2.5. Ultraluminous infrared galaxies

The Infrared Astronomical Satellite (IRAS) surveyed ~ 96% of the sky in four infrared wavelength bands centered at 12, 25, 60 and 100 µm and detected about 250 000 point sources; several thousands of them have been identified with galaxies; the majority of IRAS galaxies are starbursts, so much so that ``IRAS galaxy'' is sometimes used instead of starburst galaxy; however, some of them are AGNs ([396]).

One of the most exciting results of the IRAS survey was the discovery of galaxies of quasar-like luminosity emitting almost entirely in the infrared; they have been called ultraluminous infrared galaxies (ULIGs) and are defined as having L_FIR > 10¹² L, or L_FIR > 3.8 10⁴⁵ erg s^-1.

The FIR flux density between 42.5 and 122.5 µm has been defined as: S_FIR (erg s^-1 cm^-2) = 1.2610^-11(2.58S_60µm + S_100µm) where S_60µm and S_100µm are the IRAS flux densities (Jy) at 60 and 100 µm respectively ([190]).

QSOs with M_B = -24.0 have a luminosity, between 1 µm and 1 keV, L ~ 10^12.2 L, i.e. ULIGs have the luminosity of low luminosity QSOs.

About 30% of all ULIGs have Seyfert type optical spectra ([482]; Veilleux et al. 1999); but the fraction of Seyferts among ULIGs increases dramatically above L_FIR = 10^12.65 L; nearly one-half of the galaxies brighter than this limit present Seyfert characteristics, while only ~20% of the weaker ULIGs and 15% of the galaxies with 10¹¹ L < L_IR < 10¹² L have such characteristics (Veilleux et al. 1999). We should however be aware that at very high FIR luminosities, a number of ULIGs are enhanced either by gravitational lensing or by relativistic beaming. We have compiled a list of 12 ULIGs with L_IR > 10¹³ L; 3 are classified as starburst, 1 as a Seyfert 2 and 8 as Seyfert 1s (75% Seyferts); but two Seyfert 1s are gravitationally lensed (IRASF10214+4724 and APM08279+5255) and four are HPQs (PKS0420-01, PKS0537-441, 3C345.0 and 3C446); ignoring these six objects, the fraction of Seyferts drops to 50%. [161] showed that the space densities of ULIGs and QSOs are quite similar at small redshifts for L_bol > 10^12.35 L.

Nearly all ULIGs appear to be advanced merger systems, including those which have a Seyfert spectrum ([54]; [352]; [409]).

Merger-induced shocks are likely to leave the gas from both galaxies in dense molecular form which will rapidly cool, collapse, and fragment; thus a merger might be expected to result in a burst of star formation of exceptional intensity ([214]).

Indeed, ULIGs contain large nuclear concentrations of molecular gas detected in the millimeter lines of CO. CO maps show rotating disks of molecular gas that has been driven into the centers of the mergers; the derived gas mass, of the order of 10⁹-10¹⁰ M, is equal to the mass of molecular clouds in a large, gas-rich spiral galaxy ([106]).

There is evidence that starbursts are responsible for the IR emission of even the most luminous IRAS galaxies. Most ULIGs contain a compact but resolved (> 0.25") radio source obeying the FIR-radio correlation followed by starbursts ([92]; [398]); the FIR-mm spectra of ULIGs can be satisfactorily modelled with spherically symmetric dust clouds with high optical depths heated by a starburst ([361]; [351]).

There is ample observational evidence that ULIGs which do not have a Seyfert spectrum do not contain a hidden QSO.

Spectropolarimetric and near-infrared spectroscopic observations have shown that most, if not all, ULIGs with a Seyfert 2 spectrum contain a hidden QSO, while none of the ULIGs without a Seyfert spectrum shows any obvious sign of an obscured BLR, despite the fact that the extinction at 2 µm is 1/10 that at optical wavelength ([148]; [440]; [418]; [307]).

A starlike nucleus is seen in 15% of the near-infrared HST images of ULIGs; but these objects are those that have been classified as Seyfert 1s or 2s ([54]).

Mid-infrared spectroscopic observations (3-11.6 µm) of a number of ULIGs have not led to the discovery of Seyfert nuclei not previously known from optical spectroscopy although they can probe heavily reddened (A_V ~ 50 mag) regions; they confirmed that the fraction of Seyfert galaxies increases with ULIG luminosities ([268]).

When a QSO is found in an ULIG, its luminosity is usually too low to power the ULIG; when both an AGN and a starburst occur concurrently in an ULIG, the starburst dominates the luminosity output ([352]). [478] have been unable to detect a soft X-ray (0.1-2.4 keV) source at the position of five ULIGs (three Seyfert 2s and two starbursts), with an upper limit L_X / L_bol ~ 2.3 10^-4. In QSOs, the mean value of L_X / L_bol is ~ 0.05 ([306]); if ULIGs are powered by a hidden QSO, the observed limits imply that less than 0.005 of the X-ray QSO luminosity is scattered into our line of sight which is much smaller than the scattering fraction inferred in the optical (a few per cent). It follows that even the three observed ULIGs with a Seyfert 2 spectrum cannot contain a QSO bright enough to power the ULIG.

Near-infrared imaging of 12 ULIGs having a Seyfert spectrum (six type 1 and six type 2) reveals, in every case, an unresolved component, coincident with the optical nucleus which increasingly dominates the emission at long wavelengths. For seven of these objects only, the dereddened nuclear luminosity is sufficiently high that they could provide the bolometric luminosity ([408]).

PKS1345+12 is an ULIG containing a Seyfert 1 nucleus; it is an interacting system containing 3.310¹⁰ M of molecular gas associated with the Seyfert nucleus suggesting that the molecular gas is the fuel source rather than the AGN ([111]).

In conclusion, it seems that those ULIGs which do not have a Seyfert 1 or 2 optical spectrum do not host a burried AGN significantly contributing to the energetics of the objects and are powered by starbursts produced by the merging of two spiral galaxies. Those ULIGs which contain an AGN are also, most probably, powered by starbursts; but it is possible that, in some of them, the AGN is bright enough to significantly participate to the powering ([397]); the AGNs themselves are likely powered by the merging process ([213]; [366]).