Annu. Rev. Astron. Astrophys. 2005. 43: xxx-xxx Copyright © 2005 by Annual Reviews. All rights reserved

4.2. Cosmic Evolution

Another remarkable property of the 15 µm sources is their extremely high rates of evolution with redshift exceeding those measured for galaxies at other wavelengths and comparable to or larger than the evolution rates observed for quasars. Number counts at 15 µm show a prominent bump peaking at about 0.4 mJy. At the peak of the bump, the counts are one order of magnitude above the non-evolution models. In fact, data require a combination of a (1 + z)³ luminosity evolution and (1 + z)³ density evolution for the starburst component at redshift lower than 0.9 to fit the strong evolution. Although it has not been possible with ISOCAM to probe in detail the evolution of the infrared luminosity function, Spitzer data at 24 µm give for the first time tight constraints up to redshift 1.2 (Le Floc'h et al. 2005; Pérez-González et al. 2005). A strong evolution is noticeable and requires a shift of the characteristic luminosity L by a factor (1 + z)^4.00.5. Le Floc'h et al. (2005) find that the LIRGs and ULIRGs become the dominant population contributing to the comoving infrared energy density beyond z ~ 0.5-0.6 and represent 70% of the star-forming activity at z ~ 1. The comoving luminosity density produced by luminous infrared galaxies was more than 10 times larger at z ~ 1 than in the local Universe. For comparison, the B-band luminosity density was only three times larger at z = 1 than today. Such a large number density of LIRGs in the distant Universe could be caused by episodic and violent star-formation events, superimposed on relatively small levels of star formation activity. This idea has emerged in 1977 (Toomre 1977) and is fully developed in Hammer et al. (2005). These events can be associated to major changes in the galaxy morphologies. The rapid decline of the luminosity density from z = 1 is only partially due to the decreasing frequency of major merger events. Bell et al. (2005) showed that the SFR density at z ~ 0.7 is dominated by morphologically normal spiral, E/S0 and irregular galaxies ( 70%), while clearly interacting galaxies account for < 30%. The dominent driver of the decline is a strong decrease in SFR in morphologically undisturbed galaxies. This could be due, for example, to gas consumption or to the decrease of weak interactions with small satellites that could trigger the star formation through bars and spiral arms.

Locally 0.5% of galaxies with L_V > 10¹⁰ L have SEDs typical of LIRGs. This changes dramatically at higher redshift: in deep surveys, ISO detect about 15% of the M_B -20 galaxies (LIRGs, Hammer et al. 2005) and Spitzer detect about 30% of field galaxies (Starbursts and LIRGs, Bell et al. 2005). Thus the two populations (optical and LIR galaxies) overlap more at high z.