Analysing Hi 21-cm images from the epoch of reionization using largest cluster statistics (LCS)

Abstract

Two of the most enigmatic eras in the universe’s history are the Cosmic Dawnand Epoch ofReionization (CD-EoR). We can learn more about the early luminous sources and astrophysical processes in the Intergalactic Medium (IGM) at those times by analyzing the changes in the distribution of neutral and ionized hydrogen during these epochs. Utilizing the topological and morphological evolution of the redshifted 21-cm signal originating from the neutral hydrogen distribution throughout various CD-EoR stages, we examine these changes. The evolution of the largest ionized region (LIR) during these stages is examined using the Largest Cluster Statistics (LCS). We show that LCS is a reliable statistic that can distinguish between inside-out and outside-in reionization models by comparing their corresponding percolation transition period, the cosmic period during which the majority of the individual ionized regions become connected and form a single, infinitely long ionized region. We also use Shapefinders to examine the shape and morphology of ionized regions under various reionization models as a complement to the percolation analysis. In all the models, we observe that the largest ionized region’s volume abruptly increases in length during percolation, but its thickness and breadth essentially hold constant. We scale down the simulated 21-cm maps to a lower resolution to be consistent with the upcoming SKA1-Low. The scaled-down Hi 21-cm maps from EoR are then contaminated with simulated Gaussian noise with varying RMS values to match the upcoming square Kilometer Array (SKA) in its low frequency regime observations, specifically the SKA1-Low. Then, the maps were combined with a 3D Gaussian kernel with a range of length scales to simulate the impact of the telescope beam, and the images were examined with LCS. Further research was done to determine how the array synthesized beam impacts the 21-cm signal’s LCS analysis, particularly in light of upcoming SKA1-Low observations. We used an accurate OSKAR simulation based on the 21cmE2E-pipeline to achieve this. According to our research, the array beam introduces bias into the LCS estimation of synthetic observations, which causes the apparent percolation transition point to shift toward the later stages of reionization.