Did most present-day spirals form during the last 8 Gyrs? A formation history with violent episodes revealed by panchromatic observations

Abstract

(abridged) 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 ~10^5 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 CFRS survey. 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 38% of the total stellar mass of intermediate mass galaxies and then to account for most of the reported stellar mass formation since z=1. The high occurrence of LIRGs is easily understood only if they correspond to episodic peaks of star formation, during which galaxies are reddened through short IREs (infrared episodes). We examine how galaxy IREs can be related to the emergence at high redshift, of the abundant population of galaxies with small size, blue core and many irregularities. We show that recent merging and gas infall naturally explain both morphological changes and episodic star formation history in a hierarchical galaxy formation frame. We propose a simple scenario in which 75+-25% of intermediate mass spirals have experienced recently their last major merger event, leading to a drastic reshaping of their bulges and disks during the last 8 Gyrs.