Talks

The observations of the redshifted 21-cm signal contain a wealth of cosmological and astrophysical information. The study of this signal from the high redshift Universe provides a unique opportunity to learn about the properties of the first stars and galaxies. However, the problem is particularly challenging due to the presence of foregrounds and system noise. In this talk, I will talk about different statistical estimators to measure the cosmological 21-cm signal from radio interferometric observations. Also, I will present our recent results towards detecting this faint 21-cm signal using the uGMRT and the MWA low-frequency observations.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a drift-scan radio interferometer located at the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, British Columbia, Canada. CHIME, operating between 400 and 800 MHz, will map the redshifted 21 cm emission of neutral hydrogen between redshifts z = 0.8 − 2.5 across the northern sky. The 21cm line is a tracer of the large-scale structure of matter, whose statistics encode a well-understood standard ruler, the baryon acoustic oscillation scale. By detecting and tracking the evolution of this scale with redshift, CHIME aims to constrain the expansion history of the Universe over this crucial redshift epoch when the overall energy density of the Universe is expected to have become dominated by dark energy.

However, measuring this cosmological signal is challenging due to bright astrophysical foregrounds, which are about 4-5 orders of magnitude brighter than the cosmological HI signal.

In principle, these two signals can be separated due to their different spectral features, in which, the foreground signal is smooth in frequency, whereas the cosmological signal has spectral structure. However, this separation requires an instrument calibration at the sub-percent level accuracy. Recently, the CHIME collaboration reported the detection of cosmological 21 cm emission from a large-scale structure between redshift 0.78 and 1.43 in cross-correlation with eBOSS galaxy and quasar catalogs. In this cross-correlation measurement, a high-pass filter is applied to the frequency axis of each map pixel to remove foregrounds, which results in a measurement of the cosmological signal at the non-linear scales. I will discuss recent results in measuring the 21 cm signal in auto-correlation and some of the challenges that we have encountered. I will describe the improvements in the data processing and various data quality cuts required to measure the signal in auto-correlation. I will show the first results of the power spectrum using CHIME data.

The study of the Universe's large-scale structure and evolution has entered a new era with the emergence of neutral hydrogen intensity mapping as a powerful observational technique. HI intensity mapping (IM) offers a unique and innovative approach to cosmological research by probing the distribution of neutral hydrogen on vast scales. This technique uses radio telescopes like MeerKAT to detect the cumulative 21cm emission from neutral hydrogen. In this talk, I will introduce our current HI IM experiment with MeerKAT in the post-reionization universe and discuss our recent breakthroughs in the field. Additionally, I will present the on-the-fly (OTF) interferometric imaging capabilities with MeerKAT. Our aim is to explore cosmology through single-dish HI intensity mapping while simultaneously generating continuum images via the interferometer. This technique allows for commensal observing for both intensity mapping and interferometric imaging. We plan to survey an extensive 10,000 square degrees in the UHF band, achieving a sensitivity of 25 uJy/beam rms and a resolution of 13’’. Our partial observations have achieved an image sensitivity of approximately 140 uJy, with a goal of reaching 30 uJy using existing data. These advanced techniques are propelling the joint studies of cosmology and radio astronomy to new heights, and with the emergence of the Square Kilometre Array (SKA), the future of this field looks exceptionally promising.

The Square Kilometre Array (SKA) promises groundbreaking advances in key scientific areas, including ultra-sensitive continuum imaging and probing the cosmos through redshifted spectral lines from the epochs of cosmic dawn and reionization. However, achieving these ambitious goals requires overcoming significant challenges, particularly at low frequencies. These include ionospheric distortions, wide fields of view, complex antenna layouts, and the massive data volumes generated—all of which make calibration and imaging exceptionally difficult.

While precursor instruments have demonstrated steady progress and highlighted SKA’s immense scientific potential, the complexity of the SKA's mission demands innovative, independent approaches to deliver robust results. This unique landscape presents unparalleled opportunities for developing novel methods to address calibration and imaging challenges at scale.

In this talk, I will showcase promising advancements in tools designed for calibration and imaging. These methods offer a glimpse into how we can tackle SKA's challenges and harness its full potential, paving the way for transformative discoveries in astronomy.

I will present recent progress in constraining the 21-cm power spectrum of neutral hydrogen from the Epoch of Reionization (EoR) using the Low-Frequency Array (LOFAR), bringing us increasingly closer to the sensitivity levels predicted by standard astrophysical models. Achieving deeper limits requires not only adding additional data but also addressing systematic errors, including instrumental and ionospheric effects, as well as mitigating radio-frequency interference. In this talk, I will highlight the rapid advancements our team has achieved in tackling these obstacles, with a particular focus on the largest remaining challenge: direction-dependent gain calibration. Additionally, I will discuss the implications of our latest results, which provide new constraints on the intergalactic medium (IGM) during the EoR, derived from our deepest power-spectrum measurements across three redshifts.

We present the deepest upper limits achieved by the Murchison Widefield Array to date at redshifts z=6.5, z=6.8, and z=7. This study is based on observations centred at (RA = 0h, DEC = −27°), collected between 2013 and 2021. The analysis builds upon the systematic framework developed in Nunhokee et al. (2024), which employs intermediate data products for data quality assessment to minimise contamination in the targeted power spectrum region. The final power spectra are constructed from 221 hours of observations for z=6.5 and 226 hours for z=6.8 and z=7.0, using the Cosmological HI Power Spectrum Estimator. Our results provide the first evidence of a heated intergalactic medium (IGM) at redshifts z=6.5 to z=7.0.

The cosmological 21-cm forest, a series of absorption lines in the spectra of high-z radio-loud sources arising from the hyperfine structure of neutral hydrogen residing in the intergalactic medium (IGM), has a potential to be a unique probe of supermassive black hole growth models and the neutral IGM during the Epoch of Reionization. I will argue that the prospects of detecting the 21-cm forest signal have improved recently because of (1) recent evidence that reionization ended as late as z

The power spectrum of H I 21-cm radiation is a key probe for studying the large-scale structure of the universe and the processes of galaxy formation and evolution. However, one of the major observational challenges is the contamination by foregrounds that are orders of magnitude brighter than the redshifted 21-cm signal in the same frequency range. Foreground contamination complicates calibration, introducing residual errors that bias power spectrum estimates and add systematics. In this work, we assess the efficacy of 21-cm power spectrum estimation using uGMRT Band-3 observations of the ELAIS-N1 field. By analyzing the statistics of residual gain errors and applying additional flagging based on these statistics, we demonstrate a significant reduction in bias and systematics. Our results show that systematics at lower angular scales (ℓ < 6000) primarily arise from residual gain errors and that standard deviation consistently exceeds the bias in power spectrum estimates. We estimate that approximately 2000 hours of on-source observation time is required to detect the 21-cm signal at a redshift of 2.55 with 3-σ significance at ℓ ≈ 3000.

Additionally, we explore the impact of frequency-dependent gain calibration errors on the EoR window in the k⊥-k∥ plane, finding significant contamination due to these errors. We also develop an analytical formula for the wedge structure, allowing wedge characterization directly from MAPS without simulations. These advancements provide critical insights into overcoming challenges in 21-cm cosmology for uGMRT and SKA-like telescopes. Further analysis of residual gain effects on multifrequency angular power spectra will be presented in a companion study.

Major challenge for redshifted 21-cm detection from the epoch of Reionization and post Reionization era is the systematics arising from various signal path effects, instruments and orders of magnitude high foreground. We investigate the case of interferometric gain residuals and their imprint on the power spectrum estimators, like the Tapered Gridded Estimator, and estimate the bias and uncertainty in power spectrum estimates from the uGMRT observations. The methodology, the effect of gain errors and their origin and some possible mitigation strategies will be discussed.

Our study focuses on the evolution of the largest ionized regions (LIRs) by examining the topology and morphology of neutral hydrogen distribution across the different stages of reionization. To investigate the evolution of LIRs, we employed the largest cluster statistics (LCS). Our analysis of the LCS of the redshifted 21-cm signal from the Epoch of Reionization (EoR) has demonstrated to be a robust metric for studying the history of reionization. In our previous LCS analysis, we identified a bias introduced by the convolution of the telescopic synthesize beam of SKA1-Low with the 21-cm signal, which shifted the apparent percolation process of reionization towards the later stage of reionization. This study proposes a method to reduce this bias significantly using an optimal thresholding algorithm. We also tested the robustness of LCS under various foreground corruption conditions. Additionally, we investigated the impact of telescope noise for SKA1-Low on the LCS analysis via synthetic observations simulated by the 21cmE2E pipeline. Here, we present the initial results of the simulation on reionization history in the presence of antenna-based gain calibration errors and instrumental noise on LCS analysis using different SKA array assemblies.

MeerKLAS, the MeerKAT Large Area Synoptic Survey, aims to survey large areas of the sky with MeerKAT in order to probe cosmology using the single dish HI intensity mapping (IM) technique. The MeerKLASS project provides an additional wide continuum survey capability alongside the single-dish HI IM survey by utilizing the ‘On-the-Fly’ (OTF) mapping technique. The target is to cover 10,000 sq. deg on the UHF band with 35 uJy rms and 15’’ resolution. This OTF interferometric imaging technique enables commensal observing for intensity mapping and interferometric imaging. The target sky area will cover most of the Southern sky, overlapping with several optical/NIR wide galaxy surveys and providing an invaluable legacy dataset. We have successfully implemented an end-to-end pipeline that produces continuum images from fast scanning MeerKLASS observations. In this presentation, I will focus on the development of the MeerKLASS OTF imaging pipeline. OTF imaging comes with its own set of challenges, such as flux errors due to smearing caused by the motion of the pointing centre on the sky. I shall discuss how we have mitigated the challenges in the OTF pipeline and the consequences. The final data product for the OTF-MeerKLASS survey consists of deep continuum images obtained by combining all the data from repeated scans of the same sky patch. Finally I shall discuss early science results for the survey such as source count, clustering and multi-wavelength analysis of the OTF catalog radio sources from L and UHF-band. In future, with many fold data acquisition by the MeerKLASS survey we expect to embark on the search for slow transients and I plan to briefly discuss the prospect of such analysis. Lessons from these techniques will be valuable for the upcoming SKA-mid observations.

21-cm intensity mapping is a promising technique to probe large-scale structures in our Universe. To measure the 21-cm intensity mapping signal, we have carried out a deep radio continuum observation of the ELAIS-N1 field using the SKA pathfinder instrument uGMRT. We have used uGMRT’s high angular resolution and wide bandwidth (300-500 MHz) to make a deep image of the field, from which, we identified and removed the compact sources. We found that the residual foregrounds are still several orders of magnitude brighter than the expected 21-cm signal. In a series of subsequent works, we have systematically developed novel techniques to remove the residual foregrounds and reach the system noise. The methodology includes sidelobe suppression, RFI handling, a foreground removal technique that is robust against signal loss and the necessary ingredients for a wide bandwidth data analysis. With a mere 25 hours of observation, we have found an upper limit which is just 10 times above the expected 21-cm signal. This stringent upper limit has led us to 50 more hours of observations with uGMRT, which, combined with the refined pipeline, is expected to provide a substantial improvement and a much tighter constraint. The techniques and these results will underpin future observations with SKA.