Spectral Signatures of Quasar Ages at z~3
  Insight into quasar ages may be obtained from the proximity effect, but so
far only in a limited number of bright quasars. Based on ~2600 SDSS quasar
spectra at 2.5 <= z <= 3.5, a search for spectral voids between Ly-alpha forest
lines finds proximity zones over a wide range of radial distances. The majority
of zone sizes are less than 5 Mpc, with their numbers decreasing exponentialy
towards larger distances. After normalization by luminosities, the zone sizes
are distributed an e-folding scale of 0.64 as compared with the anticipated
values. A group of quasars are selected for their large proximity zones of >~
10 Mpc. Their composite spectrum displays strong narrow cores and large
equivalent widths in Ly-alpha and other major UV emission lines. If the
proximity zones along lines of sight are indicative of quasar ages, these
features may be the signatures of old quasars. Another group of quasars are
selected as they show no proximity zone and exhibit intrinsic absorption lines
at z_{ab} > z_{em}. They are likely young quasars and exhibit weaker narrow
emission-line components. The significant difference of spectral features
between the two groups may reflect an evolution pattern over quasars'
lifetimes.
