THE COMPOSITE SPECTRUM OF STRONG Lyα FOREST ABSORBERS

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 (approximate to 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(H) (I) approximate to 1015.4 cm(-2). Assuming typical physical conditions rho/<(rho)over bar> = 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.