Probing cosmic velocities with the pairwise kinematic Sunyaev-Zel'dovich signal in DESI Bright Galaxy Sample DR1 and ACT DR6

TL;DR

This paper measures the pairwise kinematic SZ signal using DESI BGS DR1 galaxies overlapping ACT DR6 CMB maps to probe the low-redshift velocity field. The authors implement compensated aperture photometry and calibrate the mean halo optical depth with CMB lensing and hydrodynamical priors (SIMBA), enabling a cosmological interpretation despite a degeneracy with $\tau$. They report a high-significance detection, up to $\sim5\sigma$ for certain mass and aperture choices, and translate the measurement into a constraint on the combination $H_0 f\sigma_8^2$, while highlighting the need for external priors to break degeneracies. Looking ahead, DESI DR2 is expected to roughly double the signal-to-noise, providing a powerful platform for tests of modified gravity via low-redshift velocity fields.

Abstract

We present a measurement of the pairwise kinematic Sunyaev-Zel'dovich (kSZ) signal using the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Sample (BGS) Data Release 1 (DR1) galaxy sample overlapping with the Atacama Cosmology Telescope (ACT) CMB temperature map. Our analysis makes use of $1.6$ million galaxies with stellar masses $\log M_\star/M_\odot > 10$, and we explore measurements across a range of aperture sizes ($2.1' < θ_{\rm ap} < 3.5'$) and stellar mass selections. This statistic directly probes the velocity field of the large-scale structure, a unique observable of cosmic dynamics and modified gravity. In particular, at low redshifts, this quantity is especially interesting, as deviations from General Relativity are expected to be largest. Notably, our result represents the highest-significance low-redshift ($z \sim 0.3$) detection of the kSZ pairwise effect yet. In our most optimal configuration ($θ_{\rm ap} = 3.3'$, $\log M_\star > 11$), we achieve a $5σ$ detection. Assuming that an estimate of the optical depth and galaxy bias of the sample exists via e.g., external observables, this measurement constrains the fundamental cosmological combination $H_0 f σ_8^2$. A key challenge is the degeneracy with the galaxy optical depth. We address this by combining CMB lensing, which allows us to infer the halo mass and galaxy population properties, with hydrodynamical simulation estimates of the mean optical depth, $\bar τ$. We stress that this is a proof-of-concept analysis; with BGS DR2 data we expect to improve the statistical precision by roughly a factor of two, paving the way toward robust tests of modified gravity with kSZ-informed velocity-field measurements at low redshift.

Figures (11)

• Figure 1: Right ascension and declination distribution of BGS Y1 galaxies with $\log_{10}(M_\star / M_\odot) > 10$ in the ACT footprint. In total, the sample contains 1,610,381 galaxies. Future DESI data releases, such as Y3, are expected to increase the available number of galaxies in this footprint by roughly a factor of four, further improving the statistical precision of kSZ measurements.
• Figure 2: Top panel: Number of BGS galaxies, with a stellar mass threshold of $\log_{10}(M_\star / M_\odot) > 10$, in Y1 as a function of stellar mass in differential bins. The distribution peaks near $\log_{10}(M_\star / M_\odot) \approx 10.75$, reflecting the stellar-mass completeness of the sample at the median survey redshift. Bottom panel: Number of BGS galaxies per redshift bin for six cumulative stellar-mass thresholds. Higher-mass samples trace progressively higher-redshift populations, with the mean redshift increasing from $\bar{z} \approx 0.25$ at $\log_{10}(M_\star / M_\odot) > 10$ to $\bar{z} \approx 0.36$ at $\log_{10}(M_\star / M_\odot) > 11.25$. These trends highlight the trade-off between number density and redshift reach when selecting stellar-mass subsamples. The number of galaxies in each mass bin is given in Table \ref{['tab:mass']}.
• Figure 3: Theoretical prediction for the pairwise velocity correlation function, evaluated using the colossus package Diemer18 with the Planck 2018 cosmology. The calculation is performed in linear theory, and thus is only expected to be accurate on intermediate and large scales ($r \gtrsim 30$--$40 \, \mathrm{Mpc}$). On smaller scales, the correlation is additionally suppressed by the thermal motions of galaxies, which smear structure along the line of sight. This effect can be approximated by a damping factor $1 - \exp[-r^2/(2\sigma_r^2)]$, with $\sigma_r \lesssim 10 \, \mathrm{Mpc}$, corresponding to the redshift-space distortion (RSD) smoothing scale. In comparison to the massive clusters studied in Ref. 2016MNRAS.461.3172S, where nonlinearities and higher velocity dispersions are more significant, linear theory provides a better description for the less massive galaxy populations studied here. Throughout this work, we therefore focus on scales $r > 30 \, \mathrm{Mpc}$ where the theory is expected to be robust.
• Figure 4: Measured $\kappa(\theta)$ profiles (points) and best-fit theory curves (lines) for the six BGS stellar-mass threshold samples. The amplitude of $\kappa(\theta)$ increases with stellar-mass threshold, consistent with the expectation that more massive galaxies reside in more massive halos with denser matter profiles. The theory curve is obtained using an emulator built on the five standard HOD parameters (see Section \ref{['sec:bias']} for more details).
• Figure 5: Correlation matrix of the pairwise estimator for the fiducial choice $\theta_{\rm ap} = 2.7'$ and $\log_{10} M_\star/M_\odot > 11$, obtained from 1000 bootstrap realizations of the temperature decrements with fixed positions. The dominant noise source is the CMB temperature map. Off-diagonal correlations increase at large separations, as expected from sample variance and survey geometry, while small scales remain nearly uncorrelated due to Poisson statistics. The condition number is $\sim 30$, indicating a well-behaved, invertible matrix.