SIMBAD references

For the first time, we determine a statistically significant steepening in the column density distribution function at redshifts z>4.0 (greater than 99.7% confidence). The steepening of the distribution function is due to both fewer very high column density absorbers (N_HI_≥10²¹ atoms.cm^–2) and more lower column density systems (N_HI_=2-4x10²⁰ atoms.cm^–2).

The frequency of very high column density absorbers (N_HI_≥10²¹ atoms.cm^–2) reaches a peak in the redshift range 1.5<z<4, when the universe is 10%-30% of its present age. Although the sample size is still small, the peak epoch appears to be 3.0≤z≤3.5. The highest column density absorbers disappear rapidly toward higher redshifts in the range z=3.5⟶4.7 and lower redshifts z=3.0⟶0. None with column densities logN_HI_≥21 have yet been detected at z>4, although we have increased the redshift path surveyed by ~60%.

With our current data set, the comoving mass density of neutral gas, Ω_g, appears to peak at 3.0<z<3.5, but the uncertainties are still too large to determine the precise shape of Ω_g. The statistics are consistent with a constant value of Ω_g for 2<z<4. There is still tentative evidence for a drop-off at z>4, as indicated by earlier data sets. If we define R_g*≡Ω_g/Ω_*, where R_g* is the ratio of the peak value of Ω_g to Ω_*, the mass density in galaxies in the local universe, we find values of R_g*=0.25-0.5 at z∼3, depending on the cosmology. For an Ω=1 universe with a zero cosmological constant, R_g*~0.5. For an Ω=1 universe with a positive cosmological constant (Ω_Λ=0.7, Ω_M=0.3), we find R_g*~0.25. For a universe with Ω_Λ=0 and Ω_M=0.3, we find R_g*~0.3.

Ω_g decreases with redshift for the interval z=3.5⟶0.008 for our data set, but we briefly discuss new results from Rao & Turnshek for z<1.5 that suggest that Ω_g (z<1.5) may be higher than previously determined.