The Radio Sky: Components

Introduction -- Components -- Visualization -- Links

Different physical components make the sky brightness change as you look with light of different wavelengths.

Low Frequencies

At low frequencies (long wavelengths), the dominant emission mechanism is synchrotron radiation: radio emission from electrons spiraling in the Galaxy's magnetic field. Synchrotron emits broadband radiation; it isn't concentrated at one frequency. The spectrum resembles a power law distribution, in which the flux falls off towards higher frequencies. The Haslam 408 MHz map is dominated by synchrotron.

At 1.4 GHz, however, we can see another important Galactic component: neutral Hydrogen. This atom emits radiation at a wavelength of 21 cm. The LAB HI map is dominated by hydrogen.

Middle Frequencies

As we move higher up in frequency, the Cosmic Microwave Background becomes dominant. This extragalactic emission is light left over from the Big Bang, which has cooled off since it was emitted almost 14 billion years ago. Its spectrum matches that of a blackbody with a temperature of 2.73 Kelvin. The Planck 30 GHz map is dominated by CMB, especially out of the Galactic plane.

High Frequencies

At higher frequencies (hundreds of gigahertz), we move into the "submillimeter" regime. Here, large-scale dust emission becomes very important. This radiation is thermal emission from cool dust (around 20 K) in the Galaxy. Dust has a very strong contrast ratio over the sky, because dust is mostly located in the thin disk (compare this to synchrotron, in which the relativistic electrons extend into the Galactic halo). The Planck 353 GHz map shows significant dust emission.

A schematic of the synchrotron radiation mechanism.

The CMB blackbody spectrum.

This website was created for Harvard's Astronomy 201b class: Interstellar Medium and Star Formation.