Neutral hydrogen distribution in the post reionization era through gravitational lenses

Abstract

Study of the evolution of neutral hydrogen (HI) in Inter-Galactic Medium during the post reionization era has the promise to understand the evolution of structure over cosmic time. The expected brightness temperature of the redshifted 21-cm line from the HI is rather low and hence direct observations are challenging. The synchrotron emission from our galaxy at the frequency of observation acts as a foreground to the cosmological signal. An additional challenge to detect the redshifted 21-cm signal is that this foreground is several orders of magnitude larger than the signal itself. We access the possibility of using strong gravitational lensing to enhance the 21-cm signal and detect it's power spectrum at comoving scales of 4 Mpcs and lower at a redshift range of one to three. The added advantage of the lensing is that it only enhances the cosmological 21-cm signal and not the galactic synchrotron foreground. In this work we use a visibility based formalism to calculate the lensed power spectrum of the redshifted 21-cm signal. We show that the lensing modifies the telescope sampling function by magnifying the power spectrum at the observed visibility points in the baseline plane. We use available lens models from the optical study of galaxy clusters to estimate the modified sampling function. The result suggest lensing as a promising probe for the signal. © 2020 Indian Radio Science Society.