The kinematic Sunyaev-Zel’dovich (kSZ) effect—the Doppler boosting of Cosmic Microwave Background (CMB) photons scattering off free electrons with nonzero line-of-sight velocity—is an excellent probe of the distribution of baryons in the Universe. In this paper, we measure the kSZ effect due to ionized gas traced by infrared-selected galaxies from the $unWISE$ catalog. We employ the “projected-field” kSZ estimator, which does not require spectroscopic galaxy redshifts. To suppress contributions from non-kSZ signals associated with the galaxies (e.g., dust emission and thermal SZ), this estimator requires foreground-cleaned CMB maps, which we obtain from $Planck$ and $WMAP$ data. Using a new “asymmetric” estimator that combines different foreground-cleaned CMB maps to maximize the signal-to-noise, we measure the ${\mathrm{kSZ}}^2$-galaxy cross-power spectrum for three subsamples of the $unWISE$ galaxy catalog. These subsamples peak at mean redshifts $z\approx 0.6$, 1.1, and 1.5, have average halo mass $\sim 1–5\times {10}^{13}{h}^{-1}{M}_{\odot }$, and in total contain over 500 million galaxies. After marginalizing over contributions from CMB lensing, we measure the amplitude of the kSZ signal $A_{{\mathrm{kSZ}}^2}=0.42\pm 0.31(\mathrm{stat})\pm 0.02(\mathrm{sys})$, $5.02\pm 1.01(\mathrm{stat})\pm 0.49(\mathrm{sys})$, and $8.23\pm 3.23(\mathrm{stat})\pm 0.57(\mathrm{sys})$, for the three subsamples, where $A_{{\mathrm{kSZ}}^2}=1$ corresponds to our fiducial theoretical model. The combined statistical significance of our kSZ detection exceeds $5\sigma$. Our theoretical model includes the first calculation of lensing magnification contributions to the ${\mathrm{kSZ}}^2$-galaxy cross-power spectrum, which are significant for the $z\approx 1.1$ and 1.5 subsamples. We discuss possible explanations for the excess kSZ signal associated with the $z\approx 1.1$ sample, and show that foreground contamination in the CMB maps is very unlikely to be the cause. From our measurements of $A_{{\mathrm{kSZ}}^2}$, we constrain the product of the baryon fraction $f_b$ and free electron fraction $f_{\mathrm{free}}$ to be $(f_b/0.158)(f_{\mathrm{free}}/1.0)=0.65\pm 0.24$, $2.24\pm 0.25$, and $2.87\pm 0.57$ at $z\approx 0.6$, 1.1, and 1.5, respectively, consistent with a large fraction of the cosmic baryon abundance existing in an ionized state at low redshifts.

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• Cosmic microwave background
• Sky surveys
• Large scale structure of the Universe