## Lyman forest

The spectra of high redshift galaxies suffer of a drop in flux below Ly at Å and a further decline below Ly, at Å. These drops are attributed to absorbing neutral hydrogen clouds at different redshifts between the source and the observer, forming the so-called Lyman forest in the spectra of high redshift objects.

Oke & Korycansky (1982), studying a sample of high redshift QSOs, defined two depression factors,  and , characterizing the amount of absorption between Ly and Ly and between Ly and the Lyman limit respectively: , where  and  are the observed and intrinsic fluxes per unit wavelength in the QSO restframe.  is defined in the same way as. We adopted the estimates given by Madau (1995) for the attenuation of the continuum due to line blanketing as a function of redshift. He found the flux decrements averaged over all the lines of sight to be

 (2) (3)

where Å and Å. The coefficient  relative to the Ly forest contribution is equal to . The main contribution to , representing the flux decrement caused by all Lyman series lines, belongs to Ly, Ly (Å) and Ly (Å), whose coefficients are  and . Figure 5 shows the depression factors  and  as computed from equations 2 and 3. The fact that implies that Ly and higher order lines contribute significantly to line blanketing. We have applied Madau's prescriptions to compute the opacity of IGM through the mean factors  and , whereas we approximated the Lyman continuum absorption setting  for both observed and synthetic SEDs below , where Å is the Lyman limit.

micol bolzonella

2000-12-10