacular dependence on metallicity of the 8µm to 24µm flux density ratio in starforming galaxies, observed in systematic studies by Engelbracht et al. (2005). This ratio, related to that of PAHs to VSGs, drops by almost a factor 10 when the metallicity decreases from 0.5 to 0.3 solar metallicity. More generally, the cause of the low abundance of PAHs in low metallicity galaxies (see also Hogg et al. 2005, Wu et al. 2005, Smith et al. 2007, Jackson et al. 2006) is probably linked to that of the lack of dust and of all molecules.
One major breakthrough of Spitzer is to provide MIR spectra of AIBs, and thus direct evidence of PAHs, in starburst galaxies up to z ~ 3 (Houck et al. 2005, Yan et al. 2005, 2007, Lutz et al. 2005a, b, Weedman et al. 2006a, b, Desai et al. 2006, Armus et al. 2006, Teplitz et al. 2007, Sajina et al. 2007). The majority of the objects where weak AIBs are visible, are AGN-dominated. However, several of the reported MIR spectra are almost pure AIBs and the sources are thus clearly starburst-dominated, with best spectral templates similar to regular or warmer ULIRGs. Most of the z ~ 2 objects ( 40) where AIBs have been reported, have an infrared luminosity LIR 1013 L, and are thus of the class of hyper-luminous infrared galaxies. Some of them, mostly starburst-dominated, had been detected at mm or submm wavelength (Lutz 2005b, Lonsdale et al. in preparation). They are thus sites of extreme star formation and represent a key phase in the formation of massive galaxies. Such results demonstrate the potential of using MIR AIBs to probe optically faint and infrared luminous high-z populations, and in particular to directly determine their redshifts.
8.3. Diffuse Interstellar Bands (DIBs) in galaxies
We end this section about aromatic molecules in galaxies with mentioning probable, albeit unproved, cousins of PAHs, the unidentified carriers of the `diffuse interstellar bands' (DIB). The latter are a series of broad absorption lines distributed between 4000 A and 13000 A, ubiquitously observed along sight-lines in the Milky Way which probe mostly the diffuse interstellar medium (e.g. Herbig 1995). Although several hundred DIBs are now known, and the most conspicuous ones have been observed since the dawn of interstellar spectroscopy (e.g. Merrill et al. 1934), the identification of their carriers has resisted intensive efforts during more than half a century. They are very likely gas phase carbonaceous compounds, maybe associated with metals. Possible candidates include, but are not limited to, PAHs, fullerenes, carbon nanotubes and carbon chains (e.g. Zhou et al. 2006 and references therein). Their eventual identification and understanding their physics and chemistry might provide us with a new tool for the diagnosis of the conditions in the interstellar medium.
The extension of comprehensive studies of DIBs to external galaxies is now possible with recent advances in instrumentation and telescope capabilities. They offer unique possibilities for studying very different conditions from those observed in the Milky Way, with respect to metallicity, UV intensity, gas-to-dust ratio, dust extinction properties such as the 2175 A `bump' in dust extinction, etc. Most significant works include: i) Extensive observations of sight-lines of the Magellanic Clouds (e.g. Ehrenfreund et al. 2002, Cox et al. 2005; Welty et al. 2006). On average, the DIBs are weaker by factors of almost 10 (LMC) and about 20 (SMC), compared to those typically observed in Galactic sight-lines with similar N(HI), presumably due to the lower metallicities and stronger radiation fields in the Magellanic Clouds (Welty et al. 2006). ii) Observations of several local galaxies (e.g. Sollerman et al. 2005), and especially intense starburst galaxies by Heckman & Lehnert (2000) who found that the DIBs are there remarkably similar to those in our Galaxy, while both the UV strength and the gas density are much larger than in the diffuse interstellar medium of the Milky Way. iii) The detection of several DIBs in a Damped Lyman- system at cosmological distance with zabs = 0.524 (York et al. 2006).
8.4. Molecular infrared spectral features in extragalactic dust
The molecular compounds embedded in dust grains, especially in ices, are not directly within the scope of this review of free flying molecules in galaxies. They must nevertheless be mentioned because of their relation with gaseous molecules, both as direct result of their accretion onto grains, and, more importantly, as sources of complex gaseous species in desorption of grain mantles (Section 2.4, and e.g. Tielens 2005). Verma et al. (2005) have reviewed the various ISO results about the first extragalactic detection of absorption features due to ices (H2O, CH4 and XCN) present in cold molecular components of starbursts, Seyferts and predominantly ULIRGs. For instance, in a heterogeneous sample of 103 active galaxies with high signal-to-noise mid-infrared ISO/SWS spectra, approximately 20% display absorption features attributed to ices (Spoon et al. 2002). While ice features are mostly weak or absent in the spectra of starbursts and Seyferts, they are strong in ULIRGs. The absorption dominated spectrum bears strong similarities to the spectra of embedded protostars (see e.g. Keane et al. 2001, Tielens 2005), and the depth of absorption features implies deeply embedded sources as observed with Spitzer (Fig. 4, Spoon et al. 2006, 2007), and even in the AGN (see Section 11.2).
Figure 4. (reproduced from Figs. 2 of Fosbury et al. 2007 and Spoon et al. 2006). Spitzer-IRS spectra of various classes of ULIRGs, from (featureless) AGN dominated (top), to deeply embedded (bottom). In addition to prominent features from the PAH family at 6.2, 7.7, 8.6 11.3 and 12.7 µm, and silicate absorption at 10 and 18 µm, weaker absorption becomes apparent in some sources: water ice at 5.7 µm, hydrocarbons at 6.85 and 7.2 µm, and even warm CO gas at 4.6 µm when the redshift is large enough to bring the IRS spectral coverage down to rest frame 4 µm (Spoon et al. 2006, Spoon et al. 2007)