  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.

Astrophysics