Searching for diffuse HI 21 cm signal using upgraded giant metrewave radio telescope

Abstract

Understanding the evolutionary picture of the Universe starting from high-z Uni verse (epoch of reionization, EoR: z & 6) to the present day (post-EoR era: z . 6) through the hyperfine transition of neutral hydrogen (Hi) is one of the main goal of current and future radio telescopes, like uGMRT, LOFAR, MWA, HERA, etc. The detection of the Hi signal will shed light on how galaxies formed and evolved through cosmic time. In addition to these, measuring brightness temperature fluctuations of Hi 21 cm signal in the post EoR era (z < 6) is a promising tool to study the large scale structure of the Universe in three dimensions. The measurement of the post EoR Hi 21 cm power spectrum can be used to study the large scale structure for mation, Baryon Accoustic Oscillation (BAO), neutrino mass, source clustering, etc. Several experiments, such as OWFA, CHIME, HIRAX,Tianlai, SKA-1 mid etc, are planned to detect the Hi 21 cm signal from the post-EoR epoch via intensity map ping. However, the foreground emission is several orders of magnitude brighter at low-frequencies compared to the Hi 21 cm signal. Moreover, ionospheric effects are amplified at lower frequencies, rendering the measurement of the signal extremely challenging for these experiments. In addition the observation at low-frequency will be severely affected by man-made radio frequency interference (RFI). Hence, be fore any plausible detection it is essential to develop necessary tools to overcome these barriers and to understand any other potential systematic. The challenges are nearly identical for both EoR and post-reionization experiments. So the knowledge of foregrounds at post-reionization epoch can also help us to understand the intri cacies involved in detection of the Hi signal coming from the EoR. To understand low-frequency foregrounds, systematics, RFI and ionospheric challenges and to put upper limits on post-EoR Hi signal, we observed a well known field the ELAIS N1 in northern sky using uGMRT at 300-500 MHz. We have statistically quantified the diffuse galactic synchrotron emission (DGSE) in the ELAIS-N1 field in the form of angular power spectrum using the newly de veloped Tapered Gridded Estimator (TGE). Understanding the nature of DGSE sheds light on the distribution of cosmic ray electrons and magnetic fields in our galaxy. However, it is extremely difficult to measure the power spectrum of DGSSE in the presence of bright extragalactic unresolved sources which reside at the edge of the telescope response at low-frequency. Since, primary beam is asymmetric and frequency dependent, any residual calibration error and error in modelling of bright sources can create ripple along angular and frequency direction. Also, studying DGSE will help us to build an accurate model of foregrounds to be subtracted from the data set in order to detect faint cosmological Hi signal. We have calibrated the data with and without direction-dependent calibration techniques. We have demon strated the effectiveness of TGE against the direction dependent effects by using higher tapering of field of view (FoV). We have found that diffuse Galactic syn chrotron emission (DGSE) dominates the sky, after point source subtraction, across the angular multipole range 1115 6 ` 6 5083 and 1565 6 ` 6 4754 for direction dependent and -independent calibrated visibilities respectively. Our results show that, with large tapering of FoV using TGE, one can still extract the DGSE angular power spectrum even in the presence of direction-dependent calibration affects. We have developed the flagging, calibration and imaging pipeline to reduce the wideband (200 MHz) data set of uGMRT low-frequency observation. We have cre ated the deepest image of the ELAIS-N1 field at this frequency, which covers an area of ∼ 1.8 deg2 and has a central background rms noise of ∼ 15 µJy beam−1 . We have constructed a radio source catalog containing 2528 sources and estimated the normalized Euclidean source counts (1-point statistics). The normalized source counts shows a flattening below ∼1 mJy corresponding to rise in population of star forming galaxies (SFGs) and radio-quiet AGNs. For the first time, we have estimated the spectral characteristics of the angular power spectrum of DGSE over the wide frequency bandwidth from radio interferometric observations. We have shown that it is possible to have a break in the power spectrum of DGSE as a function of frequency and found a break in the galactic synchrotron spectrum around 405 MHz and the resulting spectrum is close to Jaffe–Perola model, suggesting a continuous isotropization of the electron pitch angles (i.e., angle between the magnetic field and the electron velocity) on a timescale that is shorter than the synchrotron age (tage). However a much wider bandwidth and more high signal-to-noise data is required to confirm this finding.