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The extents of proximity zones of high-redshift quasars enable constraints onthe timescales of quasar activity, which are fundamental for understanding thegrowth of the supermassive black holes (SMBHs) that power the quasars'emission. In this study, we obtain precise estimates for the ultraviolet (UV)luminous lifetimes of ten quasars at $5.8< z< 6.5$. These objects werepre-selected to have short lifetimes based on preliminary measurements of theirproximity zone sizes, and were then targeted for high quality follow-up sub-mm,optical, and infrared observations required to increase the measurements'precision and securely determine their lifetimes. By comparing these proximityzone sizes to mock quasar spectra generated from radiative transfer simulationsat a range of different lifetimes, we deduce extremely short lifetimes $t_{\rmQ}<10^4$ yr for four objects in our sample, whereas the remaining quasars areconsistent with longer lifetimes of $t_{\rm Q}\gtrsim 10^5$ yr. These youngobjects with small proximity zones represent $\lesssim10\%$ of the quasarpopulation as a whole. We compare our results in detail to other studies ontimescales of quasar activity, which point towards an average lifetime of$t_{\rm Q}\sim10^6$ yr for the quasar population. This is consistent withfinding newly turned on quasars approximately $\sim 1-10\%$ of the time. Theseyoung quasars represent an unique opportunity to study triggering and feedbackmechanisms of SMBHs, since the onset of their UV luminous quasar phase happenedonly recently, and therefore traces of this process might still be observable.

Detecting and Characterizing Young Quasars II: Four Quasars at $z\sim 6$ with Lifetimes $<10^4$ years