Detecting and Characterizing Young Quasars II: Four Quasars at $z\sim 6$
  with Lifetimes $<10^4$ years
  The extents of proximity zones of high-redshift quasars enable constraints on
the timescales of quasar activity, which are fundamental for understanding the
growth 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 were
pre-selected to have short lifetimes based on preliminary measurements of their
proximity 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 proximity
zone sizes to mock quasar spectra generated from radiative transfer simulations
at a range of different lifetimes, we deduce extremely short lifetimes $t_{\rm
Q}<10^4$ yr for four objects in our sample, whereas the remaining quasars are
consistent with longer lifetimes of $t_{\rm Q}\gtrsim 10^5$ yr. These young
objects with small proximity zones represent $\lesssim10\%$ of the quasar
population as a whole. We compare our results in detail to other studies on
timescales of quasar activity, which point towards an average lifetime of
$t_{\rm Q}\sim10^6$ yr for the quasar population. This is consistent with
finding newly turned on quasars approximately $\sim 1-10\%$ of the time. These
young quasars represent an unique opportunity to study triggering and feedback
mechanisms of SMBHs, since the onset of their UV luminous quasar phase happened
only recently, and therefore traces of this process might still be observable.
