1. Arecibo Observatory
The derivation of the small-scale structure in the cosmic microwave background (CMB) relies on an accurate subtraction of foreground signals from the Milky Way Galaxy. Known sources include thermal emission from interstellar cirrus, galactic synchrotron emission resulting from interactions between cosmic ray electrons and magnetic fields, and electron-ion free-free emission from interstellar H II regions. Additional sources include spinning and spinning-wobbling dust grains, and emission from rotational transitions of carbon monoxide. Verschuur (2015 and references therein) showed many examples of connections, associations, and overlaps of galactic HI and CMB structure. Clark et al. (2014) showed that the long, thin filamentary features seen in the high sensitivity, high dynamic range Galactic Arecibo L-Band Feed Array (GALFA) HI survey appear to be aligned along magnetic field directions, which are inferred from the optical polarization of star light. Clark et al. (2015) took this important discovery a step further, relating those magnetic field orientations to the polarized PLANCK 353 GHz dust emission. These results imply that the neutral hydrogen in the interstellar medium is tightly coupled to the galactic magnetic field, which requires a population of electrons. Taken together, these HI results suggest a candidate for a previously unidentified foreground component that may need to be understood in order to improve our ability to measure and interpret the CMB small-scale structure. This work is supported by NASA and NSF.