Cosmological parameters sigma_8, the baryon density, and the UV
background intensity from a calibrated measurement of H I Lyman-alpha
absorption at z = 1.9
We identify a concordant model for the intergalactic medium (IGM) at redshift
z=1.9 that uses popular values for cosmological and astrophysical parameters
and accounts for all baryons with an uncertainty of 6%. We have measured the
amount of absorption, DA, in the Ly-alpha forest at redshift 1.9 in spectra of
77 QSO from the Kast spectrograph. We calibrated the continuum fits with
realistic artificial spectra, and we found that averaged over all 77 QSOs the
mean continuum level is within 1-2% of the correct value. Absorption from all
lines in the Ly-alpha forest at z=1.9 removes DA=15.1 +/- 0.7% of the flux
between 1070 and 1170 (rest) Angstroms. This is the first measurement using
many QSOs at this z, and the first calibrated measurement at any redshift.
Metal lines absorb 2.3 +/- 0.5%, and LLS absorb 1.0 +/- 0.4% leaving 11.8 +/-
1.0% from the lower density bulk of the IGM. Averaging over Delta z=0.1 or 154
Mpc, the dispersion is 6.1 +/- 0.3% including LLS and metal lines, or 3.9
(+0.5, -0.7)% for the lower density IGM alone, consistent with the usual
description of large scale structure. LLS and metal lines are major
contributors to the variation in the mean flux, and they make the flux field
significantly non-Gaussian. We find that a hydrodynamic simulation on a 1024
cubed grid in a 75.7 Mpc box reproduces the observed DA from the low density
IGM with parameters values H_o=71 km/s/Mpc, Omega_Lambda=0.73, Omega_m=0.27,
Omega_b=0.044, sigma_8=0.9 and a UV background that has an ionization rate that
is 1.08 +/- 0.08 times the prediction by Madau, Haardt & Rees (1999).Comment: Submitted to Ap