The evolution of the low-density H i intergalactic medium from z = 3.6 to 0: data, transmitted flux, and H i column density,,

ABSTRACT We present a new, uniform analysis of the H i transmitted flux (F) and H i column density (N₇\, ₈) distribution in the low-density IGM as a function of redshift z for 0 z 3. 6 using 55 HST/COS FUV (Δz = 7. 2 at z 0. 5), five HST/STIS + COS NUV (Δz = 1. 3 at z ∼ 1) and 24 VLT/UVES, and Keck/HIRES (Δz = 11. 6 at 1. 7 z 3. 6) AGN spectra. We performed a consistent, uniform Voigt profile analysis to combine spectra taken with different instruments, to reduce systematics and to remove metal-line contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z ∼ 1. Two flux statistics at 0 F 1, the mean H i flux and the flux probability distribution function (PDF), show that considerable evolution occurs from z = 3. 6 to z = 1. 5, after which it slows down to become effectively stable for z 0. 5. However, there are large sightline variations. For the H i column density distribution function (CDDF, f ∝ N ₇\, ₈^-) at (N₇\, ₈/1\, cm^{-2}) ∈ 13. 5, 16. 0, β increases as z decreases from β = 1. 60 at z ∼ 3. 4 to β = 1. 82 at z ∼ 0. 1. The CDDF shape at lower redshifts can be reproduced by a small amount of clockwise rotation of a higher-z CDDF with a slightly larger CDDF normalization. The absorption line number per z (dn/dz) shows a similar evolutionary break at z ∼ 1. 5 as seen in the flux statistics. High-N₇\, ₈ absorbers evolve more rapidly than low-N₇\, ₈ absorbers to decrease in number or cross-section with time. The individual dn/dz shows a large scatter at a given z. The scatter increases towards lower z, possibly caused by a stronger clustering at lower z.