2021ApJ...907L...7C

Measurement of fluctuations in diffuse HI 21 cm background radiation from the post-reionization epoch (z <= 6) is a promising avenue to probe the large-scale structure of the universe and understand the evolution of galaxies. We observe the European Large Area ISO Survey-North 1 (ELAIS-N1) field at 300-500 MHz using the upgraded Giant Meterwave Radio Telescope (uGMRT) and employ the "foreground avoidance" technique to estimate the HI 21 cm power spectrum in the redshift range z = 1.96-3.58. Given the possible systematics that may remain in the data, we find the most stringent upper limits on the spherically averaged 21 cm power spectra at k ∼ 1.0 Mpc^–1 are (58.87 mK)², (61.49 mK)², (60.89 mK)², and (105.85 mK)² at z = 1.96, 2.19, 2.62, and 3.58, respectively. We use this to constrain the product of neutral HI mass density (Ω_HI) and HI bias (b_HI) to the underlying dark matter density field, [Ω_HIb_HI], as 0.09, 0.11, 0.12, and 0.24 at z = 1.96, 2.19, 2.62, and 3.58, respectively. To the best of our knowledge these are the first limits on the HI 21 cm power spectra at the redshift range z = 1.96-3.58 and would play a significant role to constrain the models of galaxy formation and evolution.