arXiv subjects

Sukhdeep Singh Gill

Publications and source records attributed to Sukhdeep Singh Gill.

10 recordsLinked to original sources

arXiv2026

Overview of 21cm Experiments at high redshift with SKAO

We provide an overview of the eight SKAO Science Book chapters that motivate the Epoch of Reionisation and Cosmic Dawn experiments with SKA-Low. We describe the individual SKA-Low experiments and expected sensitivity - power spectrum, tomography, 21-cm forest, cross-correlations, building on the broad observational plan laid out in the 2015 SKA Science Book. Finally, we outline features of the telescope that will be critical for the success of EoR/CD science, e.g., beam apodization, substations, and multi-beaming.

astro-ph.CO↗

arXiv2026

Observations of the Cosmic Dawn and Epoch of Reionization with the SKAO: Observational Lessons Learned from Precursors and Pathfinder Instruments

This chapter summarizes the observational lessons learned after two decades of observations of the Cosmic Dawn (CD) and Epoch of Reionization (EoR) with SKAO pathfinders and precursors. We will describe the effort towards building accurate simulation pipelines for actual observations and summarize the approaches that different groups have taken to calibrate and mitigate systematic effects such as sky model incompleteness, limited instrument models and antenna mutual coupling. We conclude by discussing the impact that these lessons may have on the design and analysis of upcoming SKAO observations of the Cosmic Dawn and Epoch of Reionization.

astro-ph.IM↗

arXiv2026

Brisk estimator for the angular multipoles of the redshift space bispectrum

The anisotropy of the redshift space bispectrum depends upon the orientation of the triangles formed by three $\mathbf{k}$ modes with respect to the line of sight. For a triangle of fixed size ($k_1$) and shape ($μ,t$), this orientation dependence can be quantified in terms of angular multipoles $B_\ell^m(k_1,μ,t)$ which contain a wealth of cosmological information. We propose a fast and efficient FFT-based estimator that computes the bispectrum multipole moments $B_\ell^m$ of a 3D cosmological field for all possible $\ell$ and $m$ (including $m\neq 0$). The time required by the estimator to compute all multipoles from a gridded data cube of volume $N_g^3$ scales as $\sim \mathcal{O}(N_g^4)$ in contrast to the direct computation technique which requires time $\sim \mathcal{O}(N_g^6)$. Here, we demonstrate the formalism and validate the estimator using a simulated non-Gaussian field for which the analytical expressions for all the bispectrum multipoles are known. The estimated results are found to be in good agreement with the analytical predictions for all $16$ non-zero multipoles (up to $\ell= 6, m=6$). We expect the $m \neq 0$ bispectrum multipoles to significantly enhance the information available from galaxy redshift surveys and future redshifted 21-cm observations.

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arXiv2026

The size and shape dependence of the SDSS galaxy bispectrum

We have measured the spherically averaged bispectrum of the SDSS main galaxy sample, considering a volume-limited $[296.75\, \rm Mpc]^3$ data cube with mean galaxy number density $0.63 \times 10^{-3} \, {\rm Mpc}^{-3}$ and median redshift $0.102$. Our analysis considers $\sim 1.37 \times 10^{8}$ triangles, for which we have measured the binned bispectrum and analysed its dependence on the size and shape of the triangle. It spans wavenumbers $k_1=(0.075-0.434)\,{\rm Mpc}^{-1}$ for equilateral triangles, and a smaller range of $k_1$ (the largest side) for triangles of other shapes. For all shapes, we find that the measured bispectrum is well modelled by a power law $A\,\big(k_1/1\mpci\big)^{n}$, where the best-fit values of $A$ and $n$ vary with the shape. We have also analysed mock galaxy samples constructed from $Λ$CDM N-body simulations by applying a simple Eulerian bias prescription where the galaxies reside in regions where the smoothed density field exceeds a threshold. We find that the bispectrum from the mock samples with bias $b_1=1.2$ is in good agreement with the SDSS results. We further divided our galaxy sample into red and blue classes and studied the nature of the bispectrum for each category. The red galaxies exhibit higher bispectrum amplitude $A$ than the blue galaxies for all possible triangle configurations. Red galaxies are old, and their larger bispectra indicate non-linear evolutionary interactions within their environments over time, resulting in their distribution being highly clustered and more biased than younger blue galaxies.

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arXiv2026

The Epoch of Reionization 21 cm Bispectrum at $z=8.2$ from MWA data II: Smooth Component Filtering

The 21 cm bispectrum (BS) offers a powerful probe of the Epoch of Reionization (EoR), but its observational access is severely hindered by dominant astrophysical foregrounds. Considering Murchison Widefield Array (MWA) observations at $154.2~\mathrm{MHz}$ ($z=8.2$), we mitigate the foregrounds with Smooth Component Filtering (SCF) and estimate the 21 cm BS. We validate the pipeline using a simulated 21 cm signal and show that the input BS is recovered for modes $k_{\parallel} \ge [k_\parallel]_f=0.135~{\rm Mpc}^{-1}$. Applied to actual data, the SCF produces substantial foreground suppression, reducing the amplitude of the cylindrical BS $B(k_{1\perp},k_{2\perp},k_{3\perp},k_{1\parallel},k_{2\parallel})$ by $3-4$ orders of magnitude. The artifacts due to the missing frequency channels in the data are also suppressed. The resulting EoR window is significantly cleaner at small $k_{\perp}$. We adopt the region $(k_{1 \perp},k_{2 \perp},k_{3 \perp})\leq 0.026~{\rm Mpc}^{-1}$ and $(k_{1\parallel},k_{2\parallel},k_{3\parallel})>0.135~{\rm Mpc}^{-1}$ to evaluate the 3D spherical BS and constrain the EoR signal. By combining estimates over all triangle shapes, we place the lower and upper limits on the mean cube brightness temperature fluctuations $Δ^3$. The estimates are consistent with statistical fluctuations from system noise. The most stringent lower limit $Δ^3_{\rm LL}=-(1.25\times 10^4)^3~{\rm mK}^3$ and upper limit $Δ^3_{\rm UL}=(1.22\times 10^4)^3~{\rm mK}^3$ are obtained at $k_1=0.281~{\rm Mpc}^{-1}$. Additional observing time will reduce the noise level and enable substantially tighter constraints on the EoR signal.

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arXiv2025

Probing gravity beyond general relativity with bispectrum multipoles of cosmological tracers: I. Theoretical Foundations

The bispectrum, being sensitive to non-Gaussianity and mode coupling of cosmological fields induced by non-linear gravitational evolution, serves as a powerful probe for detecting deviations from General Relativity (GR). The signatures of modified gravity in the bispectrum are even more pronounced in redshift space, where anisotropies from peculiar velocities provide unbiased information on higher-order properties of gravity. We investigate the potential of all non-zero angular multipoles $B_l^m$ of redshift space bispectrum across all possible triangle configurations to probe degenerate higher-order scalar tensor (DHOST) theory. We show that the higher-order multipoles of the bispectrum with $l=2,4,6$ are more sensitive to the modifications in gravity than the spherically averaged monopole moment $l=0$. These multipoles demonstrate remarkable sensitivity to the higher-order growth history, which varies across triangle configurations, with acute triangles generally being the most sensitive to modification in GR. The values of various multipoles exhibit opposite signs in modified gravity compared to those predicted in GR, which serves as a robust indicator of the deviation from GR. We demonstrate that, unlike $l=2$ and $4$ multipoles, the $l=6$ multipoles with $m\leq 4$ are not affected by the quadratic bias and second-order tidal bias parameters, emphasising the need to leverage their capabilities in analyses. The $(l=6, m > 4)$ multipoles fail to capture the second-order growth, while all $l=8$ multipoles lack any independent information regarding modified gravity in both linear and nonlinear regimes.

astro-ph.CO↗

arXiv2025

The EoR 21-cm Bispectrum at $z=8.2$ from MWA data I: Foregrounds and preliminary upper limits

We attempt to measure the $z = 8.2$ Epoch of Reionization (EoR) 21-cm bispectrum (BS) using Murchison Widefield Array (MWA) $154.2~\mathrm{MHz}$ data. We find that $B(k_{1\perp}, k_{2\perp}, k_{3\perp}, k_{1\parallel}, k_{2\parallel})$ the 3D cylindrical BS exhibits a foreground wedge, similar to $P(k_{1\perp},k_{1\parallel})$ the 21-cm cylindrical power spectrum. However, the BS foreground wedge, which depends on $(k_{1\perp},k_{1\parallel})$, $(k_{2\perp},k_{2\parallel})$ and $(k_{3\perp},k_{3\parallel})$ the three sides of a triangle, is more complicated. Considering various foreground avoidance scenarios, we identify the region where all three sides are outside the foreground wedge as the EoR window for the 21-cm BS. However, the EoR window is contaminated by a periodic pattern of spikes that arises from the periodic pattern of missing frequency channels in the data. We evaluate the binned 3D spherical BS for triangles of all possible sizes and shapes, and present results for $Δ^3$ the mean cube brightness temperature fluctuations. The best $2σ$ upper limits we obtain for the EoR 21-cm signal are $Δ^3_{\rm UL} = (1.81\times 10^3)^3~\mathrm{mK}^3$ at $k_1 = 0.008~\mathrm{Mpc}^{-1}$ and $Δ^3_{\rm UL} = (2.04\times 10^3)^3~\mathrm{mK}^3$ at $k_1 = 0.012~\mathrm{Mpc}^{-1}$ for equilateral and squeezed triangles, respectively. These are foreground-dominated, and are many orders of magnitude larger than the predicted EoR 21-cm signal $(\sim 10^3 ~\mathrm{mK}^3)$.

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arXiv2025

A Visibility-based 21 cm Bispectrum Estimator for Radio-interferometric Data

We present a fast and scalable estimator for the binned multi-frequency angular bispectrum (MABS) and the 3D bispectrum (BS) of the redshifted 21 cm signal from radio interferometric observations. The estimator operates on gridded visibilities and leverages the FFT-based acceleration to efficiently compute the MABS and the 3D BS covering all possible triangle configurations. We present the formalism and validate the estimator using simulated visibility data for a known input model BS, considering the Murchison Widefield Array (MWA) observations with a bandwidth of $30.72$ MHz centered at $154.25$ MHz. We consider two cases, namely, without flagging, and with flagging, which has exactly the same frequency channels flagged as the actual data. We obtain estimates of the BS for a wide range of triangle shapes covering the scales $0.003 ~\mathrm{Mpc}^{-1}\leq k_1 \leq 1.258 ~\mathrm{Mpc}^{-1}$. The estimated BS shows excellent agreement with analytical predictions based on the input model BS. We find that the deviations, which are below 20\% even in the presence of flagging, are mostly consistent with the expected statistical fluctuations. This work paves the way for reliable observational estimates of the 21 cm BS for the epoch of reionization, where the signal is predicted to be highly non-Gaussian.

astro-ph.CO↗

arXiv2025

A visibility-based angular bispectrum estimator for radio-interferometric data

Considering radio-interferometric observations, we present a fast and efficient estimator to compute the binned angular bispectrum (ABS) from gridded visibility data. The estimator makes use of Fast Fourier Transform (FFT) techniques to compute the bispectrum covering all possible triangle shapes and sizes. Here, we present the formalism of the estimator and validate it using simulated visibility data for the Murchison Widefield Array (MWA) observations at $ν=154.25$ MHz. We find that our estimator is able to faithfully recover the ABS of the simulated sky signal with $\approx 10\%-15 \%$ accuracy for a wide variety of triangle shapes and sizes across the range of angular multipoles $46 \le \ell \le 1320$. In future work, we plan to apply this to actual data and also generalize it to estimate the three-dimensional redshifted 21-cm bispectrum.

astro-ph.CO↗

arXiv2023

The monopole and quadrupole moments of the Epoch of Reionization (EoR) 21-cm bispectrum

We study the monopole ($\bar{B}^0_0$) and quadrupole ($\bar{B}^0_2$) moments of the 21-cm bispectrum (BS) from EoR simulations and present results for squeezed and stretched triangles. Both $\bar{B}^0_0$ and $\bar{B}^0_2$ are positive at the early stage of EoR where the mean neutral hydrogen (HI) density fraction $\bar{x}_{\rm HI} \approx 0.99$. The subsequent evolution of $\bar{B}^0_0$ and $\bar{B}^0_2$ at large and intermediate scales $(k=0.29$ and $0.56 \, {\rm Mpc}^{-1}$ respectively) is punctuated by two sign changes which mark transitions in the HI distribution. The first sign flip where $\bar{B}^0_0$ becomes negative occurs in the intermediate stages of EoR $(\bar{x}_{\rm HI} > 0.5)$, at large scale first followed by the intermediate scale. This marks the emergence of distinct ionized bubbles in the neutral background. $\bar{B}^0_2$ is relatively less affected by this transition, and it mostly remains positive even when $\bar{B}^0_0$ becomes negative. The second sign flip, which affects both $\bar{B}^0_0$ and $\bar{B}^0_2$, occurs at the late stage of EoR $(\bar{x}_{\rm HI} < 0.5)$. This marks a transition in the topology of the HI distribution, after which we have distinct HI islands in an ionized background. This causes $\bar{B}^0_0$ to become positive. The negative $\bar{B}^0_2$ is a definite indication that the HI islands survive only in under-dense regions.

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