Constraining the baryon abundance with the kinematic Sunyaev-Zel'dovich effect: Projected-field detection using P l a n c k , W M A P , and u n W I S E

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 u n W I S E 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 P l a n c k and W M A P data. Using a new "asymmetric" estimator that combines different foreground-cleaned CMB maps to maximize the signal-to-noise, we measure the kSZ² -galaxy cross-power spectrum for three subsamples of the u n W I S E galaxy catalog. These subsamples peak at mean redshifts z ≈0.6 , 1.1, and 1.5, have average halo mass ∼1 - 5 ×10¹³ h^-1M_⊙ , 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 σ . Our theoretical model includes the first calculation of lensing magnification contributions to the kSZ² -galaxy cross-power spectrum, which are significant for the z ≈1.1 and 1.5 subsamples. We discuss possible explanations for the excess kSZ signal associated with the z ≈1.1 sample, and show that foreground contamination in the CMB maps is very unlikely to be the cause. From our 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 ≈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.