Compact steep-spectrum and peaked-spectrum radio sources
  Compact steep-spectrum (CSS) and peaked spectrum (PS) radio sources are
compact, powerful radio sources. The multi-frequency observational properties
and current theories are reviewed with emphasis on developments since the
earlier review of O'Dea (1998). There are three main hypotheses for the nature
of PS and CSS sources. (1) The PS sources might be very young radio galaxies
which will evolve into CSS sources on their way to becoming large radio
galaxies. (2) The PS and CSS sources might be compact because they are confined
(and enhanced in radio power) by interaction with dense gas in their
environments. (3) Alternately, the PS sources might be transient or
intermittent sources. Each of these hypotheses may apply to individual objects.
The relative number in each population will have significant implications for
the radio galaxy paradigm. Proper motion studies over long time baselines have
helped determine hotspot speeds for over three dozen sources and establish that
these are young objects. Multifrequency polarization observations have
demonstrated that many CSS/PS sources are embedded in a dense interstellar
medium and vigorously interacting with it. The detection of emission line gas
aligned with the radio source, and blue-shifted HI absorption and [OIII]
emission lines indicates that AGN feedback is present in these objects --
possibly driven by the radio source. CSS/PS sources with evidence of episodic
AGN over a large range of time-scales have been discussed. The review closes
with a discussion of open questions and prospects for the future.
