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

Abstract

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 $z\ensuremath{\approx}0.6$, 1.1, and 1.5, have average halo mass $\ensuremath{\sim}1--5\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ }{h}^{\ensuremath{-}1}{M}_{\ensuremath{\bigodot}}$, and in total contain over 500 million galaxies. After marginalizing over contributions from CMB lensing, we measure the amplitude of the kSZ signal ${A}_{{\mathrm{kSZ}}^{2}}=0.42\ifmmode\pm\else\textpm\fi{}0.31(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{sys})$, $5.02\ifmmode\pm\else\textpm\fi{}1.01(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.49(\mathrm{sys})$, and $8.23\ifmmode\pm\else\textpm\fi{}3.23(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.57(\mathrm{sys})$, 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 $5\ensuremath{\sigma}$. 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 $z\ensuremath{\approx}1.1$ and 1.5 subsamples. We discuss possible explanations for the excess kSZ signal associated with the $z\ensuremath{\approx}1.1$ 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 $({f}_{b}/0.158)({f}_{\mathrm{free}}/1.0)=0.65\ifmmode\pm\else\textpm\fi{}0.24$, $2.24\ifmmode\pm\else\textpm\fi{}0.25$, and $2.87\ifmmode\pm\else\textpm\fi{}0.57$ at $z\ensuremath{\approx}0.6$, 1.1, and 1.5, respectively, consistent with a large fraction of the cosmic baryon abundance existing in an ionized state at low redshifts.