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

Abstract

Extending the formalism of Datta, Bharadwaj & Choudhury for detecting ionized bubbles in redshifted 21-cm maps using a matched filtering technique, we use different simulations to analyse the impact of H I 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 Rb, the one that we are trying to detect, located at the centre, and the neutral hydrogen (H I) outside the bubble traces the underlying dark matter distribution. We consider three different possible scenarios of re-ionization, 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 re-ionization (PR) with many small ionized bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same radius Rb (PR2). The centres of the extra bubbles trace the dark matter distribution. The fourth kind of simulation uses more realistic maps based on seminumeric modelling (SM) of ionized regions. We make predictions for the currently functioning Giant Metrewave Radio Telescope (GMRT) and a forthcoming instrument, the Murchison Widefield Array (MWA) at a redshift of 6 (corresponding to an observed frequency 203 MHz) for 1000 h observations. We find that for both the SB and PR1 scenarios the fluctuating intergalactic medium restricts bubble detection to size Rb≤6 Mpc and Rb≤ 12 Mpc for the GMRT and the MWA, respectively, however large be the integration time. These results are well explained by analytical predictions. In the PR2 scenario, we find that bubble detection is almost impossible for neutral fraction xH I < 0.6 because of large uncertainty due to the H I fluctuations. Applying the matched filter technique to the SM scenario, we find that it works well even when the targeted ionized bubble is non-spherical due to surrounding bubbles and inhomogeneous recombination. We find that determining the size and positions of the bubbles is not limited by the H I 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. We also find that for bubble detection the GMRT configuration is somewhat superior to the proposed MWA.