Detecting Patchy Reionization in the Cosmic Microwave Background

Upcoming cosmic microwave background (CMB) experiments will measure temperature fluctuations on small angular scales with unprecedented precision. Small-scale CMB fluctuations are a mixture of late-time effects: gravitational lensing, Doppler shifting of CMB photons by moving electrons [the kinematic Sunyaev-Zel’dovich (KSZ) effect], and residual foregrounds. We propose a new statistic which separates the KSZ signal from the others, and also allows the KSZ signal to be decomposed in redshift bins. The decomposition extends to high redshift and does not require external data sets such as galaxy surveys. In particular, the high-redshift signal from patchy reionization can be cleanly isolated, enabling future CMB experiments to make high-significance and qualitatively new measurements of the reionization era.

Figure 1

Top panel: Solid lines show contributions to $C_L^{KK}$ from KSZ and reconstruction noise, assuming $2\mu \mathrm{K}\mathrm{arcmin}$ noise and ${\theta }_{\mathrm{FWHM}}=1^{\prime }$. Dashed lines show estimated non-Gaussian contribution to $C_L^{KK}$ from large-scale clustering of small-scale modes, computed using a model (“$\delta$ modulation”) described in the text. Middle panel: Comparison between our model for $C_L^{KK}$ and 3D large-scale structure simulations described in the text. The KSZ maps were filtered to $3000\le l\le 5000$ with no instrumental noise. Bottom panel: Lensing contribution to $C_L^{KK}$ from simulations, before and after template subtraction.

Figure 2

Forecasted $S/N$ for one-bin, two-bin, and three-bin detections as defined in the text, for varying noise level and beam size ${\theta }_{\mathrm{FWHM}}=1^{\prime }$ [blue (upper) curves] or ${\theta }_{\mathrm{FWHM}}=3^{\prime }$ [red (lower) curves].