Star formation concentration as a tracer of environmental quenching in action: a study of the eagle and c-eagle simulations

Abstract

ABSTRACT 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 time-scale 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 time-scale (median tquench > 2 Gyr). The simulated galaxies at higher redshift (z = 0.7 ∼ 2) experience faster quenching (median tquench < 2 Gyr). 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.