Constraints on the Optical Depth to Reionization from Balloon-Borne CMB Measurements

Abstract

We assess the uncertainty with which a balloon-borne experiment, nominally called Tau Surveyor ($\ddot{I}„S$), can measure the optical depth to reionization $\ddot{I}ƒ(\ddot{I}„)$ with given realistic constraints of instrument noise and foreground emissions. Using a $\ddot{I}„S$ fiducial design with six frequency bands between 150 and 380 GHz with white and uniform map noise of 7 $\hat{I}¼$K arcmin, achievable with a single mid-latitude flight, and including Planck's 30 and 44 GHz data we assess the error $\ddot{I}ƒ(\ddot{I}„)$ obtained with three foreground models and as a function of sky fraction $f_{\mathrm{sky}}$ between 40% and 54%. We carry out the analysis using both parametric and blind foreground separation techniques. We compare $\ddot{I}ƒ(\ddot{I}„)$ values to those obtained with low frequency and high frequency versions of the experiment called $\ddot{I}„S$-lf and $\ddot{I}„S$-hf that have only four and up to eight frequency bands with narrower and wider frequency coverage, respectively. We find that with $\ddot{I}„S$ the lowest constraint is $\ddot{I}ƒ(\ddot{I}„)=0.0034$, obtained for one of the foreground models with $f_{\mathrm{sky}}$=54%. $\ddot{I}ƒ(\ddot{I}„)$ is larger, in some cases by more than a factor of 2, for smaller sky fractions, with $\ddot{I}„S$-lf, or as a function of foreground model. The $\ddot{I}„S$-hf configuration does not lead to significantly tighter constraints. Exclusion of the 30 and 44 GHz data, which give information about synchrotron emission, leads to significant $\ddot{I}„$ mis-estimates. Decreasing noise by an ambitious factor of 10 while keeping $f_{\mathrm{sky}}$=40% gives $\ddot{I}ƒ(\ddot{I}„)=0.0031$. The combination of $\ddot{I}ƒ(\ddot{I}„)=0.0034$, BAO data from DESI, and future CMB B-mode lensing data from CMB-S3/S4 experiments could give $\ddot{I}ƒ(\sum m_{\hat{I}}½)=17$ meV.

12 pages, 7 Figures, reflects version published in ApJ