Are Galaxies Shy ?

Until 1996, there was little evidence that most galaxies were “shy”, i.e. that they would hide their stars behind a veil of dust and turn red when forming stars, radiating the bulk of their luminosity in the infrared (IR) at a given epoch of their history. Ten years before, IRAS had unveiled a population of luminous IR galaxies exhibiting such a “shy” behavior, the so-called LIGs and ULIGs (with 12≥ log10 (LIR/L⊙) ≥ 11 and log10 (LIR/L⊙) ≥ 12 respectively), which are responsible for the shape of the bolometric luminosity function of local galaxies above ∼ 10 L⊙ (Sanders & Mirabel 1996). But integrated over the whole local luminosity function, LIGs and ULIGs only produce ∼ 2% of the total integrated luminosity and overall only ∼ 30% of the bolometric luminosity of local galaxies is radiated in the IR above λ ∼ 5μm. The discovery of an extragalactic background in the IR at least as large as the UV-optical-near IR one, the so-called cosmic infrared background (CIRB), with the COBE satellite (Puget et al. 1996, see references in Elbaz et al. 2002b) implied that shyness must have been more common among galaxies in the past than it is today. This was confirmed with the detection of an excess of faint mid IR (MIR) galaxies by ISOCAM onboard ISO (Elbaz et al. 1999), as well as in the far IR (FIR) with ISOPHOT onboard ISO (Dole et al. 2001) and in the sub-millimeter with SCUBA at the JCMT (see Smail et al. 2001). This excess is relative to expectations based on galaxies in the local universe. It implies that galaxies were more luminous in the IR regime and/or more numerous in the past (Chary & Elbaz 2001, Franceschini et al. 2001).