Gigahertz Peaked Spectrum Galaxies Research Articles

Caltech-NRAO Stripe 82 Survey (CNSS). V. AGNs That Transitioned to Radio-loud State

Abstract A recent multiyear Caltech-NRAO Stripe 82 Survey revealed a group of objects that appeared as new radio sources after >5–20 yr of absence. They are transient phenomena with respect to the Faint Images of the Radio Sky at Twenty Centimeters survey and constitute the first unbiased sample of renewed radio activity. Here we present a follow-up, radio, optical, and X-ray study of them. The group consists of 12 sources, both quasars and galaxies with wide redshift (0.04 < z < 1.7) and luminosity ( ) distributions. Their radio properties in the first phase of activity, namely the convex spectra and compact morphology, allow them all to be classified as gigahertz-peaked spectrum (GPS) sources. We conclude that the spectral changes are a consequence of the evolution of newly born radio jets. Our observations show that over the next few years of activity the GPS galaxies keep the convex shape of the spectrum, while GPS quasars rapidly transform into flat-spectrum sources, which may result in them not being recognized as young sources. The wide range of bolometric luminosities, black hole masses, and jet powers among the transient sources indicates even greater population diversity in the group of young radio objects. We also suggest that small changes of the accretion disk luminosity (accretion rate) may be sufficient to ignite low-power radio activity that evolves on the scale of decades.

The multifrequency parsec-scale structure of PKS 2254−367 (IC 1459): a luminosity-dependent break in morphology for the precursors of radio galaxies?

We present the first multi-frequency VLBI images of PKS 2254-367, a Giga-hertz Peaked Spectrum (GPS) radio source hosted by the nearby galaxy IC 1459 (D=20.5 Mpc). PKS 2254-367 and the radio source in NGC 1052 (PKS 0238-084; D=17.2 Mpc) are the two closest GPS radio sources to us, far closer than the next closest example, PKS 1718-649 (D=59 Mpc). As such, IC 1459 and NGC 1052 offer opportunities to study the details of the pc-scale radio sources as well as the environments that the radio sources inhabit, across the electromagnetic spectrum. Given that some models for the origin and evolution of GPS radio sources require a strong connection between the radio source morphology and the gaseous nuclear environment, such opportunities for detailed study are important. Our VLBI images of PKS 2254-367 show that the previously identified similarities between IC 1459 and NGC 1052 continue onto the pc-scale. Both compact radio sources appear to have symmetric jets of approximately the same luminosity, much lower than typically noted in compact double GPS sources. Similarities between PKS 2254-367 and NGC 1052, and differences with respect to other GPS galaxies, lead us to speculate that a sub-class of GPS radio sources, with low luminosity and with jet-dominated morphologies, exists and would be largely absent from radio source surveys with ~1 Jy flux density cutoffs. We suggest that this possible low-luminosity, jet-dominated population of GPS sources could be an analog of the FR-I radio galaxies, with the higher luminosity lobe-dominated GPS sources being the analog of the FR-II radio galaxies.

Gigahertz-peaked spectrum (GPS) galaxies and quasars

The results of a comprehensive analysis of continuous radio spectra of a sample of Gigahertz-Peaked Spectrum (GPS) sources are reported. The sources are selected from a flux-density-complete sample (Sν ≥ 200 mJy at 4.8 or 5 GHz) using multifrequency measurements of the RATAN-600 radio telescope and data from the CATS astrophysical catalogs support system. The analysis revealed a very small number (1–2%) of “classical” GPS objects, which is significantly less than the expected fraction of 10%. GPS galaxies are found to have narrower and steeper radio spectra than quasars. The low-frequency part of the spectrum is seen to become steeper with increasing redshift. Galaxies and quasars at the same z have comparable angular sizes, whereas their luminosities may differ by one order of magnitude. At large redshifts there is a deficit of objects with low (several GHZ) peak frequencies. The number of GPS galaxies decreases sharply with redshift, and most of them are found at z between 0.01 and 1.81. GPS quasars are found at large redshifts, from 0.11 to 3.99. A quarter of the sample consists of blazars whose spectra may temporarily have a convex shape when the object is in active state.

Multifrequency study of GHz-peaked spectrum sources and candidates with the RATAN-600 radio telescope

Context. Gigahertz peaked spectrum (GPS) radio sources are a class of extragalactic radio sources characterized by a spectral peak in the gigahertz domain. They are a mixed class of quasars and galaxies. A large proportion of the sources studied in the literature have only few data points in the radio domain, and the determination of variability and shape of the simultaneous spectra is inadequate. Sources currently included in the GPS source lists are very heterogeneous. Aims. We present the observational results from 12 observing campaigns (carried out between 2006 and 2010) at the RATAN-600 radio telescope to obtain the simultaneous radio spectra, which is valuable and necessary to derive genuine GPS sources from flat-spectrum radio sources caught in a flaring state when their spectra are temporarily inverted. The sample contains both quasar- and galaxy-type GPS (122 sources) identified in the literature. Methods. The observations were carried out at six frequencies (1.1, 2.3, 4.8, 7.7, 11.2 and 21.7 GHz). The flux densities were measured at several epochs. A six-frequency broadband radio spectrum was obtained by observing simultaneously with an accuracy of up to a minute at 1.4, 2.7, 3.9, 6.25, 13, and 30 cm. Results. The original GPS source samples were highly contaminated. Finally, we selected 29% GPS source candidates within the sample. We found some difference in spectral properties between GPS galaxies and quasars within the sample. The GPS galaxies demonstrate a steeper spectral index in the optically thin part of the spectra. There are only relatively few (17) sources whose radio spectra strictly agree with the spectra of homogeneous self-absorbed synchrotron sources. The narrowest radio spectra are found in both ultra-high-z (z ≥ 1.8) and low-z (0.02 ≤ z ≤ 0.7, FWHM ~ 0.9) convex spectrum radio sources. The majority of quasars within this sample should be considered as flat-spectrum radio sources with a temporarily inverted spectrum, and not as genuine GPS sources. The number of truly convex-spectrum sources remains low, and the lists of GPS sources should accordingly be revised.

The X-ray view of giga-hertz peaked spectrum radio galaxies

This paper presents the X-ray properties of a flux- and volume-limited complete sample of 16 Giga-Hertz Peaked Spectrum (GPS) galaxies. This study addresses three basic questions in our understanding of the nature and evolution of GPS sources: a) What is the physical origin of the X-ray emission in GPS galaxies? b) What physical system is associated with the X-ray obscuration? c) What is the "endpoint" of the evolution of compact radio sources? We obtain a 100% (94%) detection fraction in the 0.5-2 keV (0.5-10 keV) energy band. GPS galaxy X-ray spectra are typically highly obscured. The X-ray column density is higher than the HI column density measured in the radio by a factor of 10 to 100. GPS galaxies lie well on the extrapolation to high radio powers of the correlation between radio and X-ray luminosity known in low-luminosity FRI radio galaxies. On the other hand, GPS galaxies exhibit a comparable X-ray luminosity to FRII radio galaxies, notwithstanding their much higher radio luminosity. The X-ray to radio luminosity ratio distribution in our sample is consistent with the bulk of the high-energy emission being produced by the accretion disk, as well as with dynamical models of GPS evolution where X-rays are produced by Compton upscattering of ambient photons. Further support for the former scenario comes from the location of GPS galaxies in the X-ray to O[III] luminosity ratio versus column density plane. We propose that GPS galaxies are young radio sources, which would reach their full maturity as classical FRII radio galaxies. However, column densities ~10^{22} atoms/cm/cm could lead to a significant underestimate of dynamical age determinations based on the hotspot recession velocity measurements. (abridged)

Constraints on evolutionary properties of GHz Peaked Spectrum galaxies

We have used the available samples of Gigahertz Peaked Spectrum (GPS) galaxies to investigate their evolution properties in the framework of the ``youth'' scenario. Care was taken to properly allow for the different selection criteria used to define the samples. We find that the observed redshift and peak frequency distributions can be satisfactorily accounted for in terms of simple luminosity evolution of individual sources, along the lines discussed by Fanti et al. (1995) and Begelman (1996, 1999), although the derived parameter values have large uncertainties due to ambiguities in the selection of GPS sources and to the incompleteness of redshift measurements. However the simplest self-similar model, whereby the evolution is controlled only by the radial profile of the density of the ambient medium is not good enough and one additional parameter needs to be introduced. The fit requires a decrease of the emitted power and of the peak luminosity with source age or with decreasing peak frequency, at variance with the Snellen et al. (2000) model. On the other hand, our analysis confirms the rather flat slope of the luminosity function, found by Snellen et al. (2000) who also report indications of a high luminosity break, not required by the data sets we have used. Our results suggest that the GPS galaxies are the precursors of extended radio sources with luminosities below the break of the luminosity function. No cosmological evolution of the GPS galaxy population is required by presently available data.

Multifrequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sources (94%) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify 3, 11, 7, and 2 objects as compact symmetric objects, compact doubles, core-jet and complex sources respectively. However, many of the sources classified as compact double or core-jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources, is not as clear in this faint sample.

On the evolution of young radio-loud AGN

This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self-absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large-scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of the surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario. Unfortunately, only a handful of GPS sources are known at low redshift, and an LLF can only be derived, assuming that their cosmological number density evolution is similar to that of steep spectrum sources. The LLF derived in this way is shown to be in good agreement with the proposed evolution scenario. However, the uncertainties are large, and larger, homogeneously selected samples of GPS sources are needed.

Optical spectroscopy of faint gigahertz peaked-spectrum sources

We present spectroscopic observations of a sample of faint gigahertz peaked-spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS), Redshifts have been determined for 19 (40 per cent) of the objects. The optical spectra of the GPS sources identified with low-redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by active galactic nucleus-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep-spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z similar to 2-3, It is unlikely that a significant population of low-redshift GPS quasars is missed as a result of selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift-luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio spectral properties, and hypothesize that GPS quasars are a subclass of flat-spectrum quasars.

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS) has been imaged in the optical and near-infrared, resulting in an identification fraction of 87 per cent. The R - I and R - K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R-band Hubble diagram as determined for radio-bright GPS galaxies. We find evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies. The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification, The correlation between peak frequency and redshift that has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are generally at lower redshifts than the quasars, The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.

Near-infrared imaging of gigahertz-peaked-spectrum radio galaxies: tracing the evolution of giant ellipticals?

In this Letter we present K-band data for a sample of gigahertz-peaked-spectrum (GPS) radio galaxies. We show that the dispersion of the K-band Hubble diagram is small and comparable to that of the R-band Hubble diagram for these sources. The r - K colours of GPS galaxies evolve with redshift in agreement with models for both passively evolving and non-evolving elliptical galaxies. This behaviour indicates that the optical and infrared emission from these sources is dominated by starlight with little or no contamination by light from the active nuclei. At z ,.., 1 the absolute K­ band luminosities are about a magnitude fainter than expected for passively evolving ellipticals, just as in the case for radio-quiet giant ellipticals at similar redshifts. The lack of luminosity evolution may be caused by the dynamical evolution in the galaxies counteracting a change in the mass-to-light ratio of the systems: if the galaxies undergo mergers at z > 0.5, the high-redshift systems will be less massive than those at low redshift. In contrast to powerful radio sources in general, we believe that GPS radio galaxies are representative of early-type galaxies and may be excellent objects for studying the evolution of giant ellipticals at z > 1, given their relative ease of selection.

What are the gigahertz peaked-spectrum radio sources?

view Abstract Citations (281) References (120) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS What Are the Gigahertz Peaked-Spectrum Radio Sources? O'Dea, C. P. ; Baum, S. A. ; Stanghellini, C. Abstract We discuss the astrophysical implications of recent radio and optical observations of the powerful, compact gigahertz peaked-spectrum (GPS) radio sources. The defining characteristics of these objects are (1) their peaked radio spectra with narrow spectral shape and steep high- frequency spectrum, (2) low radio (and optical?) polarization, (3) large radio luminosities, and (4) mostly compact radio structure. The nature of these enigmatic objects is still not understood. However, we present some tentative conclusions and suggest a scenario based on the existing data. The turnover in the spectrum is probably due to synchrotron self- absorption, and we assume this as the basis for our inferences from the radio spectral shape. However, as we discuss below, these sources may have higher than normal electron densities in their nuclei, and it is possible that free-free absorption plays a role in at least some sources. The result that some GPS sources have a very narrow spectral shape is consistent with the hypothesis that there is a narrow range of size scales which dominate the radio luminosity; i.e., very extended (~>100 pc) and very compact (<~0.1 pc) emission is apparently very weak if it exists at all. Based on the steep high-frequency radio spectrum and the low variability, there does not appear to be a strong beamed "jetlike" component in these objects. Instead the radio luminosity is dominated by compact (i.e., parsec scale), steep-spectrum emission ("microlobes"?). The highly inverted low-frequency radio spectrum is consistent with the hypothesis that the radio source is tightly confined. We put on a firm foundation the result that the radio polarization of these sources is systematically low. This is consistent either with a very tangled magnetic field or very large Faraday rotation measures (implying high magnetic fields and or very large electron densities). The existence of high rotation measures is tentatively found in two objects (both high-redshift quasars) but it is premature to conclude that this is a general trend in GPS sources. The optical identifications are a mixture of galaxies and quasars. As previously noted (e.g., Phillips & Mutel; Pearson & Readhead) we find that the galaxies tend to have more symmetric milliarcsecond radio structure than the quasars. CCD imaging of small samples of GPS galaxies suggest that they tend to be interacting/merging. Spectroscopy of two GPS galaxies is consistent with the hypothesis that they have unusually dense, highly reddened narrow-line regions. We suggest that GPS radio sources are formed when the radio plasma is confined on the scale of the narrow-line region by an unusually dense and clumpy ISM. In this scenario, the high electron densities are responsible for depolarizing the radio source and in some cases producing very high Faraday rotation measures. The existing (sparse) optical spectroscopic results are also consistent with the existence of a dense and dusty nuclear ISM. The GPS quasars tend to have very high redshifts (half with z ~> 3). The hosts of these radio sources may be protogalaxies with very dense and clumpy interstellar media. The lower redshift GPS galaxies may have recently cannibalized a gas-rich companion, resulting in a very dense and clumpy nuclear ISM. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170562 Bibcode: 1991ApJ...380...66O Keywords: Astronomical Polarimetry; Extragalactic Radio Sources; Quasars; Radio Spectra; Electron Density (Concentration); Luminosity; Polarization Characteristics; Astrophysics; POLARIZATION; QUASARS; RADIATION MECHANISMS; RADIO SOURCES: GALAXIES; RADIO SOURCES: SPECTRA full text sources ADS | data products SIMBAD (95) NED (95)