State of the Universe

The effect of inhomogeneous reionisation on the Lyman alpha forest

by Dr. Margherita Molaro (University of Nottingham, U.K.)

Friday, October 8, 2021 from to (Asia/Kolkata)
at Zoom link: (Meeting ID: 825 1295 6967 Passcode: 384194)
Lyman-alpha absorption features in the spectra of bright, high-redshift quasars contain a wealth of information on the evolution of the intergalactic medium (IGM) during and soon after the final stages of reionisation. By comparing observations with simulations of these absorption features in different reionisation scenarios we are therefore able to obtain information on how this process unfolded. One of the most significant challenges on the simulation side of this effort remains the implementation of photo-ionisation and -heating of the IGM by UV photons. Homogeneous UV background approximations are widely used to keep computational costs down, however, it is believed they might miss crucial fluctuations in the ionisation and temperature fields, and therefore artificially alter the simulated Lyman alpha features. It is therefore important to establish whether constraints obtained from such models might be affected by biases.

In this talk, I will review a recent work directly comparing a homogeneous UVB approximation with a hybrid-RT prescription in the Sherwood-Relics suite of IGM simulations. This study has indeed confirmed that the different treatment of the UV radiation field leads to significant differences in the 1D Lyman alpha power spectrum, at both small and large scales. More importantly, it has been highlighted that inhomogeneous reionisation effects should be accounted for in analyses of forthcoming high precision measurements of high redshift quasars if systematic biases are to be avoided. Conveniently, the same study also allowed us to develop a correction term to account for these effects that can be easily applied to existing simulations relying on homogeneous UV background approximations.