Constraining the neutral fraction of the IGM using quasar absorption spectroscopy

Abstract

The Epoch of Reionization (EoR) connects the observables to the Early Universe, providing important insights about ionization and thermal state of the Universe. Intergalactic medium (IGM) is ionized by the first luminous sources such as young galaxies and its properties can reveal timing, topology and physics of EoR. Direct measurements of observables during EoR becomes difficult primarily due to the IGM being opaque to Lyman α transmission due to high absorption, the patchiness of the reionization and even other faint 21 cm observations being dominated by the foreground effects. We aim to find whether the saturated regions arise from the UVB fluctuations or truly neutral islands. In this work, we investigate the ionization state of the IGM using dark gaps present in the quasar spectra. We consider the dark gaps are defined as the continuous regions between two successive transmission peaks lying below normalized flux Fnorm < 0.05 arising due to strong absorption by neutral hydrogen present along line of sight. We consider the minimum length of the dark gap as 1 h−1Mpc and also rebin the spectra to 1 h−1Mpc. Using EXCITE simulations, we explore three models: Early, Late, Ultra-Late Model and we also define a scaling parameter which scales the mean photoionization rate to explore larger parameter space of neutral fraction i.e. Γ12(x) = fΓ⟨Γ12⟩.