Constraining quenching timescales in galaxy clusters by forward-modelling stellar ages and quiescent fractions in projected phase space

We forward-model mass-weighted stellar ages (MWAs) and quiescent fractions in projected phase space (PPS), using data from the Sloan Digital Sky Survey, to jointly constrain an infall quenching model for galaxies in $\log (M_{\mathrm{v}\mathrm{i}\mathrm{r}}\mathrm{/}{\mathrm{M}}_{\odot })>14$ galaxy clusters at $z\sim 0$. We find the average deviation in MWA from the MWA-$M_{\star }$ relation depends on position in PPS, with a maximum difference between the inner cluster and infalling interloper galaxies of $\sim 1$ Gyr. Our model employs infall information from N-body simulations and stochastic star-formation histories from the UniverseMachine model. We find total quenching times of $t_{\mathrm{Q}}=3.7\pm 0.4$ Gyr and $t_{\mathrm{Q}}=4.0\pm 0.2$ Gyr after first pericentre, for $9<\log (M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })<10$ and $10<\log (M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })<10.5$ galaxies, respectively. By using MWAs, we break the degeneracy in time of quenching onset and timescale of star formation rate (SFR) decline. We find that time of quenching onset relative to pericentre is $t_{\mathrm{d}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{y}}=3.5_{-0.9}^{+0.6}$ Gyr and $t_{\mathrm{d}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{y}}=-0.3_{-1.0}^{+0.8}$ Gyr for our lower and higher stellar mass bins, respectively, and exponential SFR suppression timescales are ${\tau }_{\mathrm{e}\mathrm{n}\mathrm{v}}\le 1.0$ Gyr and ${\tau }_{\mathrm{e}\mathrm{n}\mathrm{v}}\sim 2.3$ Gyr for our lower and higher stellar mass bins, respectively. Stochastic star formation histories remove the need for rapid infall quenching to maintain the bimodality in the SFR of cluster galaxies; the depth of the green valley prefers quenching onsets close to first pericentre and a longer quenching envelope, in slight tension with the MWA-driven results. Taken together these results suggest that quenching begins close to, or just after pericentre, but the timescale for quenching to be fully complete is much longer and therefore ram-pressure stripping is not complete on first pericentric passage.