Astrophysics Seminar

Lyman-α(Lyα) forest in the spectra of distant quasars encodes the information of the underlying cosmic density field at the smallest scales. The modelling of the upcoming large and high-fidelity forest data using cosmological hydrodynamical simulations is computationally challenging and therefore, requires accurate semi-analytical techniques. One such approach is based on the assumption that baryonic density fields in the intergalactic medium (IGM) follow lognormal distribution. 

Keeping this in mind, we test the validity of the lognormal model in recovering the parameters characterizing IGM state, namely, the mean-density IGM temperature (T0), the slope of the temperature-density relation (γ), and the hydrogen photoionization rate (Γ12), between 2.0 ≤ z ≤ 2.7 with Sherwood smooth particle hydrodynamical simulations (SPH) data as the benchmark. These parameters are estimated through a Markov Chain Monte Carlo (MCMC) technique, using the mean and power spectrum of the transmitted flux. In this talk, I will first present the results from the MCMC runs related to the recovery of the astrophysical parameters using the lognormal model at z ∼ 2.5. We find that we are able to recover all the three IGM parameters within 1-σ of their true values in SPH. These fits remain consistent w.r.t. box size & resolution, SNR, and data seed but worsen on increasing the path length of the data. 

We then extend this work to other redshifts as well as modify the model by scaling the 1D baryonic density field by a free parameter. The modification is required to improve parameter recovery with higher path length data. We show that such a modified lognormal model is able to recover the thermal history at acceptably well for most redshifts but the recovery of photoionization rate remains significantly discrepant. Additionally, we also study the correlations in the flux statistics across different redshift bins. Studying these correlations is important since observed skewers typically span multiple redshift bins. Lastly, I will discuss the current state and future prospects of the work.