Constraints on foregrounds to detect HI 21-cm signal : uGMRT Observation Band-2 in ELAIS-N1 field

The birth of the first stars and galaxies, and their impact on the diffuse matter permeating the early Universe, is one of the final frontiers in cosmology.Half a century has passed since the first observation of Cosmic Microwave Background (CMB) radiation.However, we know almost nothing about the astrophysics of the first billion years.During this relatively brief epoch, a tiny fraction of matter condensed inside the first galaxies, forming the first stars.The light from these objects and subsequent generations of galaxies began spreading throughout the vast, web-like intergalactic medium (IGM).This culminated in the final major phase change of our Universe, cosmological reionization, when ionizing radiation stripped electrons from almost every atom in our Universe.Reionization lifted the cosmic fog, allowing visible light to finally spread throughout space.The Universe was no longer a cold, dark place.This mysterious epoch or reionization corresponds to the transition between the relative simplicity of the early Universe and the complexity of the present-day one.Over the decades, many have attempted to study the first emissions from luminous sources, such as the Cosmic Dawn (CD) and Epoch of Reionization (EoR), through ground-based radio observations using both single-dish antennas (e.g., EDGES, SARAS3) and interferometers (e.g., uGMRT, LOFAR, MWA, PAPER, HERA, NenuFAR).Radio observations provide an unobstructed view of the universe, penetrating dust clouds and revealing hidden regions.One promising probe, the 21-cm signal from the spin-flip transition of neutral hydrogen, has the potential to unveil the physics behind the epochs of the early universe.Recently, the H1 21-cm signal was detected from star-forming galaxy (SFG) using the uGMRT interferometer at a redshift of z ~ 1.3 [1].However, the detection of 21-cm signals from the reionization era remains elusive.Current-generation low-frequency radio telescopes, such as LOFAR and HERA, have endeavored to establish upper limits for the 21-cm signal.LOFAR, for instance, has accumulated data over 10 nights at redshift z ~ 9.1 [3], while HERA has gathered data over 18 nights at redshifts z = 7.9 and 10.4.Despite these extensive efforts, clear detection of the 21-cm signal has not yet been achieved.Future-generation ground-based radio telescopes, including SKA-low and SKA-mid, are expected to directly image large-scale structures from the early universe.From the uGMRT we have studied the ELAIS-N1 deep field and constraints on the parameters for the 21-cm signal in Band-3 (300-500 MHz) with direction-independent calibration [2].For the first time, we are pursuing the reionization era with the uGMRT in Band-2 (120-250 MHz).We collected 3 nights of data and calibrated it using a direction-dependent method.The final image achieved an RMS noise level of 237 Jy/beam with a resolution of 11.45 arcsec.We characterized the dominant foreground galactic diffuse synchrotron emission using visibility and image based estimation and constrained it in terms of a power-law.Using foregrounds removal technique, we further eliminated the foregrounds from the residuals after point source subtraction.We are presenting our 3 nights results for the upper limit at z = 9 using the uGMRT.