The Simons Observatory: Galactic Science Goals and Forecasts

TL;DR

The Simons Observatory aims to advance Galactic astrophysics by providing precise measurements of dust, synchrotron, and molecular cloud polarization, complementing its cosmological research with detailed Galactic science forecasts.

Contribution

This work offers the first detailed quantitative forecasts for Galactic science parameters achievable with the Simons Observatory, expanding its scientific scope beyond cosmology.

Findings

Constrain polarized dust spectral index to Δβ_d ≤ 0.01.

Improve measurement of dust-synchrotron correlation by a factor of two.

Map over 850 molecular clouds with high-resolution polarization data.

Abstract

Observing in six frequency bands from 27 to 280 GHz over a large sky area, the Simons Observatory (SO) is poised to address many questions in Galactic astrophysics in addition to its principal cosmological goals. In this work, we provide quantitative forecasts on astrophysical parameters of interest for a range of Galactic science cases. We find that SO can: constrain the frequency spectrum of polarized dust emission at a level of $\Delta\beta_d \lesssim 0.01$ and thus test models of dust composition that predict that $\beta_d$ in polarization differs from that measured in total intensity; measure the correlation coefficient between polarized dust and synchrotron emission with a factor of two greater precision than current constraints; exclude the non-existence of exo-Oort clouds at roughly 2.9$\sigma$ if the true fraction is similar to the detection rate of giant planets; map more than 850 molecular clouds with at least 50 independent polarization measurements at 1 pc resolution; detect or place upper limits on the polarization fractions of CO(2-1) emission and anomalous microwave emission at the 0.1% level in select regions; and measure the correlation coefficient between optical starlight polarization and microwave polarized dust emission in $1^\circ$ patches for all lines of sight with $N_{\rm H} \gtrsim 2\times10^{20}$ cm$^{-2}$. The goals and forecasts outlined here provide a roadmap for other microwave polarization experiments to expand their scientific scope via Milky Way astrophysics.