Tracing the gas at redshift 1.7-3.5 with the Lyα forest:: the FLO approach

We present FLO (From Lines to Overdensities), a new technique to reconstruct the hydrogen density field for the Ly alpha forest lines observed in high-resolution QSO spectra. The method is based on the hypothesis that the Ly alpha lines arise in the low to intermediate density intergalactic gas and that the Jeans length is the typical size of the Ly alpha absorbers. The reliability of FLO is tested against mock spectra obtained from cosmological simulations. The recovering algorithm gives satisfactory results in the range from the mean density to overdensities of similar to 30 and reproduces correctly the correlation function of the density field and the ID power spectrum on scales between similar to 20 and 60 comoving Mpc. A sample of Ly alpha forests from 22 high-resolution QSO spectra is analysed, covering the redshift range 1.7 less than or similar to z less than or similar to 3.5. For each line of sight, we fit Voigt profiles to the lines of the Ly alpha forest, providing the largest, homogeneous sample of fitted Ly alpha lines ever studied. The line number density evolution with redshift follows a power-law relation: dn/dz = (166 +/- 4) [(1 + z)/3.5]((2.8 +/- 0.2)) (1 sigma errors). The two-point correlation function of lines shows a signal up to separations of similar to 2 comoving Mpc; weak lines [logN(Hi) < 13.8] also show a significant clustering but on smaller scales (r less than or similar to 1.5 comoving Mpc). We estimate with FLO the hydrogen density field toward the 22 observed lines of sight. The redshift distribution of the average densities computed for each QSO is consistent with the cosmic mean hydrogen density in the analysed redshift range. The two-point correlation function and the 1D power spectrum of the 6 field are estimated. They are both consistent with the analogous results computed from hydrosimulated spectra obtained in the framework of the concordance cosmological model. The correlation function shows clustering signal up to similar to 4 comoving Mpc.