High spatial resolution spectroscopy of the Lyα absorption towards the gravitational lens system B1422+2309

We have obtained spectroscopy of individual components of the gravitational lensed quasar B1422+2309 using the Subarcsecond Imaging Spectrograph at the Canada-France-Hawan Telescope. The quasar has a redshift of 3.62, with four lensed images within a diameter of 1.3″. Components B and C have nearly identical spectra, confirming that they are the gravitationally lensed images of a single quasar. Numerous absorption lines, most of which are attributable to Lyα, are seen in both spectra. All lines detected are seen in the spectra of both components, implying that the Lyα forest clouds are larger than 0-0.15h₁₀₀^-1 kpc at z≈3.5. The equivalent widths of common lines are highly correlated, but the differences in the equivalent widths are larger than what is expected from the statistical errors in the individual lines. The scatter in the differences in equivalent widths can be accounted for if the column densities between the two lines of sight differ on average by approximately a factor of 1.5. This suggests that there are small-scale inhomogeneities in the gas density of the structures causing Lyα forest absorption.