Small‐Scale Structure at High Redshift. III. The Clumpiness of the Intergalactic Medium on Subkiloparsec Scales

Abstract

Spectra obtained with the Keck HIRES instrument of the Lyα forests in the lines of sight to the A and C components of the gravitationally lensed quasi-stellar object Q1422+231 were used to investigate the structure of the intergalactic medium at mean redshift z ~ 3.3 on subkiloparsec scales. We measured the cross-correlation amplitude between the two Lyα forests for a mean transverse separation of 120 h pc and computed the rms column density and velocity differences between individual absorption systems seen in both lines of sight. The rms differences between the velocity centroids of the Lyα forest lines were found to be less than about 400 ms-1 for unsaturated H I absorption lines with column densities in the range 12 < log N(H I) < 14.13. The rate of energy transfer into the low-density intergalactic medium on a typical scale of 100 pc seems to be lower by 3-4 orders of magnitude than the rate measured earlier for strong C IV metal absorption systems. The tight correlation between H I column density and baryonic density in the intergalactic medium was used to obtain a conservative upper limit on the rms fluctuations of the baryonic density field at z = 3.26, namely, [(Δ log ρ)2]1/2 ≤ 3.1 × 10-2 on a scale of 110 h pc. The fraction of the absorption lines that are different across the lines of sight was used to determine the filling factor of the universe for gas that has suffered recent hydrodynamic disturbances. We thereby derived upper limits on the filling factor of galactic outflows at high redshift. Short-lived, short-range ancient winds are essentially unconstrained by this method, but strong winds blowing for a substantial fraction of a Hubble time (at z = 3.3) appear to fill less than 20% of the volume of the universe.