Constraining the baryon abundance with the kinematic Sunyaev-Zel'dovich effect: Projected-field detection using Planck, WMAP, and unWISE

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 kSZ(2)-galaxy cross-power spectrum for three subsamples of the unWISE galaxy catalog. These subsamples peak at mean redshifts z approximate to 0.6, 1.1, and 1.5, have average halo mass similar to 1-5 x 10(13) h(-1)M(circle dot), and in total contain over 500 million galaxies. After marginalizing over contributions from CMB lensing, we measure the amplitude of the kSZ signal A(kSZ2) = 0.42 +/- 0.31(stat) +/- 0.02(sys), 5.02 +/- 1.01(stat) +/- 0.49(sys), and 8.23 +/- 3.23(stat) +/- 0.57(sys), for the three subsamples, where A(kSZ2) = 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 kSZ(2)-galaxy cross-power spectrum, which are significant for the z approximate to 1.1 and 1.5 subsamples. We discuss possible explanations for the excess kSZ signal associated with the z approximate to 1.1 sample, and show that foreground contamination in the CMB maps is very unlikely to be the cause. Fromour measurements of A(kSZ2), we constrain the product of the baryon fraction f(b) and free electron fraction f(free) to be (f(b)/0.158)(f(free)/1.0) = 0.65 +/- 0.24, 2.24 +/- 0.25, and 2.87 +/- 0.57 at z approximate to 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.

Automated summary

In this study, the kinematic Sunyaev-Zel'dovich (kSZ) effect was utilized to measure the distribution of baryons in the Universe using infrared-selected galaxies from the unWISE catalog. The results show the importance of detecting baryon distribution at different redshifts in the Universe, with the statistical significance of the measured signal exceeding 5 sigma.