Timing the earliest quenching events with a robust sample of massive quiescent galaxies at 2 <z <5

 We present a sample of 151 massive (M > 1010M) quiescent galaxies at 2 < z < 5, based on a sophisticated Bayesian spectral energy distribution tting analysis of the CANDELS UDS and GOODS-South elds. Our sample includes a robust sub-sample of 61 objects for which we condently exclude low-redshift and star-forming solutions. We identify 10 robust objects at z > 3, of which 2 are at z > 4. We report formation redshifts, demonstrating that the oldest objects formed at z > 6, however individual ages from our photometric data have signicant uncertainties, typically 0:5 Gyr. We
demonstrate that the UVJ colours of the quiescent population evolve with redshift at z > 3, becoming bluer and more similar to post-starburst galaxies at lower redshift. Based upon this we construct a model for the time-evolution of quiescent galaxy UVJ colours, concluding that the oldest objects are consistent with forming the bulk of their stellar mass at z 6 ~ 7 and quenching at z 5. We report spectroscopic redshifts for two of our objects at z = 3:440 and 3:396, which exhibit extremely weak Ly emission in ultra-deep VANDELS spectra. We calculate star-formation rates based on these line uxes, nding that these galaxies are consistent with our quiescent selection
criteria, provided their Ly escape fractions are > 3 and > 10 per cent respectively.We nally report that our highest redshift robust object exhibits a continuum break at 7000A in a spectrum from VUDS, consistent with our photometric redshift of zphot = 4:72+0:06-0:04. If conrmed as quiescent, this object would be the highest redshift known quiescent galaxy. To obtain stronger constraints on the times of the earliest quenching events, high-SNR spectroscopy must be extended to z & 3 quiescent objects.