A Millimeter-Wave Galactic Plane Survey With The BICEP Polarimeter

TL;DR

This paper presents millimeter-wave polarization maps of the Galactic plane from BICEP, revealing magnetic field orientations, polarization trends with frequency, and constraining Galactic dust emission models, enhancing understanding of the Milky Way's magnetic structure.

Contribution

First detailed polarization spectrum of the Galactic plane from 23 to 220 GHz combining BICEP and WMAP data, with insights into magnetic field orientation and dust emission.

Findings

Polarized emission detected across the Galactic plane within 2 degrees of the plane.

The large-scale magnetic field is aligned parallel to the Galactic plane.

Polarization fraction increases with frequency from below 50 GHz to above 90 GHz.

Abstract

In addition to its potential to probe the Inflationary cosmological paradigm, millimeter-wave polarimetry is a powerful tool for studying the Milky Way galaxy's composition and magnetic field structure. Towards this end, presented here are Stokes I, Q, and U maps of the Galactic plane from the millimeter-wave polarimeter BICEP covering the Galactic longitude range 260 - 340 degrees in three atmospheric transmission windows centered on 100, 150, and 220 GHz. The maps sample an optical depth 1 < AV < 30, and are consistent with previous characterizations of the Galactic millimeter-wave frequency spectrum and the large-scale magnetic field structure permeating the interstellar medium. Polarized emission is detected over the entire region within two degrees of the Galactic plane and indicates that the large-scale magnetic field is oriented parallel to the plane of the Galaxy. An observed trend of decreasing polarization fraction with increasing total intensity rules out the simplest model of a constant Galactic magnetic field throughout the Galaxy. Including WMAP data in the analysis, the degree-scale frequency spectrum of Galactic polarization fraction is plotted between 23 and 220 GHz for the first time. A generally increasing trend of polarization fraction with electromagnetic frequency is found, which varies from 0.5%-1.5%at frequencies below 50 GHz to 2.5%-3.5%above 90 GHz. The BICEP and WMAP data are fit to a two-component (synchrotron and dust) model showing that the higher frequency BICEP data are necessary to tightly constrain the amplitude and spectral index of Galactic dust. Furthermore, the dust amplitude predicted by this two-component fit is consistent with model predictions of dust emission in the BICEP bands.