arXiv subjects

Srikrishna Sekhar

Publications and source records attributed to Srikrishna Sekhar.

15 recordsLinked to original sources

arXiv2026

A Python-Based Peeling Framework for Radio Interferometry: Application to uGMRT 650MHz Imaging

Modern radio interferometric arrays offer high sensitivity, wide fields of view, and broad frequency coverage, but also pose significant data calibration challenges. Standard direction-independent calibration is insufficient to correct direction-dependent effects, such as ionospheric phase distortions and primary beam variations, which produce strong artifacts around bright sources and limit achievable image dynamic range. Built on standard CASA tasks, we present a Python-based direction-dependent calibration and peeling framework, demonstrated using radio continuum imaging data from the upgraded Giant Metrewave Radio Telescope (uGMRT). The framework efficiently subtracts bright-source models and suppresses their associated direction-dependent artifacts, producing significantly flattened backgrounds and improving image fidelity and faint-source detectability. We further introduce an optimized ``model-restoration'' strategy that mitigates direction-dependent artifacts while preserving the flux densities and morphologies of bright sources that are themselves of scientific interest. For fields containing multiple bright sources, sequential application of the framework systematically reduces background noise, thereby increasing sensitivity and faint-source detectability. The framework is Python-based, CASA-compatible, and can be readily applied to other mid- and low-frequency interferometric arrays. The code is publicly released with this paper.

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arXiv2025

The superMIGHTEE project: MeerKAT and GMRT Together to Unveil the Deep Radio Sky

An international team of researchers has come together to undertake an ultra-broadband exploration of the deep radio sky. The superMIGHTEE project combines data from the MIGHTEE project, using the precursor Square Kilometre Array (SKA) MeerKAT telescope in South Africa, with observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) in India to produce deep images at several $μ$Jy sensitivity over a frequency range of 200 MHz--2.5 GHz, with an angular resolution of a few arcseconds. This paper describes the initial superMIGHTEE uGMRT data release, comprising total intensity continuum images covering a total of 9.9 deg$^2$ at 650 MHz and 6.9 deg$^2$ at 400 MHz in the XMM-LSS, COSMOS, and E-CDFS deep fields. The associated radio source catalogs include 27,101 sources at 650 MHz and 10,946 sources at 400 MHz. The redshift distribution of the sources extends to $z\sim4$ with a median value of $z=1$. An overview of the broadband spectra of the sources, in combination with the MeerKAT MIGHTEE 1280 MHz data, reveals a clear change in spectral properties at the transition from an active galactic nuclei-dominated population to a population dominated by star-forming galaxies at flux densities of a few mJy. At higher frequencies, the star-forming galaxy population exhibits an optically thin synchrotron spectral index indicative of energy injection from supernovae. At lower frequencies, the spectra flatten significantly with decreasing flux density, and the fraction of sources with peaked spectra increases. This is the first superMIGHTEE uGMRT data release. Subsequent releases will include spectropolarimetric and spectral line image cubes, as well as images at lower frequencies. The goal of the superMIGHTEE ultra-wideband dataset is to enhance our understanding of the evolution of active galactic nuclei and star-forming galaxies over cosmic time, (abridged).

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arXiv2025

HI Intensity Mapping with the MIGHTEE Survey: First Results of the HI Power Spectrum

We present the first results of the HI intensity mapping power spectrum analysis with the MeerKAT International GigaHertz Tiered Extragalactic Exploration (MIGHTEE) survey. We use data covering $\sim$4 square degrees in the COSMOS field using a frequency range 962.5 MHz to 1008.42 MHz, equivalent to HI emission in $0.4<z<0.48$. The data consists of 15 pointings with a total of 94.2 hours on-source. We verify the suitability of the MIGHTEE data for HI intensity mapping by testing for residual systematics across frequency, baselines and pointings. We also vary the window used for HI signal measurements and find no significant improvement using stringent Fourier mode cuts. Averaging in the power spectrum domain, i.e. using incoherent averaging, we calculate the first upper limits from MIGHTEE on the HI power spectrum at scales 0.5 Mpc$^{-1} \lesssim k \lesssim$ 10 Mpc$^{-1}$. We obtain the best 1$σ$ upper limit of 28.6 mK$^{2}$Mpc${^3}$ on $k\sim$2 Mpc$^{-1}$. Our results are consistent with the power spectrum detected with observations in the DEEP2 field with MeerKAT. The data we use here constitutes a small fraction of the MIGHTEE survey and demonstrates that combined analysis of the full MIGHTEE survey can potentially detect the HI power spectrum at $z\lesssim0.5$ in the range 0.1 Mpc$^{-1} \lesssim k \lesssim$ 10 Mpc$^{-1}$ or quasi-linear scales.

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arXiv2024

PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: physical drivers of the molecular gas fraction, $R_{\mathrm{mol}}$

The molecular-to-atomic gas ratio is crucial to the evolution of the interstellar medium in galaxies. We investigate the balance between the atomic ($Σ_{\rm HI}$) and molecular gas ($Σ_{\rm H2}$) surface densities in eight nearby star-forming galaxies using new high-quality observations from MeerKAT and ALMA (for HI and CO, respectively). We define the molecular gas ratio as $R_{\rm mol} = Σ_{\rm H2} / Σ_{\rm HI}$ and measure how it depends on local conditions in the galaxy disks using multi-wavelength observations. We find that, depending on the galaxy, HI is detected at $>3σ$ out to 20-120 kpc in galactocentric radius ($r_{\rm gal}$). The typical radius at which $Σ_{\rm HI}$ reaches 1~$\rm M_\odot~pc^{-2}$ is $r_{\rm HI}\approx22$~kpc, which corresponds to 1-3 times the optical radius ($r_{25}$). $R_{\rm mol}$ correlates best with the dynamical equilibrium pressure, P$_{\rm DE}$, among potential drivers studied, with a median correlation coefficient of $<ρ>=0.89$. Correlations between $R_{\rm mol}$ and star formation rate, total gas and stellar surface density, metallicity, and $Σ_{\rm SFR}$/P$_{\rm DE}$ are present but somewhat weaker. Our results also show a direct correlation between P$_{\rm DE}$ and $Σ_{\rm SFR}$, supporting self-regulation models. Quantitatively, we measure similar scalings as previous works and attribute the modest differences that we find to the effect of varying resolution and sensitivity. At $r_{\rm gal} {\gtrsim}0.4~r_{25}$, atomic gas dominates over molecular gas, and at the balance of these two gas phases, we find that the baryon mass is dominated by stars, with $Σ_{*} > 5~Σ_{\rm gas}$. Our study constitutes an important step in the statistical investigation of how local galaxy properties impact the conversion from atomic to molecular gas in nearby galaxies.

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arXiv2024

MIGHTEE-HI: HI galaxy properties in the large scale structure environment at z~0.37 from a stacking experiment

We present the first measurement of HI mass of star-forming galaxies in different large scale structure environments from a blind survey at $z\sim 0.37$. In particular, we carry out a spectral line stacking analysis considering $2875$ spectra of colour-selected star-forming galaxies undetected in HI at $0.23 < z < 0.49$ in the COSMOS field, extracted from the MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust HI detection yielding an average galaxy HI mass of $M_{\rm HI}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$ at $\sim 11.8σ$. Next, we investigate the different subsamples finding a negligible difference in $M_{\rm HI}$ as a function of the galaxy overdensity. We report an HI excess compared to the full sample in satellite galaxies ($M_{\rm HI}=(11.31\pm1.22)\times 10^9$, at $\sim 10.2 σ$) and in filaments ($M_{\rm HI}=(11.62\pm 0.90)\times 10^9$. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be HI-deficient. We find the same qualitative results also when stacking in units of HI fraction ($f_{\rm HI}$). We conclude that the HI amount in star-forming galaxies at the studied redshifts correlates with the large scale structure environment.

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arXiv2023

MIGHTEE-HI: The HI mass-stellar mass relation over the last billion years

We study the $M_{\rm HI}-M_{\star}$ relation over the last billion years using the MIGHTEE-HI sample. We first model the upper envelope of the $M_{\rm HI}-M_{\star}$ relation with a Bayesian technique applied to a total number of 249 HI-selected galaxies, without binning the datasets, while taking account of the intrinsic scatter. We fit the envelope with both linear and non-linear models, and find that the non-linear model is preferred over the linear one with a measured transition stellar mass of $\log_{10}(M_\star$/$M_{\odot})$ = $9.15\pm0.87$, beyond which the slope flattens. This finding supports the view that the lack of HI gas is ultimately responsible for the decreasing star formation rate observed in the massive main-sequence galaxies. For spirals alone, which are biased towards the massive galaxies in our sample, the slope beyond the transition mass is shallower than for the full sample, indicative of distinct gas processes ongoing for the spirals/high-mass galaxies from other types with lower stellar masses. We then create mock catalogues for the MIGHTEE-HI detections and non-detections with two main galaxy populations of late- and early-type galaxies to measure the underlying $M_{\rm HI}-M_{\star}$ relation. We find that the turnover in this relation persists whether considering the two galaxy populations as a whole or separately. We note that an underlying linear relation could mimic this turnover in the observed scaling relation, but a model with a turnover is strongly preferred. Measurements on the logarithmic average of HI masses against the stellar mass are provided as a benchmark for future studies.

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arXiv2023

MIGHTEE-HI: The first MeerKAT HI mass function from an untargeted interferometric survey

We present the first measurement of the HI mass function (HIMF) using data from MeerKAT, based on 276 direct detections from the MIGHTEE Survey Early Science data covering a period of approximately a billion years ($0 \leq z \leq 0.084 $). This is the first HIMF measured using interferometric data over non-group or cluster field, i.e. a deep blank field. We constrain the parameters of the Schechter function which describes the HIMF with two different methods: $1/\rm V_{\rm max}$ and Modified Maximum Likelihood (MML). We find a low-mass slope $α=-1.29^{+0.37}_{-0.26}$, `knee' mass $\log_{10}(M_{*}/{\rm M_{\odot}}) = 10.07^{+0.24}_{-0.24}$ and normalisation $\log_{10}(ϕ_{*}/\rm Mpc^{-3})=-2.34^{+0.32}_{-0.36}$ ($H_0 = 67.4$ kms$^{-1}$ Mpc$^{-1}$) for $1/\rm V_{\rm max}$ and $α=-1.44^{+0.13}_{-0.10}$, `knee' mass $\log_{10}(M_{*}/{\rm M_{\odot}}) = 10.22^{+0.10}_{-0.13}$ and normalisation $\log_{10}(ϕ_{*}/\rm Mpc^{-3})=-2.52^{+0.19}_{-0.14}$ for MML. When using $1/\rm V_{\rm max}$ we find both the low-mass slope and `knee' mass to be consistent within $1σ$ with previous studies based on single-dish surveys. The cosmological mass density of HI is found to be slightly larger than previously reported: $Ω_{\rm HI}=5.46^{+0.94}_{-0.99} \times 10^{-4}h^{-1}_{67.4}$ from $1/\rm V_{\rm max}$ and $Ω_{\rm HI}=6.31^{+0.31}_{-0.31} \times 10^{-4}h^{-1}_{67.4}$ from MML but consistent within the uncertainties. We find no evidence for evolution of the HIMF over the last billion years.

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arXiv2022

CASA, the Common Astronomy Software Applications for Radio Astronomy

CASA, the Common Astronomy Software Applications, is the primary data processing software for the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), and is frequently used also for other radio telescopes. The CASA software can handle data from single-dish, aperture-synthesis, and Very Long Baseline Interferometery (VLBI) telescopes. One of its core functionalities is to support the calibration and imaging pipelines for ALMA, VLA, VLA Sky Survey (VLASS), and the Nobeyama 45m telescope. This paper presents a high-level overview of the basic structure of the CASA software, as well as procedures for calibrating and imaging astronomical radio data in CASA. CASA is being developed by an international consortium of scientists and software engineers based at the National Radio Astronomical Observatory (NRAO), the European Southern Observatory (ESO), the National Astronomical Observatory of Japan (NAOJ), and the Joint Institute for VLBI European Research Infrastructure Consortium (JIV-ERIC), under the guidance of NRAO.

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arXiv2022

MIGHTEE-HI: Evolution of HI scaling relations of star-forming galaxies at $z<0.5$

We present the first measurements of HI galaxy scaling relations from a blind survey at $z>0.15$. We perform spectral stacking of 9023 spectra of star-forming galaxies undetected in HI at $0.23<z<0.49$, extracted from MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope. We stack galaxies in bins of galaxy properties ($M_*$, SFR, and sSFR, with ${\rm sSFR}\equiv M_*/{\rm SFR}$), obtaining $\gtrsim 5σ$ detections in most cases, the strongest HI-stacking detections to date in this redshift range. With these detections, we are able to measure scaling relations in the probed redshift interval, finding evidence for a moderate evolution from the median redshift of our sample $z_{\rm med}\sim 0.37$ to $z\sim 0$. In particular, low-$M_*$ galaxies ($\log_{10}(M_*/{\rm M_\odot})\sim 9$) experience a strong HI depletion ($\sim 0.5$ dex in $\log_{10}(M_{\rm HI}/{\rm M}_\odot)$), while massive galaxies ($\log_{10}(M_*/{\rm M_\odot})\sim 11$) keep their HI mass nearly unchanged. When looking at the star formation activity, highly star-forming galaxies evolve significantly in $M_{\rm HI}$ ($f_{\rm HI}$, where $f_{\rm HI}\equiv M_{\rm}/M_*$) at fixed SFR (sSFR), while at the lowest probed SFR (sSFR) the scaling relations show no evolution. These findings suggest a scenario in which low-$M_*$ galaxies have experienced a strong HI depletion during the last $\sim4$ Gyr, while massive galaxies have undergone a significant HI replenishment through some accretion mechanism, possibly minor mergers. Interestingly, our results are in good agreement with the predictions of the SIMBA simulation. We conclude that this work sets novel important observational constraints on galaxy scaling relations.

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arXiv2022

A Compressed Sensing Faraday Depth Reconstruction Framework for the MeerKAT MIGHTEE-POL Survey

In this work we present a novel compute framework for reconstructing Faraday depth signals from noisy and incomplete spectro-polarimetric radio datasets. This framework is based on a compressed-sensing approach that addresses a number of outstanding issues in Faraday depth reconstruction in a systematic and scaleable manner. We apply this framework to early-release data from the MeerKAT MIGHTEE polarisation survey.

astro-ph.IM↗

arXiv2022

MeerKAT uncovers the physics of an Odd Radio Circle

Odd Radio Circles (ORCs) are recently-discovered faint diffuse circles of radio emission, of unknown cause, surrounding galaxies at moderate redshift ($z ~ 0.2-0.6). Here we present detailed new MeerKAT radio images at 1284 MHz of the first ORC, originally discovered with the Australian Square Kilometre Array Pathfinder, with higher resolution (6 arcsec) and sensitivity (~ 2.4 uJy/bm). In addition to the new images, which reveal a complex internal structure consisting of multiple arcs, we also present polarisation and spectral index maps. Based on these new data, we consider potential mechanisms that may generate the ORCs.

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arXiv2022

MIGHTEE-HI: the HI Size-Mass relation over the last billion years

We present the observed HI size-mass relation of $204$ galaxies from the MIGHTEE Survey Early Science data. The high sensitivity of MeerKAT allows us to detect galaxies spanning more than 4 orders of magnitude in HI mass, ranging from dwarf galaxies to massive spirals, and including all morphological types. This is the first time the relation has been explored on a blind homogeneous data set which extends over a previously unexplored redshift range of $0 < z < 0.084$, i.e. a period of around one billion years in cosmic time. The sample follows the same tight logarithmic relation derived from previous work, between the diameter ($D_{\rm HI}$) and the mass ($M_{\rm HI}$) of HI discs. We measure a slope of $0.501\pm 0.008$, an intercept of $-3.252^{+0.073}_{-0.074}$, and an observed scatter of $0.057$ dex. For the first time, we quantify the intrinsic scatter of $0.054 \pm 0.003$ dex (${\sim} 10 \%$), which provides a constraint for cosmological simulations of galaxy formation and evolution. We derive the relation as a function of galaxy type and find that their intrinsic scatters and slopes are consistent within the errors. We also calculate the $D_{\rm HI} - M_{\rm HI}$ relation for two redshift bins and do not find any evidence for evolution with redshift. These results suggest that over a period of one billion years in lookback time, galaxy discs have not undergone significant evolution in their gas distribution and mean surface mass density, indicating a lack of dependence on both morphological type and redshift.

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arXiv2021

Direction Dependent Corrections in Polarimetric Radio Imaging III: A-to-Z Solver -- Modeling the full Jones antenna aperture illumination pattern

In this third paper of a series describing direction dependent corrections for polarimetric radio imaging, we present the the A-to-Z solver methodology to model the full Jones antenna aperture illumination pattern (AIP) with Zernike polynomials. In order to achieve thermal noise limited imaging with modern radio interferometers, it is necessary to correct for the instrumental effects of the antenna primary beam (PB) as a function of time, frequency, and polarization. The wideband AW projection algorithm enables those corrections provided an accurate model of the AIP is available. We present the A-to-Z solver as a more versatile algorithm for the modeling of the AIP. It employs the orthonormal circular Zernike polynomial basis to model the measured full Jones AIP. These full Jones models are then used to reconstruct the full Mueller AIP repsonse of an antenna, in principle accounting for all the off-axis leakage effects of the primary beam. The A-to-Z solver is general enough to accomodate any interferometer for which holographic measurements exist, we have successfully modelled the AIP of VLA, MeerKAT and ALMA as a demonstration of its versatility. We show that our models capture the PB morphology to high accuracy within the first 1-2 sidelobes, and show the viability of full Mueller gridding and deconvolution for any telescope given high quality holographic measurements.

astro-ph.IM↗

arXiv2021

MIGHTEE-HI: The baryonic Tully-Fisher relation over the last billion years

Using a sample of 67 galaxies from the MIGHTEE Survey Early Science data we study the HI-based baryonic Tully-Fisher relation (bTFr), covering a period of $\sim$one billion years ($0 \leq z \leq 0.081 $). We consider the bTFr based on two different rotational velocity measures: the width of the global HI profile and $\rm V_{out}$, measured as the outermost rotational velocity from the resolved HI rotation curves. Both relations exhibit very low intrinsic scatter orthogonal to the best-fit relation ($σ_{\perp}=0.07\pm0.01$), comparable to the SPARC sample at $z \simeq 0$. The slopes of the relations are similar and consistent with the $ z \simeq 0$ studies ($3.66^{+0.35}_{-0.29}$ for $\rm W_{50}$ and $3.47^{+0.37}_{-0.30}$ for $\rm V_{out}$). We find no evidence that the bTFr has evolved over the last billion years, and all galaxies in our sample are consistent with the same relation independent of redshift and the rotational velocity measure. Our results set up a reference for all future studies of the HI-based bTFr as a function of redshift that will be conducted with the ongoing deep SKA pathfinders surveys.

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arXiv2018

Two procedures to flag radio frequency interference in the UV plane

We present two algorithms to identify and flag radio frequency interference (RFI) in radio interferometric imaging data. The first algorithm utilizes the redundancy of visibilities inside a UV cell in the visibility plane to identify corrupted data, while varying the detection threshold in accordance with the observed reduction in noise with radial UV distance. In the second algorithm, we propose a scheme to detect faint RFI in the visibility time-channel plane of baselines. The efficacy of identifying RFI in the residual visibilities is reduced by the presence of ripples due to inaccurate subtraction of the strongest sources. This can be due to several reasons including primary beam asymmetries and other direction dependent calibration errors. We eliminated these ripples by clipping the corresponding peaks in the associated Fourier plane. RFI was detected in the ripple-free time-channel plane but was flagged in the original visibilities. Application of these two algorithms to 5 different 150 MHz datasets from the GMRT resulted in a reduction in image noise of 20-50% throughout the field along with a reduction in systematics and a corresponding increase in the number of detected sources. However, on comparing the mean flux densities before and after flagging RFI we find a differential change with the fainter sources ($25σ<$ S $< 100$ mJy) showing a change of -6% to +1% relative to the stronger sources (S $>$ 100 mJy). We are unable to explain this effect but it could be related to the CLEAN bias known for interferometers.

astro-ph.IM↗