Observations of absorption systems in close, multiple lines of sight to gravitationally lensed QSOs can be used to infer the density fluctuations and motions of the gas clouds giving rise to the Lyman α forest phenomenon and to QSO metal absorption systems. We describe a survey of lensed QSOs with the Keck HIRES instrument and argue that one can derive limits on the frequency and impact of hydr...

Detectable flux is visible in the Lyα and Lyβ troughs of the highest redshift (z = 6.42) quasar found to date, SDSS J1148+5251. This has previously been interpreted as continuum contamination from an interloper galaxy at z = 4.94. We examine the Lyγ trough of SDSS J1148+5251 and show that this interpretation is untenable: the spectrum does not show the continuum break in a z = 4.94 galaxy expec...

It is shown that many properties of Lyα forest absorbers can be derived using simple physical arguments. Analytical expressions are derived for the density and the size of an absorber as a function of its neutral hydrogen column density, which agree well with both observations and hydrodynamical simulations. An expression is presented to compute ΩIGM from the observed column density distributio...

The spatial distribution of the Lyα forest is studied using new HST data for the quasar pair Q 1026–0045 A and B at zem = 1.438 and 1.520 respectively. The angular separation is 36 arcsec and corresponds to transverse linear separations between lines of sight of ∼300h 50 kpc (qo = 0.5) over the redshift range 0.833 < z < 1.438. From the observed numbers of coincident and anti-coincident Lyα abs...