Mapping the most massive overdensity through hydrogen (mammoth). I. Methodology

Modern cosmology predicts that a galaxy overdensity (e.g., protocluster) will be associated with a large intergalactic medium gas reservoir, which can be traced by Lyα forest absorption. We have undertaken a systematic study of the relation between Coherently Strong intergalactic Lyα Absorption systems (CoSLAs), which have the highest optical depth (τ) in the τ distribution, and mass overdensities on the scales of ~10–20 h⁻¹ comoving Mpc. On such large scales, our cosmological simulations show a strong correlation between the effective optical depth (τ eff) of the CoSLAs and the three-dimensional mass overdensity. In spectra with moderate signal-to-noise ratio, however, the profiles of CoSLAs can be confused with individual high column density absorbers. For z > 2.6, where the corresponding Lyβ is redshifted to the optical, we have developed a selection technique to distinguish between these two alternatives. We have applied this technique to ~6000 sight lines provided by Sloan Digital Sky Survey III quasar survey at z = 2.6–3.3 with a continuum-to-noise ratio greater than 8, and we present a sample of five CoSLA candidates with τ eff on 15 h⁻¹ Mpc greater than 4.5× the mean optical depth. At lower redshifts of z 1 (h⁻¹ Gpc)³. Systems traced by CoSLAs will yield a uniform sample of the most massive overdensities at z > 2 to provide stringent constraints to models of structure formation