The SAMI Galaxy Survey: using concentrated star formation and stellar population ages to understand environmental quenching

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, H_α/r_{50, 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 ≤ 10^12.5M_⊙), high-mass groups (M₂₀₀ in the range 10^12.5-14M_⊙) and clusters (M₂₀₀ > 10¹⁴M_⊙). We find the fraction of SF-concentrated galaxies increases as halo mass increases by 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 (Age_L) and mass-weighted age (Age_M) using full spectral fitting, as well as the D_n4000 and Hδ_A 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 R_e), 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 Age_L and 1.34 ± 0.56 Gyr for Age_M. 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 R_e of cluster SF-concentrated galaxies and ungrouped regular galaxies are consistent (difference of 0.19 ± 0.21 Gyr for Age_L, 0.40 ± 0.61 Gyr for Age_M), suggesting the quenching process must be rapid.