Free-Free Emission and H\(\alpha\) Radiation: Investigating the Physical Scaling Law

Abstract

We study the cosmic microwave background (CMB) because its anisotropies contain information about the birth, contents, and evolution of the universe. In order to extract information from the CMB, we need accurate models of the galactic foreground. The four known components of the galactic foreground are synchrotron radiation, thermal dust, spinning dust, and free-free emission. Free-free emission is known to be correlated with H\(\alpha\) radiation because both processes occur in regions of ionized hydrogen gas called the Warm Ionized Medium (WIM) and are directly correlated with the density of these regions. The scaling of free-free emission with H\(\alpha\) radiation depends on the temperature of the WIM. In this paper we investigate the physical basis for the scaling law between free-free emission and H\(\alpha\) radiation. We use this scaling law as a lens to examine existing estimates of free-free emission and to determine what these data suggest about the temperature of the WIM. We find that for approximately 80 percent of the sky, where extinction from dust is minimal (t < 1), H\(\alpha\) radiation and free-free emission follow the theoretically expected scaling law for temperatures that are within the known range of WIM temperatures.