1998, PASP 110: 493-532
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THE COMPACT STEEP-SPECTRUM AND GIGAHERTZ PEAKED-SPECTRUM RADIO
SOURCES
Christopher P. O'Dea
Space Telescope Science
Institute, 3700 San Martin Dr., Baltimore, MD 21218
Electronic mail: odea@stsci.edu
Abstract. I review the radio to X-ray
properties of gigahertz peaked-spectrum (GPS) and compact steep-spectrum
(CSS) sources, the current hypotheses for their origin, and their use to
constrain the evolution of powerful radio galaxies. The GPS and CSS
sources are compact, powerful radio sources with well-defined peaks in
their radio spectra (near 1 GHz in the GPS and near 100 MHz in the
CSS). The GPS sources are entirely contained within the extent of the
narrow-line region
(
1 kpc), while
the CSS sources are contained entirely within the host galaxy
( 15 kpc). The
peaks in the spectra are probably due to synchrotron self-absorption,
though free-free absorption through an inhomogeneous screen may also
play a role. The turnover frequency varies with linear size l as
m
l-0.65, suggesting a simple physical relationship
between these parameters. The radio morphologies are strikingly like
those of the large-scale classical doubles, though some sources can have
very distorted morphologies suggestive of interactions. Radio
polarization tends to be low, and in some cases the Faraday rotation
measures can be extremely large. The IR properties are consistent with
stellar populations and active galactic nucleus (AGN) bolometric
luminosity similar to that of the 3CR classical doubles. The optical
host galaxy properties (absolute magnitude, Hubble diagram, evidence for
interaction) are consistent with those of the 3CR classical doubles. CSS
sources at all redshifts exhibit high surface brightness optical light
(most likely emission-line gas) that is aligned with the radio axis. The
optical emission-line properties suggest (1) interaction of the radio
source with the emission-line gas and (2) the presence of dust toward
the emission-line regions. X-ray observations of high-redshift GPS
quasars and a couple of GPS galaxies suggest the presence of significant
columns of gas toward the nuclei. Searches for cold gas in the host
galaxies have revealed large amounts of molecular gas and smaller
amounts of atomic gas in several sources, though probably not enough to
confine the radio sources. The main competing models for the GPS and CSS
sources are that (1) they are frustrated by interaction with dense gas
in their environments and (2) they are young and evolving radio sources
that will become large-scale sources. Combining the bright GPS and CSS
samples with the 3CR results in a sample spanning a range in source size
of 105 that can be used to study source evolution. The number
density versus linear size relation is consistent with a picture in
which the sources expand with constant velocity and the radio power
drops with linear size l according to P
l-0.5. This strong evolution suggests that at least
some of the GPS and CSS sources evolve to become lower luminosity FR 1
radio sources. The GPS and CSS sources are important probes of their
host galaxies and will provide critical clues to the origin and
evolution of powerful radio sources.
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