Recent re-building of most spiral galaxies: a scenario derived from
multi-wavelength observations
François HAMMER
Observatoire de Paris/Meudon
Résumé :
Studies of distant galaxies have shown that ellipticals and large
spirals were already in place 8 Gyr ago, leading to a very modest recent
star formation in intermediate mass galaxies. This is challenged by a
recent analysis (Heavens et al, 2004) of the fossil record of the
stellar populations of 100000 nearby galaxies, which show that
intermediate mass galaxies have formed or assembled the bulk of their
stars 4 to 8 Gyr ago. Here we present direct observational evidence
supporting the Heavens et al 's findings from a long term,
multi-wavelength study of 195 z $>$ 0.4 intermediate mass galaxies, mostly
selected from the Canada France Redshift Survey (CFRS). We show that a
recent and efficient star formation is revealed at IR wavelengths since
15\% of intermediate mass galaxies at z $>$ 0.4 are indeed luminous IR
galaxies (LIRGs), a phenomenon far more common than in the local
Universe. The star formation in LIRGs is sufficient in itself to produce
40\% of the total stellar mass of intermediate mass galaxies and then to
account for most of the reported stellar mass formation since $z=1$.
Observations of distant galaxies have also the potential of resolving
their star formation and mass assembly histories. The high occurrence of
LIRGs is easily understood if they correspond to episodic peaks of star
formation, during which galaxies are reddened through short IREs
(infrared episodes). We estimate that each galaxy should experience 5
IREs from $z=1$ to $z=0.4$, assuming their characteristic timescales to be
about 0.1 Gyr. An efficient and episodic star formation is further
supported by the luminosity-metallicity relation of $z= 0.7$ emission line
galaxies, which we find to be on average, metal deficient by a factor
of 2 when compared to those of local spirals. We then examine how
galaxy IREs can be related to the emergence at high redshift, of
galaxies with small size (but not with small mass), blue core and many
irregularities. We find that recent merging and gas infall naturally
explain both morphological changes and episodic star formation history
in a hierarchical galaxy formation frame. We then test a simple scenario
in which 75\% of intermediate mass spirals have recently experienced
their last major merger event, leading to a drastic reshaping of their
bulges and disks during the last 8 Gyrs. It links in a simple manner,
distant and local galaxies, and reports for the simultaneous decreases,
during that period, of the cosmic star formation density, of the merger
rate, of the number densities of LIRGs, compact and irregular galaxies.
