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Abstract: We have measured the redshift evolution of the density of Lyman limit systemsLLS in the intergalactic medium over the redshift range 0 < z < 6. We haveused two new quasar samples to 1 improve coverage at z ~ 1, with GALEX grismspectrograph observations of 50 quasars with 0.8 < z em < 1.3, and 2 extendcoverage to z ~ 6, with Keck ESI spectra of 25 quasars with 4.17 < z em < 5.99.Using these samples together with published data, we find that the numberdensity of LLS per unit redshift, nz, can be well fit by a simple evolutionof the form nz = n 3.5 1+z-4.5^gamma, with n 3.5 = 2.80 +- 0.33 andgamma = 1.94^+0.36 -0.32 for the entire range 0 < z < 6. We have alsoreanalyzed the evolution of damped Lyman alpha systems DLAs in the redshiftrange 4 < z < 5 using our high-redshift quasar sample. We find a total of 17DLAs and sub-DLAs, which we have analyzed in combination with published data.The DLAs with log HI column density > 20.3 show the same redshift evolutionas the LLS. When combined with previous results, our DLA sample is alsoconsistent with a constant Omega DLA= 9 x 10^-4 from z = 2 to z = 5. We haveused the LLS number density evolution to compute the evolution in the mean freepath of ionizing photons. We find a smooth evolution to z ~ 6, very similar inshape to that of Madau, Haardt and Rees 1999 but about a factor of two higher.Recent theoretical models roughly match to the z < 6 data but diverge from themeasured power law at z > 6 in different ways, cautioning against extrapolatingthe fit to the mean free path outside the measured redshift range.



Author: Antoinette Songaila, Lennox L. Cowie

Source: https://arxiv.org/







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