The optimal redshift for detecting ionized bubbles in HI 21-cm maps

TL;DR

This paper identifies the optimal redshift ranges for detecting ionized bubbles during the epoch of reionization using HI 21-cm maps with GMRT and MWA, emphasizing the importance of reionization timing and instrument sensitivity.

Contribution

It determines the optimal redshift ranges for bubble detection with specific instruments and compares detection prospects under different reionization scenarios.

Findings

Optimal redshift for GMRT is ~8.1; for MWA is ~9.8.

Detection of bubbles with radius >= 30 Mpc is feasible with 1000 hrs observation.

Late reionization scenario improves detection prospects.

Abstract

The detection of individual ionized bubbles in HI 21-cm maps is one of the most promising, direct probes of the epoch of reionization(EoR). Atleast 1000 hrs of observation would be required for such a detection with either the currently functioning GMRT or the upcoming MWA. Considering the large investment of telescope time it is essential to identify the``optimal redshift'' where the prospects of a detection are most favourable. We find that the optimal redshift is determined by a combination of instrument dependent factors and the evolution of the neutral fraction x_HI. We find that the redshift range 8.1 +/- 1.1 and 9.8 +/- 1 are optimum for detecting ionized bubbles with the GMRT and MWA respectively. The prospects of a detection, we find, are more favourable in a scenario with late reionization with x_HI \approx 0.5 at z \approx 7.5 as compared to an early reionization model where x_HI \approx 0.5 at z \approx 10. In the late reionization scenario, for both instruments a 3 \sigma detection is possible for bubbles of comoving radius R_b>= 30 Mpc with 1000 hrs of observation. Future observations will either lead to the detection of ionized bubbles, or in the event of non-detection, lead to constraints on the product x_HI R_b^\gamma for the observational volume, where \gamma=1.5 and 2 for GMRT and MWA respectively.