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SÉMINAIRE DU LERMA
OBSERVATOIRE DE PARIS
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Vendredi 25 juin 2004 à 14 h


77 avenue Denfert Rochereau, Paris 14



Salle de l'Atelier, Observatoire de Paris





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.