arXiv subjects

Kavilan Moodley

Publications and source records attributed to Kavilan Moodley.

At least 19 recordsLinked to original sources

arXiv2026

The Atacama Cosmology Telescope: A demonstration of CMB lensing measurement from daytime data

We present a cosmic microwave background (CMB) lensing power spectrum analysis using daytime data (11am-11pm UTC) gathered by the Atacama Cosmology Telescope (ACT) over the period 2017-2022 (ACT Data Release 6). This dataset is challenging to analyze because the Sun heats and deforms the telescope mirror, complicating the characterization of the telescope. We perform more than one hundred null and consistency checks to ensure the robustness of our measurement and its compatibility with nighttime observations. We detect the CMB lensing power spectrum at 18$σ$ significance, with an amplitude $A_\textrm{lens} = 1.071 \pm 0.060$ with respect to the prediction from the best-fit Planck-ACT CMB power spectrum $Λ$CDM cosmology. In combination with the Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) data, this corresponds to a constraint on the amplitude of matter fluctuations $σ_8 = 0.842 \pm 0.026$. The analysis presented here is especially relevant for ground-based millimeter-wave CMB experiments at the Atacama site, paving the way for future analyses making use of both nighttime and daytime data to place tight constraints on cosmological parameters.

astro-ph.CO↗

arXiv2026

Dish Assembly Precision for HIRAX

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio interferometer array that is being deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. Mapping the southern sky, its aim is to observe neutral hydrogen (HI) through intensity mapping (IM) across the redshift range of 0.78-2.55. The observation of HI makes it possible to tomographically probe large cosmological volumes, enabling constraints on, for example, the dark energy equation of state. Systematics are a significant concern in deriving cosmological constraints from HI IM due to the presence of strong foreground signals. Instrumental effects such as dish surface deviations and feed placement errors need to be carefully controlled and monitored to preserve the sensitivity of HIRAX's redundant array configuration. These instrumental systematics cause bright foreground power to leak into the faint cosmological signal. The 6 m parabolic dishes are made from fiberglass with an embedded aluminum mesh that acts as the reflector. The feed is held in place at the prime focus (f/D = 0.21) by four fiberglass legs. This paper presents the dish surface and feed placement precision of the first 28 dishes, derived from photogrammetry metrology measurements taken during the dish fabrication, as well as from measurements in the field. While surface deviations are predominantly well within requirements, feed placement satisfies all accuracy criteria but fails precision limits; however, precision is expected to improve with a larger sample size. These results establish a baseline for modeling primary beam effects, ultimately enabling the characterization and mitigation of systematic errors in the HI IM signal with the full HIRAX array.

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arXiv2026

The Atacama Cosmology Telescope: stellar mass growth in massive galaxy clusters from DR5 over the past 7 billion years

We probe the stellar mass growth in a sample of 568 Sunyaev-Zel'dovich (SZ) selected galaxy clusters with masses greater than $2.9 \times 10^{14} \mathrm{M_{\odot}}$ and redshifts in the range $0.2 < z < 0.8$, drawn from the fifth data release of the Atacama Cosmology Telescope (ACT DR5). By utilising deep photometry from the tenth data release of the Dark Energy Camera Legacy Survey (DECaLS DR10), we construct redshift- and cluster mass-binned composite cluster stellar mass functions (SMFs), down to $M_* = 10^{9.5} \mathrm{M_{\odot}}$. This work presents the first analysis of the cluster SMF for this cluster sample at this epoch. We find that the characteristic stellar mass ($M^*$) of the cluster SMF evolves marginally from $0.55 \leq z < 0.8$, with most of the measurable growth occurring at $ 0.2 < z < 0.55$. This suggests that most of the massive galaxy population in clusters ($M_* \gtrsim 10^{10.75} \mathrm{M_{\odot}}$) is largely established by $z \sim 0.8$, with subsequent evolution driven by late-time assembly processes. The low-mass slope ($α$) of the composite cluster SMF is flat at high-$z$ ($z \sim 0.8$) but steepens at $z < 0.55$, suggesting an abundance of massive galaxies in high-$z$ clusters compared to low-$z$ clusters. We measure the evolution of cluster stellar mass fractions contained within galaxies with $M_* > 10^{9.5} \mathrm{M_{\odot}}$ between $ 0.2 < z < 0.8$, and find evidence of significant growth, by a factor of $2.5$, after accounting for the growth in cluster halo mass over this epoch.

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arXiv2026

The Atacama Cosmology Telescope: A Test of the Gravitational Force Law on Cosmological Scales Using the Kinematic Sunyaev-Zeldovich Effect

The mean pairwise velocity of massive halos reflects the gravitational force law on cosmic scales. We combine cosmic microwave background intensity maps from the Atacama Cosmology Telescope and a galaxy catalog from the Sloan Digital Sky Survey to estimate the mean pairwise velocity using the kinematic Sunyaev-Zeldovich (kSZ) effect. On scales from 30 -- 230 megaparsecs, we constrain the gravitational acceleration between pairs of halos at separation $r$ to be $g\propto 1/r^n$ with $n=2.1\pm 0.3$, which is consistent with Newtonian gravity in an expanding spacetime (\textit{i.e.}, the standard $Λ$CDM model). This constraint shows agreement with an inverse quadratic radial dependence over the large distances separating galaxy halos, as expected in standard cosmology. Upcoming surveys have the potential to rule out $n = 1$ at $10σ$ significance. Our results establish the kSZ effect as a powerful tool for testing gravity on cosmological scales.

astro-ph.CO↗

arXiv2026

Measurements of the Thermal Sunyaev-Zel'dovich Effect with ACT and DESI Luminous Red Galaxies

Cosmic Microwave Background (CMB) photons scatter off the free-electron gas in galaxies and clusters, allowing us to use the CMB as a backlight to probe the gas in and around low-redshift galaxies. The thermal Sunyaev-Zel'dovich effect, sourced by hot electrons in high-density environments, measures the thermal pressure of the target objects, shedding light on halo thermodynamics and galaxy formation and providing a path toward understanding the baryon distribution around cosmic structures. We use a combination of high-resolution CMB maps from the Atacama Cosmology Telescope (ACT) and photometric luminous red galaxy (LRG) catalogues from the Dark Energy Spectroscopic Instrument (DESI) to measure the thermal Sunyaev-Zel'dovich signal in four redshift bins from $z=0.4$ to $z=1.2$, with a combined detection significance of 19$σ$ when stacking on the fiducial CMB Compton-$y$ map. We discuss possible sources of contamination, finding that residual dust emission associated with the target galaxies is important and limits current analyses. We discuss several mitigation strategies and quantify the residual modelling uncertainty. This work complements closely related measurements of the kinematic Sunyaev-Zel'dovich and weak lensing of the same galaxies.

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arXiv2026

The Atacama Cosmology Telescope: Constraints on Local Non-Gaussianity from the ACT Cluster Catalog

We derive constraints on local-type primordial non-Gaussianity using the ACT DR6 Sunyaev--Zel'dovich cluster catalog. Modeling the redshift- and mass-dependent number counts of 1,201 clusters in the 10,347~deg$^2$ Legacy region, and accounting for survey completeness, intrinsic SZ scatter, and a weak-lensing-calibrated mass bias, we compute theoretical abundances using the Log--Edgeworth halo mass function. Assuming $Λ$CDM with well-motivated external priors, we obtain $f_{\rm NL} = 55 \pm 125$ (68% CL), consistent with Gaussian initial conditions. These constraints probe comoving scales of $5$--$10~{\rm Mpc}~h^{-1}$, complementing CMB bispectrum and scale-dependent bias measurements, which do not reach such small scales. We also find evidence for a 16.4% residual mass bias, which, although heavily informed by our adopted priors, plays a key role in matching observed and predicted counts but has negligible effect on $f_{\rm NL}$ constraints. We briefly discuss robustness of the results under relaxed priors and the prospects for next-generation SZ and lensing surveys to strengthen cluster-based tests of primordial non-Gaussianity.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope: Cross-correlation of kSZ and continuity equation velocity reconstruction with photometric DESI LRGs

Over the last year, kinematic Sunyaev--Zel'dovich (kSZ) velocity reconstruction -- the measurement of the large-scale velocity field using the anisotropic statistics of the small-scale kSZ-galaxy overdensity correlation -- has emerged as a statistically significant probe of the large-scale Universe. In this work, we perform a 2-dimensional tomographic reconstruction using ACT DR6 CMB data and DESI legacy luminous red galaxies (LRGs). We measure the cross-correlation of the kSZ-reconstructed velocity $v^{\mathrm{kSZ}}$ with the velocity inferred from the continuity equation applied to the DESI LRGs $v^{\mathrm{cont}}$ at the $\sim 10 σ$ level, detecting the signal with an amplitude with respect to our theory of $b_v = 0.339\pm 0.034$. We fit a scale-dependent galaxy bias model to our measurement in order to constrain local primordial non-Gaussianity $f_{\mathrm{NL}}^{\mathrm{loc}}$, finding {$f_{\mathrm{NL}}^{\mathrm{loc}}=-180^{+61}_{-86}$} at 67\% confidence, with $f_{\mathrm{NL}}^{\mathrm{loc}}$ consistent with zero at 95\% confidence. We also measure an auto spectrum at $2.1σ$ significance which provides a constraint on $b_v$ of $b_v=0.26_{-0.05}^{+0.11}$, which is consistent with the measurement from the cross spectrum. Our combined measurement is $b_v=0.33\pm0.03$, an $11σ$ measurement. We find a good fit of our model to the data in all cases. Finally, we use different ACT frequency combinations to explore foreground contamination, finding no evidence for foreground contamination in our velocity cross correlation. We compare to a similar measurement where $v^{\mathrm{kSZ}}$ is directly cross correlated with the large-scale galaxy field, and find signs of foreground contamination which is contained in the equal-redshift spectra.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope: DR6 Power Spectrum Foreground Model and Validation

We discuss the model of astrophysical emission at millimeter wavelengths used to characterize foregrounds in the multi-frequency power spectra of the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6), expanding on Louis et al. (2025). We detail several tests to validate the capability of the DR6 parametric foreground model to describe current observations and complex simulations, and show that cosmological parameter constraints are robust against model extensions and variations. We demonstrate consistency of the model with pre-DR6 ACT data and observations from Planck and the South Pole Telescope. We evaluate the implications of using different foreground templates and extending the model with new components and/or free parameters. In all scenarios, the DR6 $Λ$CDM and $Λ$CDM+$N_{\rm eff}$ cosmological parameters shift by less than $0.5σ$ relative to the baseline constraints. Some foreground parameters shift more; we estimate their systematic uncertainties associated with modeling choices. From our constraint on the kinematic Sunyaev-Zel'dovich power, we obtain a conservative limit on the duration of reionization of $Δz_{\rm rei} < 4.4$, assuming a reionization midpoint consistent with optical depth measurements and a minimal low-redshift contribution, with varying assumptions for this component leading to tighter limits. Finally, we analyze realistic non-Gaussian, correlated microwave sky simulations containing Galactic and extragalactic foreground fields, built independently of the DR6 parametric foreground model. Processing these simulations through the DR6 power spectrum and likelihood pipeline, we recover the input cosmological parameters of the underlying cosmic microwave background field, a new demonstration for small-scale CMB analysis. These tests validate the robustness of the ACT DR6 foreground model and cosmological parameter constraints.

astro-ph.CO↗

arXiv2025

Backlighting extended gas halos around luminous red galaxies: kinematic Sunyaev-Zel'dovich effect from DESI Y1 x ACT

The gas density profile around galaxies, shaped by feedback and affecting the galaxy lensing signal, is imprinted on the cosmic microwave background (CMB) by the kinematic Sunyaev-Zel'dovich effect (kSZ). We precisely measure this effect ($S/N\approx 10$) via velocity stacking with more than 800,000 spectroscopically confirmed luminous red galaxies (LRG) from the Dark Energy Spectroscopic Instrument (DESI) Y1 survey, which overlap with the Atacama Cosmology Telescope (ACT) Data Release 6 temperature maps over $\geq$ 4,000 deg$^2$. We explore the kSZ dependence with various galaxy parameters and find no significant trend with redshift, but clear trends with stellar mass and absolute magnitude in $g$, $r$, and $z$ bands. Our analysis suggests that the gas extends beyond the dark matter halo (99.5\% confidence, i.e. PTE = 0.005). We find a tentative preference for hydrodynamical simulation models with stronger feedback that drives gas further out (Illustris $z=0.5$, PTE = 0.37) over weaker-feedback cases (IllustrisTNG $z=0.8$, PTE = 0.045), though with limited statistical significance. In all cases, a free multiplicative amplitude was fit to the simulated profiles, and further modeling work is required to firm up these conclusions. We find consistency between kSZ profiles around spectroscopic and photometric LRG, with comparable statistical power, thus increasing our confidence in the photometric analysis. Additionally, we present the first kSZ measurement around DESI Y1 bright galaxy sample (BGS) and emission-line galaxies (ELG), whose features match qualitative expectations. Finally, we forecast $S/N \sim 50$ for future stacked kSZ measurements using data from ACT, DESI Y3, and Rubin Observatory. These measurements will serve as an input for galaxy formation models and baryonic uncertainties in galaxy lensing.

astro-ph.GA↗

arXiv2025

A cross-bispectrum estimator for CMB-HI intensity mapping correlations

Intensity mapping of 21cm emission from neutral hydrogen promises to be a powerful probe of large-scale structure in the post-reionisation epoch. However, HI intensity mapping (IM) experiments will suffer the loss of long-wavelength line-of-sight HI modes in the foreground subtraction process. This significantly reduces HI IM cross-correlations with projected large-scale structure tracers, such as CMB secondary anisotropies. Here we propose a cross-bispectrum estimator, $B^{\bar κδT_{21} δT_{21}},$ to recover the cross-correlation of the HI IM field, $δT_{21},$ with the CMB lensing field, $κ,$ constructed by correlating the position-dependent HI power spectrum with the mean overdensity traced by CMB lensing. We study the cross-bispectrum estimator in the squeezed limit and forecast its detectability based on HI IM measurements from HIRAX and CMB lensing measurements from Advanced ACT. We find that $B^{\bar κδT_{21}δT_{21}},$ in combination with the HI IM and CMB lensing auto-spectra, can place sub-percent constraints on the growth rate of fluctuations, $f,$ and the small scale amplitude of fluctuations, $ σ_8.$ The cross-bispectrum, in combination with the auto-spectra and Planck priors, improves dark energy constraints to 0.025 on $w_0$ and 0.11 on $w_a$ for flat models. These results are robust to HI foreground removal because they derive from small-scale HI modes. The HI-CMB lensing cross-bispectrum thus provides a novel way to recover HI correlations with CMB lensing and constrain cosmological parameters at a level that is competitive with next-generation galaxy redshift surveys. As a striking example of this, we find a tight constraint of 27.8 meV (29.0 meV) on the sum of neutrino masses, while varying all redshift and standard cosmological parameters within a flat $Λ$CDM ($w_0w_a$CDM) model.

astro-ph.CO↗

arXiv2025

The thermal and kinematic Sunyaev-Zeldovich effect in galaxy clusters and filaments using multifrequency temperature maps of the cosmic microwave background: Abell 399--Abell 401 cluster pair case study

We present a multifrequency and multi-instrument methodology to study the physical properties of galaxy clusters and cosmic filaments using cosmic microwave background observations. Our approach enables simultaneous measurement of both the thermal (tSZ) and kinematic Sunyaev-Zeldovich (kSZ) effects, incorporates relativistic corrections, and models astrophysical foregrounds such as thermal dust emission. We do this by jointly fitting a single physical model across multiple maps from multiple instruments at different frequencies, rather than fitting a model to a single Compton-$y$ map. We demonstrate the success of this method by fitting the Abell 399-Abell 401 galaxy cluster pair and filament system using archival data from the Planck satellite and new, targeted deep data from the Atacama Cosmology Telescope, covering 11 different frequencies over 14 maps from 30 GHz to 545 GHz. Our tSZ results are consistent with previous work using Compton-$y$ maps. We measure the line-of-sight peculiar velocities of the cluster-filament system using the kSZ effect and find statistical uncertainties on individual cluster peculiar velocities of $\lesssim $600 km s$^{-1}$, which are competitive with current state-of-the-art measurements. Additionally, we measure the optical depth of the filament component with a signal-to-noise of 8.5$σ$ and reveal hints of its morphology. This modular approach is well-suited for application to future instruments across a wide range of millimeter and sub-millimeter wavebands.

astro-ph.CO↗

arXiv2025

Primary Beam Chromaticity in HIRAX: I. Characterization from Simulations and Power Spectrum Implications

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an upcoming radio interferometric telescope designed to constrain dark energy through the 21cm intensity mapping of Baryon Acoustic Oscillations (BAO). Instrumental systematics must be controlled and carefully characterized to measure the 21cm power spectrum with fidelity and achieve high-precision constraints on the cosmological parameters. The chromaticity of the primary beam is one such complicated systematic, which can leak the power of spectrally smooth foregrounds beyond the ideal horizon limits due to the complex spatial and spectral structures of the sidelobes and the mainlobe. This paper studies the chromaticity of the HIRAX Stokes I primary beam and its effects on accurate measurements of the 21cm power spectrum. To investigate the effect of chromaticity in the 21cm power spectrum, we present a physically motivated beam modeling technique, which uses a flexible basis derived from traditional optics that can account for higher-order radial and azimuthal structures in the primary beam. We investigate the impact of imperfect knowledge of the mainlobe and sidelobes chromaticity in the power spectrum space by subtracting a simple foreground model in simulated snapshot visibilities to recover the H$\textsc{i}$ power spectrum. Additionally, we find that modeling up to the octupolar azimuthal order feature (fourth-order angular variation) in the primary beam is sufficient to reduce the leakage outside the wedge with minimal bias.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope DR6 and DESI: Structure formation over cosmic time with a measurement of the cross-correlation of CMB Lensing and Luminous Red Galaxies

We present a high-significance cross-correlation of CMB lensing maps from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) with spectroscopically calibrated luminous red galaxies (LRGs) from the Dark Energy Spectroscopic Instrument (DESI). We detect this cross-correlation at a significance of 38$σ$; combining our measurement with the Planck Public Release 4 (PR4) lensing map, we detect the cross-correlation at 50$σ$. Fitting this jointly with the galaxy auto-correlation power spectrum to break the galaxy bias degeneracy with $σ_8$, we perform a tomographic analysis in four LRG redshift bins spanning $0.4 \le z \le 1.0$ to constrain the amplitude of matter density fluctuations through the parameter combination $S_8^\times = σ_8 \left(Ω_m / 0.3\right)^{0.4}$. Prior to unblinding, we confirm with extragalactic simulations that foreground biases are negligible and carry out a comprehensive suite of null and consistency tests. Using a hybrid effective field theory (HEFT) model that allows scales as small as $k_{\rm max}=0.6$ $h/{\rm Mpc}$, we obtain a 3.3% constraint on $S_8^\times = σ_8 \left(Ω_m / 0.3\right)^{0.4} = 0.792^{+0.024}_{-0.028}$ from ACT data, as well as constraints on $S_8^\times(z)$ that probe structure formation over cosmic time. Our result is consistent with the early-universe extrapolation from primary CMB anisotropies measured by Planck PR4 within 1.2$σ$. Jointly fitting ACT and Planck lensing cross-correlations we obtain a 2.7% constraint of $S_8^\times = 0.776^{+0.019}_{-0.021}$, which is consistent with the Planck early-universe extrapolation within 2.1$σ$, with the lowest redshift bin showing the largest difference in mean. The latter may motivate further CMB lensing tomography analyses at $z<0.6$ to assess the impact of potential systematics or the consistency of the $Λ$CDM model over cosmic time.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope: High-redshift measurement of structure growth from the cross-correlation of Quaia quasars and CMB lensing from ACT DR6 and $\textit{Planck}$ PR4

We measure the amplitude of matter fluctuations over a wide range of redshifts by combining CMB lensing observations from ACT DR6 and $\textit{Planck}$ PR4 with the overdensity of quasars from Quaia, a $\textit{Gaia}$ and $\textit{unWISE}$ quasar catalog. Our analysis includes the CMB lensing power spectrum from ACT DR6, the auto-correlation of two Quaia quasar samples centered at $z \simeq 1.0$ and $z \simeq 2.1$, and their cross-correlations with CMB lensing from both ACT DR6 and $\textit{Planck}$ PR4. By performing a series of contamination and systematic null tests, we find no evidence for contamination in the lensing maps, contrary to what was suggested in previous Quaia cross-correlation analyses using $\textit{Planck}$ PR4 CMB lensing data. From the joint analysis of the quasar auto- and cross-correlations with CMB lensing, and including BOSS BAO data to break the degeneracy between $Ω_m$ and $σ_8$, we obtain $σ_8 = 0.802^{+0.045}_{-0.057}$, consistent with $Λ$CDM predictions from $\textit{Planck}$ primary CMB measurements. Combining the CMB lensing auto-spectrum with the cross-correlation measurement improves the constraint on $σ_8$ by $12\%$ relative to the lensing auto-spectrum alone, yielding $σ_8 = 0.804 \pm 0.013$. This dataset combination also enables a reconstruction of structure growth across redshifts. We infer a $12\%$ constraint on the amplitude of matter fluctuations at $z > 3$, with a measurement at the median redshift of the signal of $σ_8(\tilde{z}=5.1) = 0.146^{+0.021}_{-0.014}$, consistent with $\textit{Planck}$ at the $1.4σ$ level. These results provide one of the highest redshift constraints on the growth of structure to date.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope: A search for late-time anisotropic screening of the Cosmic Microwave Background

Since the formation of the first stars, most of the gas in the Universe has been ionized. Spatial variations in the density of this ionized gas generate cosmic microwave background anisotropies via Thomson scattering, a process known as the ``anisotropic screening'' effect. We propose and implement for the first time a new estimator to cross-correlate unWISE galaxies and anisotropic screening, as measured by the Atacama Cosmology Telescope and Planck satellite. We do not significantly detect the effect; the null hypothesis is consistent with the data at 1.7 $σ$ (resp. 0.016 $σ$) for the blue (resp. green) unWISE sample. We obtain an upper limit on the integrated optical depth within a 6 arcmin disk to be $\barτ< 0.033$ arcmin$^2$ at 95\% confidence for the blue sample and $\barτ< 0.057$ arcmin$^2$ for the green sample. Future measurements with Simons Observatory and CMB-S4 should detect this effect significantly. Complementary to the kinematic Sunyaev-Zel'dovich effect, this probe of the gas distribution around halos will inform models of feedback in galaxy formation and baryonic effects in galaxy lensing.

astro-ph.CO↗

arXiv2025

Cosmological constraints from the cross-correlation of DESI Luminous Red Galaxies with CMB lensing from Planck PR4 and ACT DR6

We infer the growth of large scale structure over the redshift range $0.4\lesssim z \lesssim 1$ from the cross-correlation of spectroscopically calibrated Luminous Red Galaxies (LRGs) selected from the Dark Energy Spectroscopic Instrument (DESI) legacy imaging survey with CMB lensing maps reconstructed from the latest Planck and ACT data. We adopt a hybrid effective field theory (HEFT) model that robustly regulates the cosmological information obtainable from smaller scales, such that our cosmological constraints are reliably derived from the (predominantly) linear regime. We perform an extensive set of bandpower- and parameter-level systematics checks to ensure the robustness of our results and to characterize the uniformity of the LRG sample. We demonstrate that our results are stable to a wide range of modeling assumptions, finding excellent agreement with a linear theory analysis performed on a restricted range of scales. From a tomographic analysis of the four LRG photometric redshift bins we find that the rate of structure growth is consistent with $Λ$CDM with an overall amplitude that is $\simeq5-7\%$ lower than predicted by primary CMB measurements with modest $(\sim2σ)$ statistical significance. From the combined analysis of all four bins and their cross-correlations with Planck we obtain $S_8 = 0.765\pm0.023$, which is less discrepant with primary CMB measurements than previous DESI LRG cross Planck CMB lensing results. From the cross-correlation with ACT we obtain $S_8 = 0.790^{+0.024}_{-0.027}$, while when jointly analyzing Planck and ACT we find $S_8 = 0.775^{+0.019}_{-0.022}$ from our data alone and $σ_8 = 0.772^{+0.020}_{-0.023}$ with the addition of BAO data. These constraints are consistent with the latest Planck primary CMB analyses at the $\simeq 1.6-2.2σ$ level, and are in excellent agreement with galaxy lensing surveys.

astro-ph.CO↗

arXiv2025

The Atacama Cosmology Telescope: Multi-probe cosmology with unWISE galaxies and ACT DR6 CMB lensing

We present a joint analysis of the CMB lensing power spectra measured from the Data Release 6 of the Atacama Cosmology Telescope and Planck PR4, cross-correlations between the ACT and Planck lensing reconstruction and galaxy clustering from unWISE, and the unWISE clustering auto-spectrum. We obtain 1.5% constraints on the matter density fluctuations at late times parametrised by the best constrained parameter combination $S_8^{\rm 3x2pt}\equivσ_8 (Ω_m/0.3)^{0.4}=0.815\pm0.012$. The commonly used $S_8\equivσ_8 (Ω_m/0.3)^{0.5}$ parameter is constrained to $S_8=0.816\pm0.015$. In combination with baryon acoustic oscillation (BAO) measurements we find $σ_8=0.815\pm 0.012$. We also present sound-horizon-independent estimates of the present day Hubble rate of $H_0=66.4^{+3.2}_{-3.7} \,\mathrm{km}\,\mathrm{s}^{-1}\mathrm{Mpc}^{-1}$ from our large scale structure data alone and $H_0=64.3^{+2.1}_{-2.4}\,\mathrm{km}\,\mathrm{s}^{-1}\mathrm{Mpc}^{-1}$ in combination with uncalibrated supernovae from Pantheon+. Using parametric estimates of the evolution of matter density fluctuations, we place constraints on cosmic structure in a range of high redshifts typically inaccessible with cross-correlation analyses. Combining lensing cross- and auto-correlations, we derive a 3.3% constraint on the integrated matter density fluctuations above $z=2.4$, one of the tightest constraints in this redshift range and fully consistent with a $Λ$CDM model fit to the primary CMB from Planck. Combining with primary CMB observations and using the extended low redshift coverage of these combined data sets we derive constraints on a variety of extensions to the $Λ$CDM model including massive neutrinos, spatial curvature, and dark energy. We find in flat $Λ$CDM $\sum m_ν<0.12$ eV at 95% confidence using the LSS data, BAO measurements from SDSS and primary CMB observations.

astro-ph.CO↗

arXiv2024

The Atacama Cosmology Telescope: Semi-Analytic Covariance Matrices for the DR6 CMB Power Spectra

The Atacama Cosmology Telescope Data Release 6 (ACT DR6) power spectrum is expected to provide state-of-the-art cosmological constraints, with an associated need for precise error modeling. In this paper we design, and evaluate the performance of, an analytic covariance matrix prescription for the DR6 power spectrum that sufficiently accounts for the complicated ACT map properties. We use recent advances in the literature to handle sharp features in the signal and noise power spectra, and account for the effect of map-level anisotropies on the covariance matrix. In including inhomogeneous survey depth information, the resulting covariance matrix prescription is structurally similar to that used in the $\textit{Planck}$ Cosmic Microwave Background (CMB) analysis. We quantify the performance of our prescription using comparisons to Monte Carlo simulations, finding better than $3\%$ agreement. This represents an improvement from a simpler, pre-existing prescription, which differs from simulations by $\sim16\%$. We develop a new method to correct the analytic covariance matrix using simulations, after which both prescriptions achieve better than $1\%$ agreement. This correction method outperforms a commonly used alternative, where the analytic correlation matrix is assumed to be accurate when correcting the covariance. Beyond its use for ACT, this framework should be applicable for future high resolution CMB experiments including the Simons Observatory (SO).

astro-ph.CO↗