1.4 Luminous Infrared Galaxies

One of the most important discoveries from extragalactic observations at mid and far-IR wavelengths has been the identification of a class of "infrared galaxies", objects that emit more energy in the infrared than at all other wavelengths combined. IRAS (Neugebauer, 1984, see also Section B.3) surveyed $\sim$96% of the sky, producing an initial IRAS Point Source Catalogue (PSC) with a completeness limit of $\sim$0.5 Jy at 12, 25 and 60 $\mu$m and $\sim$1.5 Jy at 100 $\mu$m. It contained $\sim$20000 galaxies, the majority of which had not been previously catalogued.

At luminosities above $10^{11}\,L_\odot$, infrared galaxies become the dominant population of extragalactic objects in the local Universe ( $z \leq 0.3$), being more numerous than optically-selected starburst and Seyfert galaxies and quasi-stellar objects at comparable bolometric luminosity.

The bulk of the luminosity produced in galaxies bolometrically more luminous than $\sim 4 \, L^\star$, i.e. with $L_{bol} > 10^{11} \, L_\odot$, appears to be produced in objects that are heavily obscured by dust.

Although LIRGs comprise the dominant population of extragalactic objects at $L_{bol} > 10^{11}\,L_\odot$, they are still very rare. The infrared luminosity function suggests that only one object with $L_{ir}>10^{12}\,L_\odot$ will be found out to a redshift of $\sim 0.033$, and indeed, Arp220 ($z=0.018$) is the only ULIRG within this volume. The total infrared luminosity from LIRGs in the IRAS Bright Galaxy Survey (BGS) is only $\sim$6% of the infrared emission in the local Universe (Soifer, 1991). Figure 1.5 shows how the shape of the mean spectral energy distribution (SED) varies for galaxies with increasing total infrared luminosity. The ratio $f_{60}/f_{100}$ increases while $f_{12}/f_{25}$ decreases with increasing infrared luminosity. Figure 1.5 also shows that the observed range of over 3 orders of magnitude in $L_{ir}$ for infrared-selected galaxies is accompanied by less than a factor 3-4 change in the optical luminosity.

Various models of the infrared emission (Rowan-Robinson, 1986; Helou, 1986; Rowan-Robinson, 1993) have suggested that in lower luminosity "normal" galaxies the secondary peak in the mid-infrared is due to emission from small dust grains near hot stars, while the stronger peak at $\lambda \geq 100-200$ $\mu$m represents emission dominated by dust from infrared cirrus ( $T_D \leq 20$K) heated by the older stellar population. In more infrared luminous galaxies, a starburst component emerges ( $T_D \sim 30-60$K) with a peak closer to 60 $\mu$m, plus, in Seyfert galaxies, an even warmer component ( $T_D \sim 150-250$K) peaking near 20 $\mu$m, presumably representing warm dust directly heated by the AGN.

Figure 1.5: Variation of the Mean SEDs with Increasing Infrared Luminosity for a 60 $\mu$m Sample of Infrared Galaxies. From Sanders (1996).