Images IV: strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies from z~0.8

Abstract

Context: Intermediate mass galaxies (> 10¹⁰ M) at z ~ 0.6 are the likeliest progenitors of the present-day, numerous population of spirals. There is growing evidence that they have evolved rapidly in the last 6 to 8 Gyr, and likely already have formed a significant fraction of their stellar mass, often showing perturbed morphologies and kinematics. Aims: We have gathered a representative sample of 88 such galaxies and have provided robust estimates of their gas phase metallicity. Methods: We used moderate spectral resolution spectroscopy at VLT/FORS2 with an unprecedentedly high S/N allowing us to remove biases coming from interstellar absorption lines and extinction, to establish robust values of R₂₃ = ([OII] λ3727 + [OIII]λλ4959, 5007)/H β . Results: We definitively confirm that the predominant population of z ~ 0.6 starbursts and luminous IR galaxies (LIRGs) are on average two times less metal rich than the local galaxies at a given stellar mass. We do find that the metal abundance of the gaseous phase of galaxies evolves linearly with time, from z = 1 to z = 0 and after comparing with other studies, from z = 3 to z = 0. Combining our results with the reported evolution of the Tully Fisher relation, we find that such an evolution requires that ~30% of the stellar mass of local galaxies have been formed through an external supply of gas, thus excluding the closed box model. Distant starbursts & LIRGs have properties (metal abundance, star formation efficiency & morphologies) similar to those of local LIRGs. Their underlying physics is likely dominated by gas infall, probably through merging or interactions. Conclusions: Our study further supports the rapid evolution of z ~ 0.4-1 galaxies. Gas exchange between galaxies is likely the main cause of this evolution.