The extended epoch of galaxy formation: Age dating of ∼3600 galaxies with 2 < z < 6.5 in the VIMOS Ultra-Deep Survey

In this paper we aim at improving constraints on the epoch of galaxy formation by measuring the ages of 3597 galaxies with reliable spectroscopic redshifts 2 <= z <= 6 : 5 in the VIMOS Ultra Deep Survey (VUDS). We derive ages and other physical parameters from the simultaneous fitting with the GOSSIP + software of observed UV rest-0frame spectra and photometric data from the u band up to 4.5 mu m using model spectra from composite stellar populations. We perform extensive simulations and conclude that at z >= 2 the joint analysis of spectroscopy and photometry, combined with restricted age possibilities when taking the age of the Universe into account, substantially reduces systematic uncertainties and degeneracies in the age derivation; we find that age measurements from this process are reliable. We find that galaxy ages range from very young with a few tens of million years to substantially evolved with ages up to 1.5 Gyr or more. This large age spread is similar for di ff erent age definitions including ages corresponding to the last major star formation event, stellar mass-0weighted ages, and ages corresponding to the time since the formation of 25% of the stellar mass. We derive the formation redshift z(f) from the measured ages and find galaxies that may have started forming stars as early as z(f) similar to 15. We produce the formation redshift function (FzF), the number of galaxies per unit volume formed at a redshift z(f), and compare the FzF in increasing observed redshift bins finding a remarkably constant FzF. The FzF is parametrized with (1 + z)(zeta), where zeta similar or equal to 0.58 +/- 0.06, indicating a smooth increase of about 2 dex from the earliest redshifts, z similar to 15; to the lowest redshifts of our sample at z similar to 2. Remarkably, this observed increase in the number of forming galaxies is of the same order as the observed rise in the star formation rate density (SFRD). The ratio of the comoving SFRD with the FzF gives an average SFR per galaxy of similar to 7-17 M-circle dot/yr at z similar to 4 6, in agreement with the measured SFR for galaxies at these redshifts. From the smooth rise in the FzF we infer that the period of galaxy formation extends all the way from the highest possible formation redshifts that we can probe at z similar to 15 down to redshifts z similar to 2. This indicates that galaxy formation is a continuous process over cosmic time, with a higher number of galaxies forming at the peak in SFRD at z similar to 2 than at earlier epochs.