Search

We present tomographic measurements of structure growth using cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck cosmic microwave background (CMB) lensing maps with the unWISE Blue and Green galaxy samples, which span the redshift ranges 0.2 ≲ z ≲ 1.1 and 0.3 ≲ z ≲ 1.8, respectively. We improve on prior unWISE cross-correlations not just by making use of the new, high-precision ACT DR6 lensing maps, but also by including additional spectroscopic data for redshift calibration and by analyzing our measurements with a more flexible theoretical model. We determine the amplitude of matter fluctuations at low redshifts (z ≃ 0.2-1.6), finding S 8 ≡ σ 8 ( Ω m / 0.3 ) 0.5 = 0.813 ± 0.021 using the ACT cross-correlation alone and S 8 = 0.810 ± 0.015 with a combination of Planck and ACT cross-correlations; these measurements are fully consistent with the predictions from primary CMB measurements assuming standard structure growth. The addition of baryon acoustic oscillation data breaks the degeneracy between σ 8 and Ωm , allowing us to measure σ 8 = 0.813 ± 0.020 from the cross-correlation of unWISE with ACT and σ 8 = 0.813 ± 0.015 from the combination of cross-correlations with ACT and Planck. These results also agree with the expectations from primary CMB extrapolations in ΛCDM cosmology; the consistency of σ 8 derived from our two redshift samples at z ∼ 0.6 and 1.1 provides a further check of our cosmological model. Our results suggest that structure formation on linear scales is well described by ΛCDM even down to low redshifts z ≲ 1.

The early dark energy (EDE) scenario aims to increase the value of the Hubble constant (H0) inferred from cosmic microwave background (CMB) data over that found in the standard cosmological model (ΛCDM), via the introduction of a new form of energy density in the early Universe. The EDE component briefly accelerates cosmic expansion just prior to recombination, which reduces the physical size of the sound horizon imprinted in the CMB. Previous work has found that nonzero EDE is not preferred by Planck CMB power spectrum data alone, which yield a 95% confidence level (C.L.) upper limit fEDE99.7% C.L.: fEDE=0.091-0.036+0.020, with H0=70.9-2.0+1.0 km/s/Mpc (both 68% C.L.). From a model-selection standpoint, we find that EDE is favored over ΛCDM by these data at roughly 3σ significance. In contrast, a joint analysis of the full Planck and ACT data yields no evidence for EDE, as previously found for Planck alone. We show that the preference for EDE in ACT alone is driven by its TE and EE power spectrum data. The tight constraint on EDE from Planck alone is driven by its high-ℓ TT power spectrum data. Understanding whether these differing constraints are physical in nature, due to systematics, or simply a rare statistical fluctuation is of high priority. The best-fit EDE models to ACT and Planck exhibit coherent differences across a wide range of multipoles in TE and EE, indicating that a powerful test of this scenario is anticipated with near-future data from ACT and other ground-based experiments.

We use Atacama Cosmology Telescope (ACT) observations at 98 GHz (2015–2019), 150 GHz (2013–2019), and 229 GHz (2017–2019) to perform a blind shift-and-stack search for Planet 9. The search explores distances from 300 au to 2000 au and velocities up to 6.′3 per year, depending on the distance (r). For a 5 Earth-mass Planet 9 the detection limit varies from 325 au to 625 au, depending on the sky location. For a 10 Earth-mass planet the corresponding range is 425 au to 775 au. The predicted aphelion and most likely location of the planet corresponds to the shallower end of these ranges. The search covers the whole 18,000 square degrees of the ACT survey. No significant detections are found, which is used to place limits on the millimeter-wave flux density of Planet 9 over much of its orbit. Overall we eliminate roughly 17% and 9% of the parameter space for a 5 and 10 Earth-mass Planet 9, respectively. These bounds approach those of a recent INPOP19a ephemeris-based analysis, but do not exceed it. We also provide a list of the 10 strongest candidates from the search for possible follow-up. More generally, we exclude (at 95% confidence) the presence of an unknown solar system object within our survey area brighter than 4–12 mJy (depending on position) at 150 GHz with current distance 300 au < r < 600 au and heliocentric angular velocity , corresponding to low-to-moderate eccentricities. These limits worsen gradually beyond 600 au, reaching 5–15 mJy by 1500 au.

The Sunyaev-Zel'dovich (SZ) effect introduces a specific distortion of the blackbody spectrum of the cosmic microwave background (CMB) radiation when it scatters off hot gas in clusters of galaxies. The frequency dependence of the distortion is only independent of the cluster redshift when the evolution of the CMB radiation is adiabatic. Using 370 clusters within the redshift range 0.07 ≲ z ≲ 1.4 from the largest SZ-selected cluster sample to date from the Atacama Cosmology Telescope, we provide new constraints on the deviation of CMB temperature evolution from the standard model α =0.017-0.032+0.029, where T(z)=T0(1+z)1-α. This result is consistent with no deviation from the standard adiabatic model. Combining it with previous, independent data sets we obtain a joint constraint of α = -0.001 ± 0.012. Attributing deviation from adiabaticity to the decay of dark energy, this result constrains its effective equation of state weff=-0.998-0.010+0.008.

Advances in cosmic microwave background (CMB) science depend on increasing the number of sensitive detectors observing the sky. New instruments deploy large arrays of superconducting transition-edge sensor (TES) bolometers tiled densely into ever larger focal planes. High multiplexing factors reduce the thermal loading on the cryogenic receivers and simplify their design. We present the design of focal-plane modules with an order of magnitude higher multiplexing factor than has previously been achieved with TES bolometers. We focus on the novel cold readout component, which employs microwave SQUID multiplexing (μmux). Simons Observatory will use 49 modules containing 70,000 bolometers to make exquisitely sensitive measurements of the CMB. We validate the focal-plane module design, presenting measurements of the readout component with and without a prototype detector array of 1728 polarization-sensitive bolometers coupled to feedhorns. The readout component achieves a 95% yield and a 910 multiplexing factor. The median white noise of each readout channel is 65 pA/√Hz. This impacts the projected SO mapping speed by

We present arcminute-resolution intensity and polarization maps of the Galactic center made with the Atacama Cosmology Telescope. The maps cover a 32 deg2 field at 98, 150, and 224 GHz with |l| ≤ 4 , |b| ≤ 2 . We combine these data with Planck observations at similar frequencies to create coadded maps with increased sensitivity at large angular scales. With the coadded maps, we are able to resolve many known features of the Central Molecular Zone (CMZ) in both total intensity and polarization. We map the orientation of the plane-of-sky component of the Galactic magnetic field inferred from the polarization angle in the CMZ, finding significant changes in morphology in the three frequency bands as the underlying dominant emission mechanism changes from synchrotron to dust emission. Selected Galactic center sources, including Sgr A∗, the Brick molecular cloud (G0.253+0.016), the Mouse pulsar wind nebula (G359.23-0.82), and the Tornado supernova remnant candidate (G357.7-0.1), are examined in detail. These data illustrate the potential for leveraging ground-based cosmic microwave background polarization experiments for Galactic science.

We measured the cross-correlation between galaxy weak lensing data from the Kilo Degree Survey (KiDS-1000, DR4) and cosmic microwave background (CMB) lensing data from the Atacama Cosmology Telescope (ACT, DR4) and the Planck Legacy survey. We used two samples of source galaxies, selected with photometric redshifts, (0.1 < zB < 1.2) and (1.2 < zB < 2), which produce a combined detection significance of the CMB lensing and weak galaxy lensing cross-spectrum of 7.7σ. With the lower redshift galaxy sample, for which the cross-correlation was detected at a significance of 5.3σ, we present joint cosmological constraints on the matter density parameter, ωm, and the matter fluctuation amplitude parameter, σ8, marginalising over three nuisance parameters that model our uncertainty in the redshift and shear calibration as well as the intrinsic alignment of galaxies. We find our measurement to be consistent with the best-fitting flat ΛCDM cosmological models from both Planck and KiDS-1000. We demonstrate the capacity of CMB weak lensing cross-correlations to set constraints on either the redshift or shear calibration by analysing a previously unused high-redshift KiDS galaxy sample (1.2 < zB < 2), with the cross-correlation detected at a significance of 7σ. This analysis provides an independent assessment for the accuracy of redshift measurements in a regime that is challenging to calibrate directly owing to known incompleteness in spectroscopic surveys.

The scattering of cosmic microwave background (CMB) photons off the free-electron gas in galaxies and clusters leaves detectable imprints on high resolution CMB maps: the thermal and kinematic Sunyaev-Zel'dovich effects (tSZ and kSZ respectively). We use combined microwave maps from the Atacama Cosmology Telescope DR5 and Planck in combination with the CMASS (mean redshift ⟨z

The thermal and kinematic Sunyaev-Zel'dovich effects (tSZ, kSZ) probe the thermodynamic properties of the circumgalactic and intracluster medium (CGM and ICM) of galaxies, groups, and clusters, since they are proportional, respectively, to the integrated electron pressure and momentum along the line of sight. We present constraints on the gas thermodynamics of CMASS (constant stellar mass) galaxies in the Baryon Oscillation Spectroscopic Survey using new measurements of the kSZ and tSZ signals obtained in a companion paper [Schaan et al.]. Combining kSZ and tSZ measurements, we measure within our model the amplitude of energy injection ϵM⋆c2, where M⋆ is the stellar mass, to be ϵ=(40±9)×10-6, and the amplitude of the nonthermal pressure profile to be αNth

Two recent large data releases for the Atacama Cosmology Telescope (ACT),called DR4 and DR5, are available for public access. These data includetemperature and polarization maps that cover nearly half the sky at arcminuteresolution in three frequency bands; lensing maps and component-separated mapscovering ~ 2,100 deg^2 of sky; derived power spectra and cosmologicallikelihoods; a catalog of over 4,000 galaxy clusters; and supporting ancillaryproducts including beam functions and masks. The data and products aredescribed in a suite of ACT papers; here we provide a summary. In order tofacilitate ease of access to these data we present a set of Jupyter IPythonnotebooks developed to introduce users to DR4, DR5, and the tools needed toanalyze these data. The data products (excluding simulations) and the set ofnotebooks are publicly available on the NASA Legacy Archive for MicrowaveBackground Data Analysis (LAMBDA); simulation products are available on theNational Energy Research Scientific Computing Center (NERSC).

Two recent large data releases for the Atacama Cosmology Telescope (ACT),called DR4 and DR5, are available for public access. These data includetemperature and polarization maps that cover nearly half the sky at arcminuteresolution in three frequency bands; lensing maps and component-separated mapscovering ~ 2,100 deg^2 of sky; derived power spectra and cosmologicallikelihoods; a catalog of over 4,000 galaxy clusters; and supporting ancillaryproducts including beam functions and masks. The data and products aredescribed in a suite of ACT papers; here we provide a summary. In order tofacilitate ease of access to these data we present a set of Jupyter IPythonnotebooks developed to introduce users to DR4, DR5, and the tools needed toanalyze these data. The data products (excluding simulations) and the set ofnotebooks are publicly available on the NASA Legacy Archive for MicrowaveBackground Data Analysis (LAMBDA); simulation products are available on theNational Energy Research Scientific Computing Center (NERSC).

We present high signal-to-noise measurements (up to 12$\sigma$) of theaverage thermal Sunyaev Zel'dovich (tSZ) effect from optically selected galaxygroups and clusters and estimate their baryon content within a 2.1$^\prime$radius aperture. Sources from the Sloan Digital Sky Survey (SDSS) BaryonOscillation Spectroscopic Survey (BOSS) DR15 catalog overlap with 3,700 sq.deg. of sky observed by the Atacama Cosmology Telescope (ACT) from 2008 to 2018at 150 and 98 GHz (ACT DR5), and 2,089 sq. deg. of internal linear combinationcomponent-separated maps combining ACT and $\it{Planck}$ data (ACT DR4). Thecorresponding optical depths, $\bar{\tau}$, which depend on the baryon contentof the halos, are estimated using results from cosmological hydrodynamicsimulations assuming an AGN feedback radiative cooling model. We estimate themean mass of the halos in multiple luminosity bins, and compare the tSZ-based$\bar{\tau}$ estimates to theoretical predictions of the baryon content for aNavarro-Frenk-White profile. We do the same for $\bar{\tau}$ estimatesextracted from fits to pairwise baryon momentum measurements of the kinematicSunyaev-Zel'dovich effect (kSZ) for the same data set obtained in a companionpaper. We find that the $\bar{\tau}$ estimates from the tSZ measurements inthis work and the kSZ measurements in the companion paper agree within$1\sigma$ for two out of the three disjoint luminosity bins studied, while theydiffer by 2-3$\sigma$ in the highest luminosity bin. The optical depthestimates account for one third to all of the theoretically predicted baryoncontent in the halos across luminosity bins. Potential systematic uncertaintiesare discussed. The tSZ and kSZ measurements provide a step towards empiricalCompton-$\bar{y}$-$\bar{\tau}$ relationships to provide new tests of clusterformation and evolution models.

We present a 5.4$\sigma$ detection of the pairwise kinematicSunyaev-Zel'dovich (kSZ) effect using Atacama Cosmology Telescope (ACT) and$\it{Planck}$ CMB observations in combination with Luminous Red Galaxy samplesfrom the Sloan Digital Sky Survey (SDSS) DR15 catalog. Results are obtainedusing three ACT CMB maps: co-added 150 GHz and 98 GHz maps, combiningobservations from 2008-2018 (ACT DR5), which overlap with SDSS DR15 over 3,700sq. deg., and a component-separated map using night-time only observations from2014-2015 (ACT DR4), overlapping with SDSS DR15 over 2,089 sq. deg. Comparisonsof the results from these three maps provide consistency checks in relation topotential frequency-dependent foreground contamination. A total of 343,647galaxies are used as tracers to identify and locate galaxy groups and clustersfrom which the kSZ signal is extracted using aperture photometry. We considerthe impact of various aperture photometry assumptions and covariance estimationmethods on the signal extraction. Theoretical predictions of the pairwisevelocities are used to obtain best-fit, mass-averaged, optical depth estimatesfor each of five luminosity-selected tracer samples. A comparison of thekSZ-derived optical depth measurements obtained here to those derived from thethermal SZ effect for the same sample is presented in a companion paper.

We present a 5.4$\sigma$ detection of the pairwise kinematicSunyaev-Zel'dovich (kSZ) effect using Atacama Cosmology Telescope (ACT) and$\it{Planck}$ CMB observations in combination with Luminous Red Galaxy samplesfrom the Sloan Digital Sky Survey (SDSS) DR15 catalog. Results are obtainedusing three ACT CMB maps: co-added 150 GHz and 98 GHz maps, combiningobservations from 2008-2018 (ACT DR5), which overlap with SDSS DR15 over 3,700sq. deg., and a component-separated map using night-time only observations from2014-2015 (ACT DR4), overlapping with SDSS DR15 over 2,089 sq. deg. Comparisonsof the results from these three maps provide consistency checks in relation topotential frequency-dependent foreground contamination. A total of 343,647galaxies are used as tracers to identify and locate galaxy groups and clustersfrom which the kSZ signal is extracted using aperture photometry. We considerthe impact of various aperture photometry assumptions and covariance estimationmethods on the signal extraction. Theoretical predictions of the pairwisevelocities are used to obtain best-fit, mass-averaged, optical depth estimatesfor each of five luminosity-selected tracer samples. A comparison of thekSZ-derived optical depth measurements obtained here to those derived from thethermal SZ effect for the same sample is presented in a companion paper.

We present high signal-to-noise measurements (up to 12$\sigma$) of theaverage thermal Sunyaev Zel'dovich (tSZ) effect from optically selected galaxygroups and clusters and estimate their baryon content within a 2.1$^\prime$radius aperture. Sources from the Sloan Digital Sky Survey (SDSS) BaryonOscillation Spectroscopic Survey (BOSS) DR15 catalog overlap with 3,700 sq.deg. of sky observed by the Atacama Cosmology Telescope (ACT) from 2008 to 2018at 150 and 98 GHz (ACT DR5), and 2,089 sq. deg. of internal linear combinationcomponent-separated maps combining ACT and $\it{Planck}$ data (ACT DR4). Thecorresponding optical depths, $\bar{\tau}$, which depend on the baryon contentof the halos, are estimated using results from cosmological hydrodynamicsimulations assuming an AGN feedback radiative cooling model. We estimate themean mass of the halos in multiple luminosity bins, and compare the tSZ-based$\bar{\tau}$ estimates to theoretical predictions of the baryon content for aNavarro-Frenk-White profile. We do the same for $\bar{\tau}$ estimatesextracted from fits to pairwise baryon momentum measurements of the kinematicSunyaev-Zel'dovich effect (kSZ) for the same data set obtained in a companionpaper. We find that the $\bar{\tau}$ estimates from the tSZ measurements inthis work and the kSZ measurements in the companion paper agree within$1\sigma$ for two out of the three disjoint luminosity bins studied, while theydiffer by 2-3$\sigma$ in the highest luminosity bin. The optical depthestimates account for one third to all of the theoretically predicted baryoncontent in the halos across luminosity bins. Potential systematic uncertaintiesare discussed. The tSZ and kSZ measurements provide a step towards empiricalCompton-$\bar{y}$-$\bar{\tau}$ relationships to provide new tests of clusterformation and evolution models.

We present ΛCDM cosmological parameter constraints obtained from delensed microwave background power spectra. Lensing maps from a subset of DR4 data from the Atacama Cosmology Telescope (ACT) are used to undo the lensing effect in ACT spectra observed at 150 and 98 GHz. At 150 GHz, we remove the lensing distortion with an effective efficiency of 30% (TT), 30% (EE), 26% (TE) and 20% (BB); this results in detections of the delensing effect at 8.7σ (TT), 5.1σ (EE), 2.6σ (TE), and 2.4σ (BB) significance. The combination of 150 and 98 GHz TT, EE, and TE delensed spectra is well fit by a standard ΛCDM model. We also measure the shift in best-fit parameters when fitting delensed versus lensed spectra; while this shift does not inform our ability to measure cosmological parameters, it does provide a three-way consistency check among the lensing inferred from the best-fit parameters, the lensing in the CMB power spectrum, and the reconstructed lensing map. This shift is predicted to be zero when fitting with the correct model since both lensed and delensed spectra originate from the same region of sky. Fitting with a ΛCDM model and marginalizing over foregrounds, we find that the shift in cosmological parameters is consistent with zero. Our results show that gravitational lensing of the microwave background is internally consistent within the framework of the standard cosmological model.

This paper presents a maximum-likelihood algorithm for combining sky maps with disparate sky coverage, angular resolution and spatially varying anisotropic noise into a single map of the sky. We use this to merge hundreds of individual maps covering the 2008–2018 ACT observing seasons, resulting in by far the deepest ACT maps released so far. We also combine the maps with the full Planck maps, resulting in maps that have the best features of both Planck and ACT: Planck’s nearly white noise on intermediate and large angular scales and ACT’s high-resolution and sensitivity on small angular scales. The maps cover over 18 000 square degrees, nearly half the full sky, at 100, 150 and 220 GHz. They reveal 4 000 optically-confirmed clusters through the Sunyaev Zel’dovich effect (SZ) and 18 500 point source candidates at > 5σ, the largest single collection of SZ clusters and millimeter wave sources to date. The multi-frequency maps provide millimeter images of nearby galaxies and individual Milky Way nebulae, and even clear detections of several nearby stars. Other anticipated uses of these maps include, for example, thermal SZ and kinematic SZ cluster stacking, CMB cluster lensing and galactic dust science. The method itself has negligible bias. However, due to the preliminary nature of some of the component data sets, we caution that these maps should not be used for precision cosmological analysis. The maps are part of ACT DR5, and will be made available on LAMBDA no later than three months after the journal publication of this article, along with an interactive sky atlas.

We construct cosmic microwave background lensing mass maps using data from the 2014 and 2015 seasons of observations with the Atacama Cosmology Telescope (ACT). These maps cover 2100 square degrees of sky and overlap with a wide variety of optical surveys. The maps are signal dominated on large scales and have fidelity such that their correlation with the cosmic infrared background is clearly visible by eye. We also create lensing maps with thermal Sunyaev-Zel'dovich contamination removed using a novel cleaning procedure that only slightly degrades the lensing signal-to-noise ratio. The cross-spectrum between the cleaned lensing map and the BOSS CMASS galaxy sample is detected at 10σ significance, with an amplitude of A = 1.02 ± 0.10 relative to the Planck best-fitting Lambda cold dark matter cosmological model with fiducial linear galaxy bias. Our measurement lays the foundation for lensing cross-correlation science with current ACT data and beyond.

We use gravitational lensing of the cosmic microwave background (CMB) to measure the mass of the most distant blindly selected sample of galaxy clusters on which a lensing measurement has been performed to date. In CMB data from the the Atacama Cosmology Telescope and the Planck satellite, we detect the stacked lensing effect from 677 near-infrared-selected galaxy clusters from the Massive and Distant Clusters of WISE Survey (MaDCoWS), which have a mean redshift of zñ = 1.08. There are currently no representative optical weak lensing measurements of clusters that match the distance and average mass of this sample. We detect the lensing signal with a significance of 4.2s. We model the signal with a halo model framework to find the mean mass of the population from which these clusters are drawn. Assuming that the clusters follow Navarro–Frenk–White (NFW) density profiles, we infer a mean mass of M500cñ = (1.7 + 0.4) ´ 1014 M*. We consider systematic uncertainties from cluster redshift errors, centering errors, and the shape of the NFW profile. These are all smaller than 30% of our reported uncertainty. This work highlights the potential of CMB lensing to enable cosmological constraints from the abundance of distant clusters populating ever larger volumes of the observable universe, beyond the capabilities of optical weak lensing measurements.

Optimal analyses of many signals in the cosmic microwave background (CMB) require map-level extraction of individual components in the microwave sky, rather than measurements at the power spectrum level alone. To date, nearly all map-level component separation in CMB analyses has been performed exclusively using satellite data. In this paper, we implement a component separation method based on the internal linear combination (ILC) approach which we have designed to optimally account for the anisotropic noise (in the 2D Fourier domain) often found in ground-based CMB experiments. Using this method, we combine multifrequency data from the Planck satellite and the Atacama Cosmology Telescope Polarimeter (ACTPol) to construct the first wide-area (≈2100 sq. deg.), arcminute-resolution component-separated maps of the CMB temperature anisotropy and the thermal Sunyaev-Zel'dovich (tSZ) effect sourced by the inverse-Compton scattering of CMB photons off hot, ionized gas. Our ILC pipeline allows for explicit deprojection of various contaminating signals, including a modified blackbody approximation of the cosmic infrared background (CIB) spectral energy distribution. The cleaned CMB maps will be a useful resource for CMB lensing reconstruction, kinematic SZ cross-correlations, and primordial non-Gaussianity studies. The tSZ maps will be used to study the pressure profiles of galaxies, groups, and clusters through cross-correlations with halo catalogs, with dust contamination controlled via CIB deprojection. The data products described in this paper are available on LAMBDA.

We present the temperature and polarization angular power spectra measured by the Atacama Cosmology Telescope Polarimeter (ACTPol). We analyze night-time data collected during 2013-14 using two detector arrays at 149 GHz, from 548 deg2 of sky on the celestial equator. We use these spectra, and the spectra measured with the MBAC camera on ACT from 2008-10, in combination with planck and wmap data to estimate cosmological parameters from the temperature, polarization, and temperature-polarization cross-correlations. We find the new ACTPol data to be consistent with the ΛCDM model. The ACTPol temperature-polarization cross-spectrum now provides stronger constraints on multiple parameters than the ACTPol temperature spectrum, including the baryon density, the acoustic peak angular scale, and the derived Hubble constant. The new ACTPol data provide information on damping tail parameters. The joint uncertainty on the number of neutrino species and the primordial helium fraction is reduced by 20% when adding ACTPol to Planck temperature data alone.

We use microwave temperature maps from two seasons of data from the Atacama Cosmology Telescope at 146 GHz, together with the "Constant Mass" CMASS galaxy sample from the Baryon Oscillation Spectroscopic Survey to measure the kinematic Sunyaev-Zel'dovich (kSZ) effect over the redshift range z=0.4-0.7. We use galaxy positions and the continuity equation to obtain a reconstruction of the line-of-sight velocity field. We stack the microwave temperature at the location of each halo, weighted by the corresponding reconstructed velocity. We vary the size of the aperture photometry filter used, thus probing the free electron profile of these halos from within the virial radius out to three virial radii, on the scales relevant for investigating the missing baryons problem. The resulting best fit kSZ model is preferred over the no-kSZ hypothesis at 3.3 and 2.9σ for two independent velocity reconstruction methods, using 25,537 galaxies over 660 square degrees. The data suggest that the baryon profile is shallower than the dark matter in the inner regions of the halos probed here, potentially due to energy injection from active galactic nucleus or supernovae. Thus, by constraining the gas profile on a wide range of scales, this technique will be useful for understanding the role of feedback in galaxy groups and clusters. The effect of foregrounds that are uncorrelated with the galaxy velocities is expected to be well below our signal, and residual thermal Sunyaev-Zel'dovich contamination is controlled by masking the most massive clusters. Finally, we discuss the systematics involved in converting our measurement of the kSZ amplitude into the mean free electron fraction of the halos in our sample.