We study environmental quenching in the Eagle/C-Eagle cosmological hydrodynamic simulations over the last 11 Gyr (i.e. z = 0 − 2). The simulations are compared with observations from the SAMI Galaxy Survey at z = 0. We focus on satellite galaxies in galaxy groups and clusters ($10^{12}\, \rm M_{\odot }$ ≲ M200 < $3 \times 10^{15}\, \rm M_{\odot }$). A star-formation concentration index [C-index =log10(r50, SFR/r50, rband)] is defined, which measures how concentrated star formation is relative to the stellar distribution. Both Eagle/C-Eagle and SAMI show a higher fraction of galaxies with low C-index in denser environments at z = 0 − 0.5. Low C-index galaxies are found below the SFR-M⋆ main sequence (MS), and display a declining specific star formation rate (sSFR) with increasing radii, consistent with “outside-in” environmental quenching. Additionally, we show that C-index can be used as a proxy for how long galaxies have been satellites. These trends become weaker at increasing redshift and are absent by z = 1 − 2. We define a quenching timescale tquench as how long it takes satellites to transition from the MS to the quenched population. We find that simulated galaxies experiencing “outside-in” environmental quenching at low redshift (z = 0 ∼ 0.5) have a long quenching timescale (median tquench > 2 Gyr). The simulated galaxies at higher redshift (z = 0.7 ∼ 2) experience faster quenching (median tquench < 2Gyr). At z ≳ 1 − 2 galaxies undergoing environmental quenching have decreased sSFR across the entire galaxy with no “outside-in” quenching signatures and a narrow range of C-index, showing that on average environmental quenching acts differently than at z ≲ 1.

We study environmental quenching using the spatial distribution of current star-formation and stellar population ages with the full SAMI Galaxy Survey. By using a star-formation concentration index [C-index, defined as log10(r50, Hα/r50, cont)], we separate our sample into regular galaxies (C-index≥−0.2) and galaxies with centrally concentrated star-formation (SF-concentrated; C-index < −0.2). Concentrated star-formation is a potential indicator of galaxies currently undergoing ‘outside-in’ quenching. Our environments cover ungrouped galaxies, low-mass groups (M200 ≤ 1012.5M⊙), high-mass groups (M200 in the range 1012.5 − 14M⊙) and clusters (M200 > 1014M⊙). We find the fraction of SF-concentrated galaxies increases as halo mass increases with 9 ± 2 per cent, 8 ± 3 per cent, 19 ± 4 per cent and 29 ± 4 per cent for ungrouped galaxies, low-mass groups, high-mass groups and clusters, respectively. We interpret these results as evidence for ‘outside-in’ quenching in groups and clusters. To investigate the quenching time-scale in SF-concentrated galaxies, we calculate light-weighted age (AgeL) and mass-weighted age (AgeM) using full spectral fitting, as well as the Dn4000 and HδA indices. We assume that the average galaxy age radial profile before entering a group or cluster is similar to ungrouped regular galaxies. At large radius (1–2 Re), SF-concentrated galaxies in high-mass groups have older ages than ungrouped regular galaxies with an age difference of 1.83 ± 0.38 Gyr for AgeL and 1.34 ± 0.56 Gyr for AgeM. This suggests that while ‘outside-in’ quenching can be effective in groups, the process will not quickly quench the entire galaxy. In contrast, the ages at 1-2 Re of cluster SF-concentrated galaxies and ungrouped regular galaxies are consistent (difference of 0.19 ±0.21 Gyr for AgeL, 0.40 ± 0.61 Gyr for AgeM), suggesting the quenching process must be rapid.