The Star Formation History as a Function of Type: Constraints from Galaxy Counts

Deep galaxy counts are among the best constraints on the cosmic star formation history (SFH) of galaxies. Using various tracers, the evolution of the star formation activity may now be followed on a wide range of redshifts (0 ≤ z ≤ 4) covering most of the history of the Universe. Two incompatible interpretations of the observations are currently competing. After applying star formation rate (SFR) conversion factors to the CFRS, Hα or ISO samples, many authors conclude to a strong increase (≃ a factor 10) of the SFR from z = 0 to z =1. They also find some evidence for a peak at z ≃ 1and for a rapid decrease at higher redshifts. On the other side, the Hawaii deep surveys favor only a mild increase between z =0 and 1 (Cowie et al.,1996, 1999).In this paper, we tackle this problem from the point of view of the modelist of the spectral evolution of galaxies. To understand the reason for these discrepant interpretations, we consider three classes of galaxies: E/S0(`early-type'); Sa-Sbc (`intermediate-type'); Sc-Sd, irregulars and bursting dwarfs (`late-type').We use the new version of our evolutionary synthesis code, Pégase (Fioc and Rocca-Volmerange, 2000, in preparation), which takes into account metallicity and dust effects. The main results are: i) Late-type galaxies contribute significantly to the local SFR, especially bursting dwarfs (Fioc and Rocca-Volmerange, 1999). Because of that, the cosmic SFR can not decrease by a factor 10 from z = 1 to 0.This is in agreement with Cowie et al., 1999's result. ii) The SFR of intermediate-type galaxies has strongly decreased sincez = 1. Though the decrease is less than what find Lilly et al., 1996, this suggests that the CFRS and Hα surveys are dominated by such bright early spirals. The limits in surface brightness and magnitudes of the observed samples may be the main reason for this selection. iii) The contribution of early-type galaxies increases rapidly from z =1 to their redshift of formation (≥ 2-3 for cosmological reasons). Their intense star formation rates at high-z give strong constraints on early ionization phases, primeval populations or metal enrichments.