Polarization fraction of Planck Galactic cold clumps and forecasts for the Simons Observatory

Abstract

We measure the polarization fraction of a sample of $6282$ Galactic cold clumps at $353\mathrm{GHz}$, consisting of $Planck$ Galactic cold clump (PGCC) catalogue category 1 objects (flux densities measured with signal-to-noise ratio $(S/N)>4$). We find the mean-squared polarization fraction at $353\mathrm{GHz}$ to be $⟨{\Pi }^2⟩=[4.79\pm 0.44]\times 10^{-4}$ equating to an $11\sigma$ detection of polarization. We test if the polarization fraction depends on the clumps' physical properties, including flux density, luminosity, Galactic latitude and physical distance. We see a trend towards increasing polarization fraction with increasing Galactic latitude, but find no evidence that polarization depends on the other tested properties. The Simons Observatory, with an angular resolution of order $1\mathrm{arcmin}$ and noise levels between $22$ and $54$ $\mu \mathrm{K}-\mathrm{arcmin}$ at high frequencies, will substantially enhance our ability to determine the magnetic field structure in Galactic cold clumps. At $\ge 5\sigma$ significance, we predict the Simons Observatory will detect at least $\sim 12,000$ cold clumps in intensity and $\sim 430$ cold clumps in polarization. This number of polarization detections would represent a two orders of magnitude increase over the current $Planck$ results. We also release software that can be used to mask these Galactic cold clumps in other analyses.

arXiv comment12 pages, 12 figures, accepted by MNRAS; section 5 number counts revised and improved

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