Quasars and the transverse proximity effect

Abstract

Quasars, first identified in the 1960s, have been used as cosmological probes to study the Inter-Galactic Medium. This includes the process of its metal enrichment and how material is clustered at different scales in the high redshift Universe. The effect of quasars on their environment can help shed light on the variability of their luminosity and anisotropies in the geometry of the quasar radiation field. One way to probe the effects of quasars on their environment is through the Transverse Proximity Effect (TPE). The TPE should be visible as a diminution of the Lyman-alpha forest absorption in the spectrum of a background quasar, at a foreground quasar's redshift, when the two quasars are transversely separated by < 5 Mpc. Alternatively, the TPE can also be detected as a local enhancement of the ionising radiation field proximate to the foreground quasar, via the strength of different atomic species in the background quasar's spectrum. This thesis aims to use the CFHTLS data to search for quasar pairs at z > 2.2 having small transverse separations on the sky by using an efficient selection method, which we prove is capable of detecting numerous rare proximate quasar pairs. The best pairs can be used to detect the TPE by analysing the metal lines in the spectrum of the background quasar, a technique demonstrated in this thesis that is superior to simply using the diminution of the Lyman-alpha forest absorption as a detection tool. Such studies can reveal important information regarding quasar lifetimes and opening angles when conducted on a statistically significant scale. We utilise an effective statistical technique, Kernel Density Estimation (KDE), to select quasars from the photometric data sets using the ugriz colour space. The KDE algorithm has been successfully applied to the CFHTLS data, detecting 1,000 quasars at z > 2.2, using the AAOmega spectrograph on the Anglo-Australian Telescope. We have detected 40 TPE candidates, with one standout candidate in our sample due to its small angular separation (11.9"), with the foreground quasar at z = 2.23 and background quasar at z = 2.56. We have followed-up this best pair using MIKE (echelle spectrograph) on Magellan. The background quasar's spectrum has revealed two metal absorption systems lying close to the redshift of the foreground quasar (z = 2.23). We have identified absorption features due to HI, CIV, CII, SiIV and SiII in our data. Photoionisation modelling of these absorbers using MAPPINGS III, based on the column densities of the metal lines, has revealed that the absorbers are being ionised by the foreground quasar, i.e. we detect the TPE as an enhancement in the ionisation, above the UV background, proximate to the foreground quasar. The fact that the quasar is affecting both the metal line absorbers which lie within its ionisation region suggests that its radiation cone has a half angle that is quite wide (> 45 degrees). Since the transverse separation between our two quasars is ~100 kpc, we find that the minimum lifetime of our foreground quasar is ~ 0.3 Myr. Show more