Annu. Rev. Astron. Astrophys. 1996. 34: 749-792 Copyright © 1996 by Annual Reviews. All rights reserved

3. REDSHIFT SURVEYS

A more complete description of the properties of infrared galaxies became possible only after the determination of redshifts for relatively large unbiased samples of infrared selected objects. Table 2 lists the major published redshift catalogs for IRAS galaxies [Saunders et al. (1990) provides a good reference for IRAS galaxy surveys prior to 1990].

Table 2.IRAS galaxy redshift surveys

Name	Flux limit(s)	Area	Sources a	Reference b
		all sky c
RBGS	f60 5.24 Jy	|b| > 5°	602 P	Sanders et al. 1996a
1.2 Jy Survey	f60 1.2 Jy	|b| > 10°	5321 P	Fisher et al. 1995
1 Jy ULIGs	f60 1.0 Jy	|b| > 30°	115 F	Kim & Sanders 1996
QDOT	f60 0.59 Jy	|b| > 10°	2387 F	Lawrence et al. 1996
12 µm Survey	f12 0.22 Jy	|b| > 25°	893 F	Rush et al. 1993
		small area
2 Jy Survey &	f60 2.0 Jy	1072 deg2	70 P	Smith et al. 1987
Bootes Void	f60 0.75 Jy	1423 deg2	379 P	Strauss & Huchra 1988
KOS-KOSS	f60 0.6 Jy	142 deg2	63 P	Vader & Simon 1987b
NGW	f60 0.5 Jy	844 deg2	389 P	Lawrence et al. 1986
FSS-z	f60 0.2 Jy	1310 deg2	~ 3600 F	Oliver et al. 1996
Pointed Obs	f60 0.15 Jy	18 deg2	66 A	Lonsdale & Hacking 1989
NEPR	f60 0.05 Jy	6 deg2	76 D	Ashby et al. 1996
		color selected
AGN Candidates	1 > f25 / f60 > 0.27	|b| > 20°	563 P	de Grijp et al. 1992
WEO	3 > f25 / f60 > 0.25	|b| > 30°	187 P	Low et al. 1988
Tepid FIRGs	f25 / f60 < 0.27		53 P	Armus et al. 1989
	f60 / f100 > 0.78
60 µm Peakers	1 > f25 / f60 > 0.25	|b| > 10°	51 P	Vader et al. 1993
	f60 / f100 > 1
a IRAS Catalogs: (P) PSC (1988), (F) FSC (Moshir et al. 1992), (A) Pointed Observations (Young et al. 1986), (D) Deep Survey (Hacking & Houck 1987).
b References for earlier versions of surveys: RBGS - Soifer et al. 1986, 1987b, 1989 (BGS); Sanders et al. 1995 (BGS-Part II), 1.2 Jy Survey - Strauss et al. 1992 (1.936 Jy Survey), QDOT - Lawrence et al. 1989 (QCD), 12 µm Survey - Spinoglio & Malkan 1989, Tepid FIRGs - Heckman et al. 1987.
c 1 Jy ULIGs - |b| > 30° and -40°, QDOT-1 in 6 random source selection.

3.1 Luminosity Functions

A comparison of the luminosity function of infrared bright galaxies with other classes of extragalactic objects is shown in Figure 1. At luminosities below 10¹¹ L, IRAS observations confirm that the majority of optically selected objects are relatively weak far-infrared emitters (Bothun et al. 1989, Knapp et al. 1989, Devereux & Young 1991, Isobe & Feigelson 1992). Surveys of Markarian galaxies (Deutsch & Willner 1986, Mazzarella & Balzano 1986, Mazzarella et al. 1991, Bicay et al. 1995) confirm that both Markarian starbursts and Seyferts have properties (e.g. f₆₀/f₁₀₀ and L_ir / L_B ratios) closer to infrared selected samples as does the subclass of optically selected interacting galaxies (e.g. Bushouse 1987, Kennicutt et al. 1987, Bushouse et al. 1988, Sulentic 1989); however relatively few objects in optically selected samples are found with L_ir > 10^11.5 L.

The high luminosity tail of the infrared galaxy luminosity function is clearly in excess of what is expected from the Schechter function. A better description (e.g. Soifer et al. 1987b) is a double power law with slope -1 at low luminosity, changing to a slope of ~ -2.35 at L_bol 10^10.3 L. For L_bol = 10¹¹-10¹² L, LIGs are as numerous as Markarian Seyferts and ~ 3 times more numerous than Markarian starbursts. Ultraluminous infrared galaxies (hereafter ULIGs: L_ir > 10¹² L) appear to be ~ 2 times more numerous than optically selected QSOs, the only other previously known population of objects with comparable bolometric luminosities.

Figure 1. The luminosity function for infrared galaxies compared with other extragalactic objects. References: IRAS RBGS (Sanders et al. 1996a), IRAS 1-Jy Survey of ULIGs (Kim 1995), Palomar-Green QSOs (Schmidt & Green 1983), Markarian starbursts and Seyfert galaxies (Huchra 1977), and normal galaxies (Schechter 1976). Determination of the bolometric luminosity for the optically selected samples was as described in Soifer et al. (1986), except for the adoption of a more accurate bolometric correction for QSOs of 11.8 x L(0.43 µm) (Elvis et al. 1994).

Although LIGs comprise the dominant population of extragalactic objects at L_bol > 10¹¹ L, they are still relatively rare. For example, Figure 1 suggests that only one object with L_ir > 10¹² L will be found out to a redshift of ~ 0.033, and indeed, Arp 220 (z = 0.018) is the only ULIG within this volume. The total infrared luminosity from LIGs in the IRAS Bright Galaxy Survey (BGS) is only ~ 6% of the infrared emission in the local Universe (Soifer & Neugebauer 1991).

Comparison of the space density of ULIGs in the 1-Jy Survey with ``local'' ULIGs from the BGS provides some evidence for possible strong evolution in the luminosity function at the highest infrared luminosities. Assuming pure density evolution of the form (z) (1 + z)ⁿ, Kim (1995) found n ~ 7 ± 3 for the complete 1-Jy sample of ULIGs, the uncertainty being influenced primarily by the small range of redshift (z_max = 0.27) and the apparent effects of local large scale structure: Nearly all of the evidence for strong evolution comes from ULIGs at flux levels f₆₀ = 1-2 Jy corresponding to sources at z 0.13. No evidence for evolution is found for the subsample of 2 Jy ULIGs, i.e. n = 3.8 ± 3 (Kim & Sanders 1996). These results appear to be consistent with previous debates in the literature which find n ~ 5.6-7 for redshift surveys with flux limits f₆₀ ~ 0.5 Jy (Saunders et al. 1990, Oliver et al. 1995) but only n ~ 3-4 for surveys with flux limits f₆₀ 1.5 Jy (Fisher et al. 1992), and with analyses of IRAS extragalactic source counts (Hacking et al. 1987, Lonsdale & Hacking 1989, Lonsdale et al. 1990, Gregorich et al. 1995) that show evidence for strong evolution only at relatively low flux levels (f₆₀ 1 Jy). More definitive tests of whether the luminosity function for ULIGs indeed evolves strongly, and how this may compare, for example, with the strong evolution seen for the most luminous QSOs (e.g. Schmidt & Green 1983), will need to wait for future more sensitive infrared surveys.

3.2 Spectral Energy Distributions

The infrared properties for the complete IRAS BGS have been summarized and combined with optical data to determine the relative luminosity output from galaxies in the local Universe at wavelengths ~ 0.1-1000 µm (Soifer & Neugebauer 1991). Figure 2 uses data from Sanders et al. (1996a, b) and Kim (1995) to illustrate how the shape of the mean spectral energy distribution (SED) varies for galaxies with increasing total infrared luminosity. Systematic variations are observed in the mean infrared colors; the ratio f₆₀/f₁₀₀ increases while f₁₂ / f₂₅ decreases with increasing infrared luminosity. Figure 2 also illustrates that the observed range of over 3 orders of magnitude in L_ir for infrared-selected galaxies is accompanied by less than a factor of 3-4 change in the optical luminosity.

Figure 2. Variation of the mean SEDs (from submillimeter to UV wavelengths) with increasing Lir for a 60 µm sample of infrared galaxies. (Insert) Examples of the subset (~ 15%) of ULIGs with ``warm'' infrared color (f25 / f60 > 0.3). Data for the three objects (1 - the powerful Wolf-Rayet galaxy IRAS 01003-2238, 2 - the ``infrared QSO'' IRAS 07598+6508, 3 - the optically selected QSO I Zw 1) are from Sanders et al. (1988b).

Various models of the infrared emission (e.g. Helou 1986, Rowan-Robinson 1986, Rowan-Robinson & Efstathiou 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 100-200 µm represents emission dominated by dust from infrared cirrus (T<sub>D</sub> 20 K) heated substantially by the older stellar population. In more infrared luminous galaxies a ``starburst'' component emerges (T_D ~ 30-60 K) with a peak closer to 60 µm, plus, in Seyfert galaxies, an even warmer component (T_D ~ 150-250 K) peaking near 25 µm, presumably representing warm dust directly heated by the AGN.

Sanders et al. (1988b) showed that a small but significant fraction of ULIGs, those with ``warm'' (f<sub>25</sub> / f<sub>60</sub> > 0.3) infrared colors, have SEDs with mid-infrared emission (~ 5-40 µm) over an order of magnitude stronger than the larger fraction of ``cooler'' ULIGs. These warm galaxies (Figure 2 insert), which appear to span a wide range of classes of extragalactic objects including powerful radio galaxies (PRGs: L_408MHz 10²⁵ W Hz^-1) and optically selected QSOs, have been used as evidence for an evolutionary connection between ULIGs and QSOs (e.g. Sanders et al. 1988a, b). This connection is strengthened by IRAS data for QSOs (Figure 3), which shows that the mean SED of optically selected QSOs is dominated by thermal emission from an infrared/submillimeter bump (~ 1-300 µm) in addition to the ``big blue bump'' (~ 0.05-1 µm); the former is typically 30% as strong as the latter and is presumably thermal emission from dust in an extended circumnuclear disk surrounding the active nucleus.

Figure 3. Mean spectral energy distributions from radio to X-ray wavelengths of optically selected radio-loud and radio-quiet QSOs (Sanders et al. 1989a), and Blazars (Impey & Neugebauer 1988).