A MILLIMETER-WAVE GALACTIC PLANE SURVEY WITH THE BICEP POLARIMETER

Abstract

In addition to its potential to probe the Inflationary cosmological paradigm,\nmillimeter-wave polarimetry is a powerful tool for studying the Milky Way\ngalaxy's composition and magnetic field structure. Towards this end, presented\nhere are Stokes I, Q, and U maps of the Galactic plane from the millimeter-wave\npolarimeter BICEP covering the Galactic longitude range 260 - 340 degrees in\nthree atmospheric transmission windows centered on 100, 150, and 220 GHz. The\nmaps sample an optical depth 1 < AV < 30, and are consistent with previous\ncharacterizations of the Galactic millimeter-wave frequency spectrum and the\nlarge-scale magnetic field structure permeating the interstellar medium.\nPolarized emission is detected over the entire region within two degrees of the\nGalactic plane and indicates that the large-scale magnetic field is oriented\nparallel to the plane of the Galaxy. An observed trend of decreasing\npolarization fraction with increasing total intensity rules out the simplest\nmodel of a constant Galactic magnetic field throughout the Galaxy. Including\nWMAP data in the analysis, the degree-scale frequency spectrum of Galactic\npolarization fraction is plotted between 23 and 220 GHz for the first time. A\ngenerally increasing trend of polarization fraction with electromagnetic\nfrequency is found, which varies from 0.5%-1.5%at frequencies below 50 GHz to\n2.5%-3.5%above 90 GHz. The BICEP and WMAP data are fit to a two-component\n(synchrotron and dust) model showing that the higher frequency BICEP data are\nnecessary to tightly constrain the amplitude and spectral index of Galactic\ndust. Furthermore, the dust amplitude predicted by this two-component fit is\nconsistent with model predictions of dust emission in the BICEP bands.\n