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 % .
The amount of absorption by H I in the IGM provides the best evidence on the physical conditions in the IGM , especially the combination of the mean gas density , the density fluctuations , the intensity of the ionizing flux , and the level of ionization .
We have measured the amount of absorption , known as the flux decrement , DA , in the Ly \alpha forest at redshift 1.9 .
We used spectra of 77 QSO that we obtained with 250 km s ^ { -1 } resolution from the Kast spectrograph on the Lick observatory 3m telescope .
We fit the unabsorbed continua to these spectra using b-splines .
We also fit equivalent continua to 77 artificial spectra that we made to match the real spectra in all obvious ways : redshift , resolution , S/N , emission lines and absorption lines .
The typical relative error in our continuum fits to the artificial spectra is 3.5 % .
Averaged over all 77 QSOs the mean level is within 1–2 % of the correct value , except at S/N < 6 where we systematically placed the continuum too high .
We then adjusted the continua on the real spectra to remove this bias as a function of S/N and a second smaller bias .
Absorption from all lines in the Ly \alpha forest at z = 1.9 removes DA ( z=1.9 ) = 15.1 \pm 0.7 % of the flux at rest frame wavelengths 1070 < \lambda _ { r } < 1170 Å .
This is the first measurement using many QSOs at this z , and the first calibrated measurement at any redshift .
Using similar methods on 1225 < \lambda _ { r } < 1500 Å we find metal lines absorb an average 1.6 % the flux , increasing slightly as the rest frame wavelength \lambda _ { r } decreases because more types of spectral lines contribute and there is more C IV at lower redshifts .
We estimate that the metal lines absorb 2.3 \pm 0.5 % of the flux in the Ly \alpha forest at z=1.9 .
The absorption from Ly \alpha alone then has DA = 12.8 \pm 0.9 % .
The Ly \alpha lines of Lyman limit systems with column densities log N _ { HI } > 17.2 cm ^ { -2 } are responsible for a DA = 1.0 \pm 0.4 % at z = 1.9 .
These lines arise in higher density regions than the bulk of the IGM Ly \alpha absorption , and hence they are harder to simulate in the huge boxes required to represent the large scale variations in the IGM .
If we subtract these lines , for comparison with simulations of the lower density bulk of the IGM , we are left with DA = 11.8 \pm 1.0 % .
The mean DA in segments of individual spectra with \Delta z = 0.1 , or 153 Mpc comoving at z = 1.9 , has a large dispersion , \sigma = 6.1 \pm 0.3 % including Lyman limit systems ( LLS ) and metal lines , and \sigma ( \Delta z = 0.1 ) = 3.9 ^ { +0.5 } _ { -0.7 } % for the Ly \alpha from the lower density IGM alone , excluding LLS and metal lines .
This is consistent with the usual description of large scale structure and accounts for the large variations from QSO to QSO .
Although the absorption at z = 1.9 is mostly from the lower density IGM , the Ly \alpha of LLS and the metal lines are both major contributors to the variation in the mean flux on 153 Mpc scales at z = 1.9 , and they make the flux field significantly different from a random Gaussian field with an enhanced probability of a large amount of absorption .
We find that a hydrodynamic simulation on a 1024 ^ { 3 } grid in a 75.7 Mpc box reproduces the observed DA from the low density IGM alone when we use popular parameters values H _ { o } = 71 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { b } = 0.044 , \Omega _ { m } = 0.27 , { \Omega _ { \Lambda } = 0.73 } , { \sigma _ { 8 } = 0.9 } and a UV background ( UVB ) that has an ionization rate per H I atom of \Gamma _ { 912 } = ( 1.44 \pm 0.11 ) \times 10 ^ { -12 } s ^ { -1 } .
This is 1.08 \pm 0.08 times the prediction by Madau , Haardt & Rees ( 1999 ) with 61 % from QSOs and 39 % from stars .