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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.