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

Abstract

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 $\sim 10 \sigma$ 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 $2.1\sigma$ 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 $11\sigma$ 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.