Circular Polarization of the CMB: A probe of the First stars

Abstract: While it is revealed that the Cosmic Microwave Background (CMB) is linearly polarized at 10 % level, it is predicted that there exists no significant intrinsic source for circular polarization (CP) in the standard cosmology. However, during the propagation through a magnetised plasma, the CP of the CMB could be produced via the Faraday conversion (FC). The FC converts a pre-existing linear polarization into CP in presence of a magnetic field with relativistic electrons. In this paper, we focus on the FC due to supernova remnants of the first stars, also called Pop III stars. We derive an analytic form for the angular power spectrum of the CP of the CMB generated by the general FC. We apply this result to the case of the FC triggered by explosions of the first stars and evaluate the angular power spectrum, CV V . We show that the amplitude of l(l + 1)C_l^V V /(2pi) > 10^-2 micro Kelvin squared for l > 100, with only one Pop III star per halo, the age of Pop III SN remnants as 104 years and frequency of CMB observation as 1 GHz. We expect the CP of the CMB to be a very promising probe of the yet unobserved first stars, primarily due to the expected high signal along with an unique frequency dependence.