Study of inter-galactic medium in the epoch of reionization era from simulated HI 21cm images using 2D wavelet scattering transform

Abstract

Epoch of Reionization (EoR) is a crucial phase when the Intergalactic Medium (IGM), primarily composed of neutral hydrogen (HI), began to undergo ionization due to the influence of high-energy photons emitted by various sources. This epoch holds critical significance because it sheds light on the timing and nature of the first luminous sources, provides context for interpreting high-redshift observations affected by intervening IGM, and serves as a test for dark matter and galaxy formation models, which must produce sources consistent with reionization constraints. Observations so far haven’t been able to probe the exact duration, timing of reionization & the characteristics of the primary ionizing sources. The HI 21-cm signal holds significant promise as a direct means of investigating these critical aspects of this epoch. By directly measuring HI 21-cm signal at different redshifts along with its spatial distribution in the IGM, we can trace the history of reionization as a three dimensional movie of the early universe. The upcoming Square Kilometer Array (SKA) telescope is expected to be capable of such 21-cm observations. SKA is expected to make tomographic images of the neutral hydrogen distribution during the Epoch of Reionization via the redshifted 21-cm signal. Non-Gaussian information from these observations can reveal, in principle, the nature of ionizing sources and the dominant astrophysical processes in the Intergalactic medium. We apply Wavelet Scattering Transform (WST) to analyze simulated HI 21-cm images & extract information from 21-cm images to help answer these questions about the IGM. We consider different reionization sources in our work which emit UV, soft X-ray & hard X-ray photons. We have also considered the effects of halo mass dependent power-law efficiency and non-uniform recombination. We compare the WST statistic with power spectrum. We demonstrate that 2D WST outperforms 3D spherically averaged power spectrum when it comes to its ability to probe non-Gaussian information. We further quantify the impact of light cone effect on WST Analysis by generating lightcones from our simulated coeval cubes and found that lightcone results follow the same trends as those of the simulated coeval cubes, which is expected from the 2D nature of WST. Impact of telescope resolution and rms noise effects on WST coefficients has been studied at varying rms noise levels and source models. WST coefficients at intermediate scales show promising capacity to distinguish source models.