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 < log(L[IR]/Lsol < 11 and L[IR]/Lsol > 12 respectively), which are responsible for the shape of the bolometric luminosity function of local galaxies above ~ 10^11 Lsol. 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 ~ 5 microns. 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 implied that shyness must have been more common among galaxies in the past than it is today. New results from the Infrared Space Observatory suggest that the bulk of present-day stars formed at a time when their host galaxies experienced such a phase of shyness 5 to 10 Gyr ago, i.e. between z= 0.5 and 2. The shyness of galaxies appears to be induced by galaxy encounters.