ICTS

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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.

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.

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I will introduce 21cmFASTv4, a new version of the public simulation package for interpreting multi-tracer observations of the first billion years. 21cmFASTv4 features a new, efficient halo finder as well as a flexible semi-empirical framework to connect galaxies to host halos. Built with Bayesian inference in mind, it allows for self-consistent forward models of multiple tracer fields (e.g. 21cm, galaxies, CMB anisotropies, line intensity maps). I show some recent examples of the power of this approach, including quantifying which galaxy surveys are the most promising for a cross-correlation with 21cm, and characterizing galaxies using recent JWST observations.

Observations of the redshifted 21-cm signal measure the state of the intergalactic medium. However, forward modelling of this signal depends mostly on parameters of the sources and thus inference frameworks put constraints on source properties. Given the considerable uncertainties on the properties of the sources of reionization, it would be good to be able to infer IGM properties from 21-cm observations. In this talk I will address our recent attempts at developing such a model.

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With the increased sensitivity and field of view of SKA pathfinders, dynamic radio imaging (imaging the time axis) is becoming a burgeoning field, yielding rich new discoveries of transients and variable sources. MeerKAT is capable of reaching sub-150 uJy image rms in an 8s integration, which opens up studies of variability on much shorter timescales than was possible with previous radio interferometers. This also has important implications for interferometric SETI, since any potential technosignatures would be a subset of such transient events.

At the same time, imaging at such short timescales introduces its own substantial challenges. Instrumental effects that tend to average out in a traditional long synthesis observation can become limiting for dynamic imaging if not addressed correctly. I will discuss these challenges and present MeerKAT dynamic imaging of Jupiter’s radiation belts, which have led to the serendipitous discovery of a pulsar-class object named the PARROT (pulsar with abnormal refraction recurring on odd timescales).

This work has led to the development of (and given the name to) a more general dynamic imaging pipeline, developed in collaboration with the Breakthrough Listen initiative. The PARROT pipeline is capable of detecting short-duration transients in imaging data, and yielding light curves and dynamic spectra for thousands of field sources en masse. We are already starting to use it to “mine” existing archival MeerKAT data, yielding a couple of new discoveries. The longer-term plan is to develop the PARROT pipeline to a state where it can be run in real-time, commensally with any MeerKAT imaging observation. This would open the door to transient event triggers -- something that has never been done with a radio interferometer before.    

The MeerKAT Exploration of Relics, Giant Halos, and Extragalactic Radio Sources (MERGHERS) survey is a multi-semester MeerKAT programme targeting a homogeneously selected sample of galaxy clusters with the aim of probing the cosmic and mass evolution of diffuse cluster radio emission, such as radio halos and radio relics. Almost all statistical cluster samples studied for diffuse radio emission are restricted to low-redshift, high-mass clusters due to historical telescope sensitivity limitations. MERGHERS is designed in statistically complete tiers, to build up to a large statistically significant cluster sample across wide redshift and mass ranges, as yet unprobed at mid-MHz and GHz ranges. The MERGHERS target samples are extracted from the Atacama Cosmology Telescope’s fifth data release catalogue (ACT DR5), where galaxy clusters are detected via the Sunyaev-Zel’dovich effect, providing a mass-selected sample with a well defined selection function. The MERGHERS pilot study (Knowles et al. 2021) consisted of 13 ACT clusters and served as an observing strategy testbed, to ensure potential problems with limited uv-coverage could be adequately addressed before the full program began. The pilot study, which focused on likely merging clusters based on the projected separation
between the SZ peak and the cluster’s brightest cluster galaxy, produced several new diffuse emission detections and confirmed the observational strategy moving forward. The first tier of the MERGHERS project focuses on a statistical sample of 25 massive ACT DR5 clusters in the redshift range 0.4 < z < 0.6, selected while being blind to the cluster dynamical state so as not to bias observed diffuse emission occurrence rates. The second tier data, lowering the mass range within the same redshift bin has also been observed, taking the current statistical sample up to 56 systems. In this talk I will discuss the observational results of the first tier MERGHERS sample, and highlight some interesting serendipitous findings in the preliminary tier two images.

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Understanding the evolution of active galactic nuclei (AGN) and host galaxies across cosmic epochs is one of the key science drivers of extragalactic surveys. Although, obscuration poses a challenge to detect the complete population of AGN across cosmic epochs. The merger induced dust-obscured galaxies (DOGs) are supposedly potential hosts of AGN. However, detection of AGN in DOGs is challenging due to high absorption of optical, UV and X-ray emission arising from the AGN. Radio emission is insensitive to dust obscuration, and hence, radio continuum surveys are efficient means to detect radio AGN hosted in DOGs. I shall discuss the role of multi-frequency radio continuum surveys in uncovering the AGN population in obscured environments up to large redshifts, and therefore, shedding new insights to the cosmic evolution of AGN.

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Cosmic inflation is currently the most promising paradigm for explaining the fine-tuned initial conditions of the standard Big Bang model. A class of inflation models that involves bursts of particle productions during inflation predicts bump-like features on the power spectrum of primordial density perturbations. This talk will present the imprints of these primordial features on the cosmological 21 cm signal at the high-redshift universe. Using semi-numerical simulations, we demonstrate that the 21 cm signal measurements with the average profile and power spectrum can provide unique constraints on inflationary models that predict such features. Furthermore, we discuss how to distinguish the effects of reionization parameters from the imprints of primordial features on the 21 cm signal, offering insights into both early universe physics and cosmic reionization.

Photons between the Lyman-α and Lyman-limit frequencies, produced in the large scale structures during the Cosmic Dawn, redshift into the neutral IGM and undergo multiple scatterings by the HI atoms, coupling the HI spin temperature to the gas kinetic temperature. However, during this process the Lyman-α photons also exchange energy with the medium. The photons which redshift into the Lyman-α line (continuum photons) heat up the medium, while the photons which are injected at the line centre (injected photons) cause cooling, assuming the profile near Lyman-α line centre to be in quasi-static equilibrium. However, we find that the time-scale to reach this equilibrium are often under-estimated in the literature. Thermal feedback due to evolving IGM temperature delays this equilibrium. Additionally, the life time of the first stars in the Universe (PopIII stars) is comparable to the time scales for reaching quasi-static equilibrium (1–10 Myr). In our analysis we have also found a new equilibrium profile for the continuum photons which is reached on the expansion time scales (∼100 Myr). We also calculate the final equilibrium temperature of the IGM for a combination of continuum and injected photons. However, this balance shifts when the source switches off, and the residual photons in the IGM can heat the medium above the equilibrium temperatures. In this talk I will discuss times scales for the temperature evolution of the IGM during Cosmic Dawn and it’s effect on the 21-cm signal.

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Cosmic dawn and epoch of reionization, the period during which the first stars and galaxies formed and reionized the universe, can be studied using the 21-cm line transition of the neutral hydrogen atom. The sky-averaged global 21-cm signal component corresponding to 6 < z < 34 is redshifted to metre wavelengths and lies between 40 to 200 MHz.

However, detecting this signal poses significant challenges due to strong foreground emissions and systematic effects introduced by antenna instrumentation. The antenna's primary beam and impedance, which also depend on the properties of the observing environment, play an influential role in distorting the signal. In this presentation, we explore the intricate relationship between antenna properties and signal detectability. We identify key parameters influencing signal detection and estimate biases across different 21-cm signal models. We use realistic simulations of SARAS antenna characteristics. Our analysis finds correlations between beam chromaticity and spectral profile biases. We also find stringent requirements for transfer function corrections, which can otherwise prohibit detection prospects. We finally explore a range of critical parameters that allow robust signal detection.

Furthermore, we share insights from recent RFI surveys and SARAS deployments in the Trans-Himalayan region of Ladakh and the Andaman Islands. This includes an overview of the Radio Frequency Interference (RFI) environment at the surveyed sites and the methodologies used for data reduction and analysis. Our findings underscore the importance of site-specific environmental factors in enhancing detection prospects for the global 21-cm signal

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21-cm Cosmology from Dark Ages

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Despite its great success, the LCDM model faces several theoretical and observational challenges. In this review, we explore some of the alternative models and investigate their possible detectable imprints on the 21-cm observations.

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