Probing the physical state of the intergalactic medium and Quasar environments

Abstract

Soon after the discovery of the first quasars, a small decrement in the spectrum of a quasar shortward of its Lyα emission line was measured. Recognizing the decrement was due to the Lyα resonance absorption of radiation from the quasar’s continuum by intervening neutral hydrogen, it turns out that the number density of neutral hydrogen was exceedingly small, implying that either the galaxy formation was an extraordinarily efficient process, sweeping up all but a tiny residue of the primordial hydrogen, or that the gas in intergalactic space, the Intergalactic Medium (IGM), is ionized by UV background (UVB) radiation field. The UVB ionization scenario is backed by its multiple excellent determinations made using space and ground-based instruments. The UV-radiation from quasars also creates excess ionization around it, commonly known as the quasars proximity effect. Its extent is dependent on the strength of the UVB radiation along with the quasar’s luminosity. However, many previous studies have shown that the quasar’s environment has excess clustering, as a result, it is difficult to interpret the proximity effect due to lack of agreed-upon density profile around the quasars over megaparsec (Mpc) length scales and/or the possibility of an-isotropic radiation field of quasars. We have addressed these issues in detail in this thesis by using recently improved UVB radiation field measurements available from the independent method in conjunction with the availability of a large sample of quasar spectra.