Detecting ionized bubbles in redshifted 21-cm maps

Abstract

The reionization of the Universe, it is believed, occurred by the growth of ionized regions (bubbles) in the neutral intergalactic medium. We study the possibility of detecting these bubbles in radio-interferometric observations of redshifted neutral hydrogen (HI) 21-cm radiation. The signal (<1 mJy) will be buried in noise and foregrounds, the latter being at least a few orders of magnitude stronger than the signal. We develop a visibility based formalism that uses a filter to optimally combine the entire signal from a bubble while minimizing the noise and foreground contributions. This formalism makes definite predictions on the ability to detect an ionized bubble or conclusively rule out its presence in a radio-interferometric observation. We make predictions for the currently functioning Giant Metre-Wave Radio Telescope (GMRT) and a forthcoming instrument, the Mileura Widefield Array (MWA) at a frequency of 150 MHz (corresponding to a redshift of 8.5). For both instruments, we show that a 3σ detection will be possible for a bubble of comoving radius Rb≥ 40 Mpc (assuming it to be spherical) in 100 h of observation and Rb≥ 22 Mpc in 1000 h of observation, provided the bubble is at the centre of the field of view. In both these cases, the filter effectively removes the expected foreground contribution so that it is below the signal, and the system noise is the deciding criteria. We find that there is a fundamental limitation on the smallest bubble that can be detected arising from the statistical fluctuations in the HI distribution. Assuming that the HI traces the dark matter, we find that it will not be possible to detect bubbles with Rb < 8 Mpc using the GMRT and Rb < 16 Mpc using the MWA, however, large be the integration time.