Reading the Radio Rainbow
While optical astronomers classify stars by color and absorption lines, radio astronomers characterize sources by their spectral index — how flux density changes with frequency. This single parameter reveals the emission mechanism: synchrotron radiation from relativistic electrons, thermal bremsstrahlung from ionized gas, or a combination. Plotting flux vs. frequency on logarithmic axes turns power-law spectra into straight lines whose slope is the spectral index α.
Synchrotron Emission
The dominant radio emission mechanism in the extragalactic sky is synchrotron radiation, produced by relativistic electrons spiraling in magnetic fields. The resulting spectrum follows S ∝ ν^α with α typically around -0.7. The spectrum steepens at high frequencies as electrons radiate away their energy (synchrotron aging), providing a clock for the age of radio plasma in galaxy lobes and supernova remnants.
Spectral Turnovers
At sufficiently low frequencies, synchrotron sources become opaque to their own radiation — synchrotron self-absorption causes the spectrum to turn over with S ∝ ν^(5/2). The turnover frequency depends on the source size and magnetic field: more compact sources turn over at higher frequencies. Gigahertz-peaked spectrum (GPS) sources are thought to be young radio galaxies whose jets are still confined within the host galaxy.
From Surveys to Science
Large radio surveys at multiple frequencies — from LOFAR at 150 MHz through VLA at 1.4 GHz to ALMA at 300 GHz — measure spectral indices for millions of sources. These classifications enable population studies, identify rare objects like high-redshift radio galaxies, and separate AGN from star-forming galaxies. The forthcoming SKA will survey the radio spectrum with unprecedented sensitivity and angular resolution.