VIS³COS. III. Environmental effects on the star formation histories of galaxies at z ∼ 0.8 seen in [O II], Hδ, and D_n4000

Abstract

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 D_n4000 to trace recent, medium-, and long-term star formation histories and investigate the effect of stellar mass and local environment on them. By studying trends in individual and composite galaxy spectra, we find that stellar mass and environment play a role in the observed galactic properties. Galaxies with low stellar mass (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). Galaxies with intermediate stellar mass (10.5 <  log₁₀ (M⋆/M⊙) < 11) have similar Hδ absorption profiles in all environments, but show an indication of enhanced [OII] emission in intermediate-density environments. This indicates that field galaxies with low stellar mass and filament galaxies with high stellar mass 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 D_n4000 increases from 1.09 ± 0.01 to 1.56 ± 0.03 with increasing stellar mass (from ∼10^(9.25) to ∼10^(11.35) M⊙). For the dependence on the environment, we find that at fixed stellar mass, |EW_([OII])| is tentatively lower in environments with higher density. We find for D_n4000 that the increase with stellar mass is sharper in denser environments, which indicates that these environments may accelerate galaxy evolution. Moreover, we find higher D_n4000 values in denser environments at fixed stellar mass, suggesting that galaxies are on average older and/or more metal rich in these dense environments. This set of tracers depicts a scenario where the most massive galaxies have, on average, the lowest specific star formation rates 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 specific star formation rate) at intermediate densities may lead to quenching because we find that the quenched fraction increases sharply from the filament to cluster-like regions at similar stellar masses.

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