Kinematic Sunyaev-Zel’dovich Effect with Projected Fields: A Novel Probe of the Baryon Distribution with Planck, WMAP, and WISE Data

The kinematic Sunyaev-Zel’dovich (KSZ) effect—the Doppler boosting of cosmic microwave background (CMB) photons due to Compton scattering off free electrons with nonzero bulk velocity—probes the abundance and the distribution of baryons in the Universe. All KSZ measurements to date have explicitly required spectroscopic redshifts. Here, we implement a novel estimator for the KSZ—large-scale structure cross-correlation based on projected fields: it does not require redshift estimates for individual objects, allowing KSZ measurements from large-scale imaging surveys. We apply this estimator to cleaned CMB temperature maps constructed from Planck and WMAP data and a galaxy sample from the Wide-field Infrared Survey Explorer (WISE). We measure the KSZ effect at $3.8\sigma –4.5\sigma$ significance, depending on the use of additional WISE galaxy bias constraints. We verify that our measurements are robust to possible dust emission from the WISE galaxies. Assuming the standard $\mathrm{\Lambda }$ cold dark matter cosmology, we directly constrain $(f_b/0.158)(f_{\text{free}}/1.0)=1.48\pm 0.19$ (statistical error only) at redshift $z\approx 0.4$, where $f_b$ is the fraction of matter in baryonic form and $f_{\text{free}}$ is the free electron fraction. This is the tightest KSZ-derived constraint reported to date on these parameters. Astronomers have long known that baryons do not trace dark matter on $\sim$ kiloparsec scales and there has been strong evidence that galaxies are baryon poor. The consistency between the $f_b$ value found here and the values inferred from analyses of the primordial CMB and big bang nucleosynthesis verifies that baryons approximately trace the dark matter distribution down to $\sim$ megaparsec scales. While our projected-field estimator is already competitive with other KSZ approaches when applied to current data sets (because we are able to use the full-sky WISE photometric survey), it will yield enormous signal-to-noise ratios when applied to upcoming high-resolution, multifrequency CMB surveys.

Figure 1

Cross-power spectrum of filtered, squared $T_{\text{clean}}$ map with the WISE galaxies (the green circles). The thick blue, dashed black, and thin red curves show the best-fit KSZ, CMB lensing, and $\mathrm{KSZ}+\text{lensing}$ power spectra, respectively. The KSZ signal is detected at $3.8\sigma$ significance. No external galaxy bias constraint is used in these fits; including a prior from cross-correlating the WISE galaxies with Planck CMB lensing maps produces nearly identical best-fit results (see Table 1), with a KSZ significance of $4\sigma –4.5\sigma$. For visual purposes only, the magenta squares show the residual excess left after subtracting the best-fit lensing template from the data points.

Figure 2

Dust null tests. (Top panel) Cross-correlation of $T_{\text{clean}}$ with the WISE galaxies. This verifies that any mean emission (e.g., dust, radio, or TSZ) of the galaxies is removed in $T_{\text{clean}}$. (Bottom panel) Cross-correlation of $(T_{\text{clean}}{T}_{\text{dust}})$ with the WISE galaxies. This further verifies that any WISE-galaxy-correlated dust emission in $T_{\text{clean}}$ is sufficiently removed. Rescaling $T_{\text{dust}}$ from 545 GHz to 100–217 GHz (a factor of $\approx 400–500$) yields a dust contribution to the data points in Fig. 1 of $\lesssim 0.003\mu {\mathrm{K}}^2$, well below the statistical errors.