Cosmic radio waves originate in interstellar gas by three distinct mechanisms:
- Line emission (from hydrogen).
- Thermal emission by free-free electron transitions.
- Non-thermal process believed to be synchrotron emission.
Observations, particularly of the line emission, have provided a great extension of knowledge of the distribution and motion of interstellar gas in galaxies. This includes data on rotation leading to estimates of mass distribution, on spiral structure, on the shape of the galactic disk, and on the relative proportions of gas and stars in different parts of our galaxy and in different galaxies.
The synchrotron emission hypothesis implies the existence, in places where non-thermal cosmic radio waves originate, of interstellar magnetic fields and high energy electrons. Two cases of special interest are the remnants of supernovae, which are strong radio sources and may be the primary sources of cosmic rays, and galactic coronas, vast emitting regions which have been found to extend far beyond the stars in our own and some external galaxies.
The discovery of radio galaxies has provided a fresh approach to cosmology. They appear to be observable far beyond current optical limits and, although no definite cosmological information has yet been established from radio evidence, it is probable that significant results may be obtained in the not distant future.
So do you think cosmic radiowaves are messages from other species in other galaxies that we are yet to decipher, or are they simply naive waves of radiation washing across the cosmos with as much of a message as the waves lapping on the shore?