Slow-then-rapid quenching as traced by tentative evidence for enhanced metallicities of cluster galaxies at z ∼ 0.2 in the slow quenching phase

Aims: As large-scale structures in the Universe develop with time, environmental effects become more and more important as a star formation quenching mechanism. Since the effects of environmental quenching are more pronounced in denser structures that form at later times, we seek to constrain environmental quenching processes using cluster galaxies at z < 0.3.
Methods: We explored seven clusters from the Local Cluster Substructure Survey (LoCuSS) at 0.15 < z < 0.26 with spectra of 1965 cluster members in a mass-complete sample from the ACReS (Arizona Cluster Redshift Survey) Hectospec survey covering a region that corresponds to about three virial radii for each cluster. We measured fluxes of [O II] λ 3727, Hβ, [O III] λ 5007, Hα, and [N II] λ 6584 emission lines of cluster members, enabling us to unambiguously derive O/H gas metallicities. We also measured star formation rates (SFRs) from extinction-corrected Hα fluxes. We compared our cluster galaxy sample with a field sample of 705 galaxies at similar redshifts observed with Hectospec as part of the same survey.
Results: We find that star-forming cluster and field galaxies show similar median specific SFRs in a given mass bin of 1 - 3.2 × 10¹⁰ M_⊙ and 3.2 - 10 × 10¹⁰ M_⊙, respectively. But their O/H values are displaced, in the lower mass bin, to higher values (significance 2.4σ) at projected radii of R < R₂₀₀ compared with galaxies at larger radii and in the field. The comparison with metallicity-SFR-mass model predictions with inflowing gas indicates a slow-quenching scenario in which strangulation is initiated when galaxies pass R ∼ R₂₀₀ by stopping the inflow of gas. We find tentative evidence that the metallicities of cluster members inside R₂₀₀ are thereby increasing, but their SFRs are hardly affected for a period of time because these galaxies consume available disk gas. We use the observed fraction of star-forming cluster galaxies as a function of clustercentric radius compared to predictions from the Millennium simulation to constrain quenching timescales to be 1-2 Gyr, which is defined as the time between the moment the galaxy passes R₂₀₀ until complete quenching of star formation. This is consistent with a slow-then-rapid quenching scenario. Slow quenching (strangulation) starts when the gas inflow is stopped when the galaxy passes R₂₀₀ with a phase in which cluster galaxies are still star forming, but they show elevated metallicities tracing the ongoing quenching. This phase lasts for 1-2 Gyr, and meanwhile the galaxies travel to denser inner regions of the cluster. This is followed by a "rapid" phase, i.e., a rapid complete quenching of star formation due to the increasing ram pressure toward the cluster center that can also strip the cold gas in massive galaxies.