and Cosmology

5.1 Introduction
The synchrotron spectrum is self-absorbed at low
frequencies, i.e., the optical depth for absorption due
to the synchrotron process is close to or larger than
unity. In this case, the spectrum becomes flatter and, for
small ν, it may even rise. In the limiting case of a high
optical depth for self-absorption, we obtain S ν ∝ ν 2.5 for
ν → 0. The extended radio components are optically
thin at cm wavelength, so that α ∼ 0.7, whereas the
compact core component is often optically thick and
thus self-absorbed, which yields α ∼ 0, or even inverted
so that α