COS : III . environmental effects on the star formation histories of galaxies at z ∼ 0 . 8 seen in [ O ii ] , H δ , and D n 4000 ?

We present spectroscopic observations of 466 galaxies in and around a superstructure at z ∼ 0.84 targeted by the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS3COS). We use [Oii]λ3727, Hδ, and Dn4000 to trace the recent, mid-, and longterm star formation histories and investigate how stellar mass and the local environment impacts those. By studying trends both in individual and composite galaxy spectra, we find that both stellar mass and environment play a role in the observed galactic properties. Low stellar mass galaxies (10 < log10 (M?/M ) < 10.5) in the field show the strongest Hδ absorption. Similarly, the massive population (log10 (M?/M ) > 11) shows an increase in Hδ absorption strengths in intermediate-density environments (e.g. filaments). Intermediate stellar mass galaxies (10.5 < log10 (M?/M ) < 11) have similar Hδ absorption profiles in all environments, but show a hint of enhanced [Oii] emission at intermediate-density environments. This hints that low stellar mass field galaxies and high stellar mass filament galaxies are more likely to have experienced a recent burst of star formation, while galaxies of the intermediate stellar-mass show an increase of star formation at filament-like densities. We also find that the median [Oii] equivalent width (|EW[OII] |) decreases from 27 ± 2 Å to 2.0+0.5 −0.4 Å and Dn4000 increases from 1.09 ± 0.01 to 1.56 ± 0.03 with increasing stellar mass (from ∼ 109.25 to ∼ 1011.35 M ). Concerning the dependence on the environment, we find that at fixed stellar mass |EW[OII] | is tentatively lower in higher density environments. Regarding Dn4000, we find that the increase with stellar mass is sharper in denser environments, hinting that such environments may accelerate galaxy evolution. Moreover, we find larger Dn4000 values in denser environments at fixed stellar mass, suggesting that galaxies are on average older and/or more metal-rich in such dense environments. This set of tracers depicts a scenario where the most massive galaxies have, on average, the lowest sSFRs and the oldest stellar populations (age & 1 Gyr, showing a mass-downsizing effect). We also hypothesize that the observed increase in star formation (higher EW[OII] | , higher sSFR) at intermediate densities may lead to quenching since we find the quenched fraction to increase sharply from the filament to cluster-like regions at similar stellar masses.