We present a new method for probing the physical conditions and metal enrichment of the Intergalactic Medium : the composite spectrum of Ly \alpha forest absorbers .
We apply this technique to a sample of 9480 Ly \alpha absorbers with redshift 2 < z < 3.5 identified in the spectra of 13,279 high-redshift quasars from the Sloan Digital Sky Survey ( SDSS ) Fifth Data Release ( DR5 ) .
Absorbers are selected as local minima in the spectra with 2.4 < \tau _ { Ly \alpha } < 4.0 ; at SDSS resolution ( \approx 150 { km } { s } ^ { -1 } FWHM ) , these absorbers are blends of systems that are individually weaker .
In the stacked spectra we detect seven Lyman-series lines and metal lines of O vi , N v , C iv , C iii , Si iv , C ii , Al ii , Si ii , Fe ii , Mg ii , and O i .
Many of these lines have peak optical depths of < 0.02 , but they are nonetheless detected at high statistical significance .
Modeling the Lyman-series measurements implies that our selected systems have total H i column densities N _ { HI } \approx 10 ^ { 15.4 } { cm } ^ { -2 } .
Assuming typical physical conditions \rho / \bar { \rho } = 10 , T = 10 ^ { 4 } -10 ^ { 4.5 } { { K } } , and [ Fe/H ] = -2 yields reasonable agreement with the line strengths of high-ionization species , but it underpredicts the low-ionization species by two orders of magnitude or more .
This discrepancy suggests that the low ionization lines arise in dense , cool , metal-rich clumps , present in some absorption systems .