Abstract:
In recent years, the Lyman-α forest in quasar spectra has
been used, together with N-body simulations, to determine the underlying
matter distribution in the intergalactic medium (IGM). One of the key
parameters to be known in order to compare observations and numerical
simulations is the mean HI absorption in the IGM. To derive the latter, one
has first to fit the quasar continuum.We have observed 20 high redshift and
highly luminous QSOs (mV ≤ 17.5 and 2.40 ≤ zem ≤ 3.91) at intermediate
spectral resolution, with either EMMI (ESO Multi-Mode Instrument)
on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de
Haute-Provence), and applied different methods of determining the QSO
continuum to this QSO sample. We have measured the amount of absorption,
known as the flux decrement, DA, in the Lyman-α forest for these
different methods and compared the results. In addition, we have compared
DA values measured along the same lines of sight observed at high
and intermediate spectral resolutions.We discuss the systematics resulting
from the use of automatic continuum fitting methods.
