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GPS (Gigahertz Peaked-Spectrum) galaxies and quasars are powerful
radio sources with a convex radio spectrum that peaks between 0.5 and several
tens of GHz (observer's frame). GPS objects are characterized by a small size
(less than 1 kpc) and low variability amplitude.
Because of their small angular sizes the structure of these sources can be
resolved only partially even in VLBI observations (figure 1).
Figure 1: left - generic synchrotron spectrum;
right - a classical GPS source's example - galaxy J1408+28.
For a detailed description of the properties of GPS objects see
(Ch. P. O'Dea, S. A. Baum, and C. Stanghellini, Astrophys.
J. 380, 66, 1991).
Most of our knowledge about GPS objects was obtained statistically.
The samples studied often include quasars with a peak in the radio
spectrum (hereafter referred to as FSRQ - Flat Spectrum Radio Quasars).
The conclusions so far obtained are influenced by selection effects due to
natural restrictions on increasing the sample size simultaneously
with expanding the frequency interval.
Analysis of the nature of GPS sources are further complicated by the lack
of homogeneous samples covering a wide frequency range and allowing their
intrinsic properties to be determined.
GPS galaxies are characterized by low redshifts
(0.1  z
1) and they have a rather low
radio variability compared to the GPS quasars
(figure 2).
Furthermore, GPS galaxies have spectra peaking at lower frequencies
and it is more common for them to have a symmetric structure when observed
with VLBI. Quasars of this type usually exhibit a more
complex or a core-jet type structure
(C. Stanghellini, D. Dallacasa, Ch. P. O'Dea, et al.,
Astronom. and Astrophys. 377, 377, 2001).
Figure 2. The redshift - peak frequency - radio-spectrum width
(z - 
intrinsic - FWHM) relation for all the objects of the
sample whose spectra peak at radio frequencies.
The peak frequency is in the rest frame of the source.
This relation is characteristic
of the distribution of compact objects with relatively
uniform synchrotron emission. The relation covers all
objects (467), including those caught at the time of
activity, when radio emission from a compact nuclear
region dominates, and the radio spectrum becomes
temporarily convex.
It can be clearly seen in the figure
that the radio sources with the narrowest spectra
(FWHM 1.1)
are located at the redshift domains z < 1 and z > 3.
The currently favored view is that the galaxy-type
GPS sources are intrinsically small due to their young
age. These objects are believed to be the progenitors of extended
radio sources, as corroborated by kinematic and spectral studies,
which yield ages on the order of 103-105 yr.
Another scenario explains the small linear sizes of GPS galaxies by
their dense environment
(S. A. Baum, Ch. P. O'Dea, D. W. Murphy, and
A. G. de Bruyn, Astronom. and Astrophys. 232, 19, 1990):
such sources are not young, but remain small because of the external
pressure preventing their expansion. Small linear sizes of
GPS quasars are sometimes explained by projection effects
(C. Stanghellini, Publ. Astronom. Soc. Australia 20,
118, 2003).
The results of simultaneous measurements in many parts of the electromagnetic
spectrum are used extensively in recent studies. This brought about
the emergence of new techniques and approaches to AGN studies.
Bai and Lee (J. M. Bai and M. G. Lee, Jr. Korean Astron. Soc. 38,
125, 2005) suggested, based on X-ray measurements, that GPS quasars are
blazars in a dense gas and dust environment. That is why, although their
jets are oriented at a small angle to the line of sight, these sources
do not exhibit blazar properties (flat radio spectrum and radioemission
variations with amplitudes up to several tens of percent). The nature of GPS
quasars is not yet entirely understood.
Most of the conclusions and hypotheses were based on small samples, detailed
studies of several objects, or samples contaminated by objects of other
types because of classification errors.
Studies of GPS objects usually involve the analysis of the following
parameters of their radio spectra:
- peak frequency in the observer's
(obs) or rest
(intrinsic)
frame;
- spectral indices below
(
 below) and above
(above)
the peak frequency, which characterize the optically thick and thin
emission region, respectively, and
- the full width at half maximum FWHM of the fitted spectra
in frequency decades.
In some cases constraints are imposed on the peak frequency for GPS
objects:
0.5
obs
10 GHz.
Objects with peak frequencies below 0.5 or above 10 GHz belong to the CSS
(Compact Steep Spectrum) and HFP (High-Frequency Peakers)
classes (D. Dallacasa, C. Stanghellini, M. Centonza, and
R. Fanti, Astronom. and Astrophys. 363, 887, 2000),
respectively. Researchers often use the notion of a "classical" or
"canonical" spectrum of a GPS object
(W. H. de Vries, P. D. Barthel, and Ch. P. O'Dea,
Astronom. and Astrophys. 321, 105, 1997).
These are spectra whose form is most similar to that of the theoretical
spectra corresponding to synchrotron emission of a homogeneous
object with self-absorption at low frequencies
(K. I. Kellermann and I. I. Pauliny-Toth, Annu. Rev.
Astronom. Astrophys. 19, 373, 1981).
For example, Vries et al. adopt as the "canonical" GPS spectrum the radio
spectrum with
(below) and
(above)
equal to +0.5 and -0.7, respectively.
O'Dea et al. consider the spectral width FWHM
1.2 to be one of the
parameters of the "classical" GPS.
We found differences between the spectral properties
of GPS galaxies and quasars. Their common
feature is a rather simple form of the radio spectrum.
Unlike other compact extragalactic radio sources, they have no close-to-zero
spectral indices. Spectral parts above and below the peak frequency for GPS
sources are typically steep.
There are statistically significant differences in the
average values of
above
and FWHM between the subgroups galaxies and quasars of our sample.
This result confirms the average spectral parameters found
earlier based on the monitoring of a limited list of GPS
sources at the RATAN-600 radio telescope.
In 2006-2011 systematic monitoring of 122 candidate GPS was carried
out at the RATAN-600 radio telescope
(Table 1).
As a result, simultaneous broadband
radio spectra (at 1.1, 2.3, 4.8, 7.7, 11.2, and 21.7 GHz) and
some preliminary results have been obtained.
Only 25% of the objects of this sample were found to have the properties
of a "classical" GPS. Highfrequency spectral indices and widths of radio
spectra differ statistically for GPS galaxies and quasars
(M. G. Mingaliev, Yu. V. Sotnikova, I. Torniainen, et
al., Astronom. and Astrophys. 544, A25, 2012).
It is possible that different physical mechanisms or ambient conditions,
rather than only the jet position angle, play the crucial role in the
formation of the spectra of GPS galaxies and quasars.
We expanded the sample of GPS objects to perform a further comparison
with our results obtained in 2006-2011. To this end, we selected GPS
candidates from a flux-density complete sample
(S
 200 mJy
at 4.8 or 5 GHz) based on the CATS database
(O. V. Verkhodanov, S. A. Trushkin, and V. N. Chernenkov,
Baltic Astronomy 6, 275, 1997).
In total, the sample contains about 5000 sources.We selected 467 objects
with convex radio spectra, which we considered to be GPS candidates.
We adopt the spectral parameters for the GPS candidates observed at the
RATAN-600 during the 2006-2011 period from our previous paper
(M. G. Mingaliev, Yu. V. Sotnikova, I. Torniainen, et
al., Astronom. and Astrophys. 544, A25, 2012).
In this paper we report the results of a comprehensive study of a sample
of GPS candidates, which includes bright objects of both the
Northern and Southern hemisphere.
We compiled a new flux-density complete sample of GPS candidates
(112 objects) based on an analysis of the data obtained with RATAN-600
and the available catalogs (CATS). A comprehensive study of these objects
led us to the following conclusions:
We selected a total of 112 GPG candidates among the 467 objects with
peaked spectra, which makes up about 2% of the entire sample of objects with
fluxes
S
200 mJy at 4.8 or 5 GHz.
Only 45 of the selected GPS candidates strictly satisfy the criteria
for classical GPS. This amounts to about 1% of the entire sample,
which is significantly less than expected: GPS objects are believed to
constitute about one tenth of the bright extragalactic sources
(at centimeter-wave frequencies).
Our analysis of the parameters of the radio spectra revealed that GPS
galaxies have narrower spectra and higher high-frequency spectral indices
than quasars. The low-frequency spectral index increases with redshift
z and its values are comparable for the two types of objects.
The number of GPS galaxies in the sample decreases sharply with redshift,
starting with z = 1. Galaxies and quasars have comparable angular
sizes at the same z, whereas their luminosities may differ
by one order of magnitude.
We find a deficit of objects
with low peak frequencies (several GHz) at large redshifts.
It is possible that there are no objectswith large
synchrotron self-absorbing components at large z.
Various indirect estimates confirm the presence of
medium with high density of emitting particles in the
circumnuclear regions of GPS objects.
Within the framework of this study we formulated a number of specific
problems, which require a separate investigation of GPS galaxies and quasars
(the study of the physical conditions in circumnuclear regions, accretion
rates, etc.), as well as modelling of synchrotron emission mechanisms in
homogeneous objects with the given spectral parameters determined
in this paper.
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