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

We present a new, uniform analysis of the H I transmitted flux (F) and H I column density (N-HI) distribution in the low-density IGM as a function of redshift z for 0 < z < 3.6 using 55 HST/COS FUV (Delta z = 7.2 at z < 0.5), five HST/STIS + COS NUV (Delta z = 1.3 at z similar to 1) and 24 VLT/UVES, and Keck/HIRES (Delta z = 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 metalline contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z similar to 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 alpha N-H I(-beta)) at log(N-H I/1 cm(-2)) is an element of [13.5, 16.0], beta increases as z decreases from beta = 1.60 at z similar to 3.4 to beta = 1.82 at z similar to 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 similar to 1.5 as seen in the flux statistics. High-NH, absorbers evolve more rapidly than low-N(H I )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.

Automated summary

A new uniform analysis of the H I transmitted flux and column density distribution in the low-density IGM as a function of redshift was presented. The study confirmed previous conclusions and revealed evolution in flux and column density distribution with decreasing redshift, as well as variations in sightlines. High-NH absorbers were found to evolve more rapidly than low-N(H I ) absorbers, decreasing in number or cross-section over time.