Impact of light-cone effect on EoR 21-cm image analysis using largest cluster statistics (LCS)

Abstract

Epoch of reionization (EoR) is the time period in which neutral matter, primarily composed of neutral hydrogen, present in the Intergalactic medium (IGM) has been gradually ionized by the luminous sources. Lack of observations during EoR makes this epoch least understood one. The redshifted 21-cm signal emitted due to spinflip transition in neutral Hydrogen is one of the promising probes to understand the evolution of topology and morphology of ionized or neutral regions, thereby the properties and distribution of ionizing sources during EoR. The low-frequency band of upcoming Square Kilometre Array (SKA) is expected to have enough sensitivity to detect 21-cm signal and produce tomographic images of this signal with cosmic time or observation frequency. To assess the statistical properties of this 21-cm signal, various methods have been developed. Fourier statistics e.g. power spectrum helps us to understand the fluctuations of this field at different length scales but won’t be able to provide the information about phase of this signal. Image analysis techniques like percolation analysis, superpixel method etc. retains the phase information of the signal. Once the ionized bubbles are identified with image analysis techniques then summary statistics e.g. bubble size distributions (BSDs) and Largest Cluster Statistics (LCS) will help us to understand the properties like evolution of morphology of IGM, nature of photon sources responsible for ionizing the neutral matter present in the IGM etc. Previous works have demonstrated that different reionization scenarios can be differentiated from each other by using percolation analysis on the simulated coeval maps with LCS as a key statistic for the analysis. The LCS technique can be used for “percolation analysis” which gives the information about the percolation process (where all the ionized regions coalesce into infinitely connected one single large ionized region). However this analysis has never been done considering the impact of the finite light travel time on the signal (light-cone effect), which is inherent to any radio interferometric observations of the EoR. In this work for the first time we employ the LCS on the simulated light-cone tomographic 21-cm maps with the aim of doing the percolation analysis. We use moving-volume method on the light-cone maps to quantify the impact of the light cone effect on the percolation analysis. We also repeat our analysis for various kinds of reionization scenarios. Our results show that, it is possible to distinguish between different reionization scenarios even from the light cone LCS analysis.