The spatial extent and correlations of QSO absorbers

The lines of sight to QSOs are powerful probes of large-scale structure from redshifts corresponding to the most distant QSOs to the local universe. In this thesis, spectroscopy of QSO pairs and groups are used to study superclustering at high redshift and to estimate the tranverse dimensions of the Lyα absorbers. We present high resolution (∼30km s⁻¹) echelle spectra obtained with the CTIO 4-m telescope of the wide QSO pair Tol 1037—2704 (z(em) = 2.193) and Tol 1038—2712 (z(em) = 2.331), as well as two neighboring quasars. The quasars exhibit a large number of apparently correlated C IV absorption systems over a narrow redshift range 1.48 ≤ 2 ≤ 2.15 which is thought to be produced by an intervening supercluster. The velocity correlation function of C IV absorbers distributed among the four lines of sight show significant clustering signal on comoving scales out to ∼30 h⁻¹ Mpc at redshift z ∼ 2 (h ≡ H₀/100 km s⁻¹ Mpc⁻¹; q₀ = 0.5). The spatial correlation function shows a marginally significant peak on scales of < 18h⁻¹ Mpc. The clustering amplitude on these scales is larger than that predicted by current theories of the formation of large scale structure. We present spectroscopy of three close pairs of quasars with angular separations 10" to 2' in order to measure the sizes of the Lyα forest absorbers from scales of a few tens of kpc out to hundreds of kpc. Ground-based estimates of the pair Q1343+2640.A (z(em) = 2.029) and B (z(em) = 2.031) imply a characteristic radius of the Lyα absorbers of ∼100 h⁻¹ kpc at z ≃ 2. Ultraviolet FOS spectra of the pair Q0107—025A (z(em) = 0.956) and B (z(em) = 0.952) in the redshift range 0.5 < 2 < 0.9 show a number of Lyα absorption features common to both spectra as well as several features which are not in common, and imply characteristic radii of 400 h⁻¹ kpc to bigger than 1 h⁻¹ Mpc. Furthermore, the rms velocity difference between the common systems between the two lines of sight is only about 100 km s⁻¹ These measurements lead to a picture of absorbing clouds that are larger in extent than previously thought and surprisingly quiescent. Using a new statistical technique, we tested the relative likelihood of three geometric models, namely, spherical absorbers, with and without a distribution in size, as well as filamentary and disk-like absorbers. Spherical absorbers with uniform radius cannot represent the observations and are ruled out. Randomly-inclined disks and filaments match the data comparably, with disks being slightly favored over filaments. Our results are in remarkable agreement with hydrodynamical simulations in which the Lyα absorption is found to arise in diverse structures with coherence lengths as great as 1 Mpc. Finally, we present FOS observations of a second pair of quasars, LB 9605 (z(em) = 1.834) and LB 9612 (z(em) = 1.898), over the redshift range 1.1 < z < 1.7 from which we placed an 95% confidence upper limit on the radii of the Lyα absorbers of 280 h⁻¹ kpc. The estimates span the redshift range 0.5 < z < 2, corresponding to roughly a third of the age of the universe, and provide tantalizing evidence for evolutionary growth in the size of the Lyα absorber