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ABSTRACT  In the absence of complex astrophysical processes that characterize the reionization era, the 21‐cm emission from neutral hydrogen (H  i ) in the post‐reionization epoch is believed to be an excellent tracer of the underlying dark matter distribution. Assuming a background cosmology, it is modelled through (i) a bias function  b ( k ,  z ), which relates H  i  to the dark matter distribution and (ii) a mean neutral fraction () which sets its amplitude. In this paper, we investigate the nature of large‐scale H  i  bias. The post‐reionization H  i  is modelled using gravity only  N ‐body simulations and a suitable prescription for assigning gas to the dark matter haloes. Using the simulated bias as the fiducial model for H  i  distribution at  z ≤ 4, we have generated a hypothetical data set for the 21‐cm angular power spectrum ( C  ℓ ) using a noise model based on parameters of an extended version of the Giant Metrewave Radio Telescope (GMRT). The binned  C  ℓ  is assumed to be measured with S/N ≳ 4 in the range 400 ≤ℓ≤ 8000 at a fiducial redshift  z = 2.5. We explore the possibility of constraining  b ( k ) using the principal component analysis on these simulated data. Our analysis shows that in the range 0.2 &lt;  k  &lt; 2 Mpc −1 , the simulated data set cannot distinguish between models exhibiting different  k ‐dependences, provided 1 ≲ b ( k ) ≲ 2 which sets the 2σ limits. This justifies the use of linear bias model on large scales. The largely uncertain  is treated as a free parameter resulting in degradation of the bias reconstruction. The given simulated data are found to constrain the fiducial  with an accuracy of ∼4 per cent (2σ error). The method outlined here could be successfully implemented on future observational data sets to constrain  b ( k ,  z ) and  and thereby enhance our understanding of the low‐redshift Universe.

Constraining large‐scale H i bias using redshifted 21‐cm signal from the post‐reionization epoch