Probing the circumgalactic medium at high-redshift using composite BOSS spectra of strong Lyman α forest absorbers

We present composite spectra constructed from a sample of 242 150 Lyman alpha (Ly alpha) forest absorbers at redshifts 2.4 < z < 3.1 identified in quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS) as part of Data Release 9 of the Sloan Digital Sky Survey III. We select forest absorbers by their flux in bins 138 km s(-1) wide (approximately the size of the BOSS resolution element). We split these absorbers into five samples spanning the range of flux -0.05 <= F < 0.45. Tests on a smaller set of high-resolution spectra show that our three strongest absorption samples would probe circumgalactic regions (projected separation < 300 proper kpc and |Delta v| < 300 km s(-1)) in about 60 per cent of cases for very high signal-to-noise ratio. Within this subset, weakening Ly alpha absorption is associated with decreasing purity of circumgalactic selection once BOSS noise is included. Our weaker two Ly alpha absorption samples are dominated by the intergalactic medium. We present composite spectra of these samples and a catalogue of measured absorption features from H i and 13 metal ionization species, all of which we make available to the community. We compare measurements of seven Lyman series transitions in our composite spectra to single line models and obtain further constraints from their associated excess Lyman limit opacity. This analysis provides results consistent with column densities over the range 14.4 less than or similar to log(N-HI) less than or similar to 16.45. We compare our measurements of metal absorption to a variety of simple single-line, single-phase models for a preliminary interpretation. Our results imply clumping on scales down to similar to 30 pc and near-solar metallicities in the circumgalactic samples, while high-ionization metal absorption consistent with typical IGM densities and metallicities is visible in all samples.