We live inside the Milky Way, and so whenever we observe distant galaxies or the CMB, we also see light emitted by our own Galaxy. At higher frequencies, we detect the heat radiated by tiny interstellar dust grains warmed by the ambient starlight. At lower frequencies, we observe “synchrotron” radiation, the energy radiated by relativistic electrons as they spiral in the magnetic fields of the Milky Way. While it may be a source of contamination for doing cosmology, this “foreground” light from our Galaxy is interesting in its own right for the information it contains on the dust, gas, and magnetic fields in the Milky Way. This material is the interstellar medium– the stuff between the stars. 

Giant clouds of gas and dust are the birthplaces of stars and their associated solar systems, but the process by which interstellar material becomes dense enough to collapse and form stars is still poorly understood. The role of magnetic fields in this process is a particularly tricky open question, because measuring the polarized light that lets us trace magnetic fields requires very sensitive telescopes. With the Simons Observatory, we will be able to map magnetic fields in unprecedented detail from cold, dense clouds to diffuse interstellar space. We can also study the physical mechanisms behind Galactic emission, answering such questions as: whether dust grains come in distinct compositional varieties or are more or less homogenous; how much the population of energetic cosmic-ray electrons changes depending on location in the Galaxy; and how interstellar magnetic fields influence the turbulent motions of interstellar gas.