Simulating the impact of HI fluctuations on matched filter search for ionized bubbles in redshifted 21 cm maps

TL;DR

This paper evaluates how HI fluctuations affect the detection of ionized bubbles in 21 cm maps using matched filtering, revealing detection limits and the robustness of the technique under various reionization scenarios.

Contribution

It extends previous formalism to analyze HI fluctuation impacts on bubble detection, incorporating realistic semi-numeric simulations and different reionization models.

Findings

Detection limits are R_b<=6 Mpc (GMRT) and R_b<=12 Mpc (MWA) due to HI fluctuations.

Matched-filter technique effectively detects non-spherical bubbles despite inhomogeneous conditions.

Instrument resolution, not HI fluctuations, limits bubble size and position determination in certain scenarios.

Abstract

Extending the formalism of Datta, Bharadwaj & Choudhury (2007) for detecting ionized bubbles in redshifted 21 cm maps using a matched-filtering technique, we use different simulations to analyze the impact of HI fluctuations outside the bubble on the detectability of the bubble. In the first three kinds of simulations there is a spherical bubble of comoving radius R_b, the one that we are trying to detect, located at the center, and the neutral hydrogen (HI) outside the bubble traces the underlying dark matter distribution. We consider three different possible scenarios of reionization, i.e., (i) there is a single bubble (SB) in the field of view (FoV) and the hydrogen neutral fraction is constant outside this bubble (ii) patchy reionization with many small ionized bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same radius $R_b$ (PR2). The fourth kind of simulation uses more realistic maps based on semi-numeric modelling (SM) of ionized regions. We find that for both the SB and PR1 scenarios the fluctuating IGM restricts bubble detection to size R_b<= 6 Mpc and R_b<= 12 Mpc for the GMRT and the MWA respectively, however large be the integration time. These results are well explained by analytical predictions. Large uncertainty due to the HI fluctuations restricts bubble detection in the PR2 scenario for neutral fraction x_HI<0.6. The matched-filter technique works well even when the targeted ionized bubble is non-spherical due to surrounding bubbles and inhomogeneous recombination (SM). We find that determining the size and positions of the bubbles is not limited by the HI fluctuations in the SB and PR1 scenario but limited by the instrument's angular resolution instead, and this can be done more precisely for larger bubble (abridged).