Number-density evolution of Lyman-alpha line

This image shows the number-density evolution of the Lyman alpha lines, observed with UVES in the quasars HE22-28 and QSO J2233-606 . 

The latter is the QSO at the centre of the southern Hubble Deep Field (HDF-South), with z(em) = 2.238 The two filled symbols at the centre of the diagram represent the number density of hydrogen lines at N(HI) = 10 (13.64-16) cm -2. The measurements from HE22-28 (to the right; average z = 2.12) and J2233-606 (left; average z = 1.96) fall in the middle of the little studied redshift interval, l1.6 dN/dz = 11.75 (1+z) 2.01. The new determinations based on the UVES observations seem to be in reasonable agreement with the Keck measurements at higher redshifts, indicating a similar number density evolution. Contrarily, there is a discrepancy with the measurements at redshift z.