Classical Double Sources Research Articles

Compact symmetric objects—newborn radio galaxies?

We review the question of the age of Compact Symmetric Objects (CSOs); defined as lobe-dominated sources smaller than 1 kpc in overall size. We show that the evidence increasingly points to these objects being very young (<10 4 yr old). Evidence from spectral aging, energy supply arguments and, most convincingly, from long term VLBI kinematic studies, is all consistent with the ‘youth’ scenario for CSOs. From VLBI kinematic studies hotspot advance speeds in CSOs are found to be 0.1 to 0.3 c and external densities estimated from ram pressure balance are ≈1 cm −3. The separate question of the subsequent evolution of CSOs and whether they are the progenitors of classical double sources is, in contrast, not yet definitively answered. However it is found that the numbers of CSOs in flux limited samples is to first order what would be expected under such a scenario. The detailed differences in CSO population density between the data and model predictions might be resolved in various ways. Possibly not all CSOs evolve into large sources, or some sources show recurrent activity or, most likely, the simplest source evolution models need modification.

A Detailed Investigation of Projection Effects Relevant to the Study of Powerful Classical Double Radio Sources

A detailed investigation of projection effects involved in the study of classical double sources is presented here. Theoretical calculations of the way projection effects enter empirically determined parameters are studied. It is shown that some parameters, such as the lobe width, are rather insensitive to the projection angle, while other parameters, such as the ambient gas density, are more sensitive. The deviation of the observed value from the true value of any given parameter is usually not significant when the projection angle is greater than about 60° and is significant only at very small projection angle, at which the symmetry axis of the source is pointing close to the line of sight to the observer. Applications of the theoretical model to a sample of classical double radio sources yield the following principal results: 1. A comparison between the radio-loud quasars and the radio galaxies in the sample suggests that their average projection angles are very similar. The average projection angle of the radio-loud quasars in the sample is greater than 35°-40° at the 3 σ level, assuming that the average projection angle of radio galaxies is 70° or above. 2. On average, the low-redshift radio-loud quasars in the sample have much lower lobe surface brightness and lower nonthermal pressure than the low-redshift radio galaxies, a result which cannot be explained by projection effects. This is consistent with the fact that the radio-loud quasars appear to be fatter than the radio galaxies, as previously noted by Leahy, Muxlow, & Stephens. 3. The study of departures of the magnetic field strength from that estimated using the minimum-energy conditions suggests that the projection angle of Cygnus A is greater than about 40° (at 3 σ) to 55° (at 2 σ), in agreement with the independent results of Sorathia et al. These departures also indicate that the projection angles of most of the sources in the sample, including galaxies and quasars, are likely to be greater than about 30° (at 3 σ) to 40° (at 2 σ). This result does not depend on any assumptions about the average projection angle of radio galaxies and is consistent with result 1 stated above. Thus, it is very unlikely that projection significantly affects key parameters, such as the ambient gas temperature and density, for the sources studied here.

A Detailed Investigation of Radio Power Selection Effects Relevant to the Study of Classical Double Radio Sources

A sample of powerful classical double radio sources, also known as FR II sources, has been assembled to study the properties of the radio sources and their gaseous environments, as well as the redshift evolution of these properties. The possibility that the observed redshift dependences of various parameters of interest are caused by radio power selection effects is investigated here using statistical tools such as two-parameter fitting and partial rank correlation analysis. It is found that the Mach number of lobe advance, the lobe propagation velocity, and the ambient gas density of the FR II sources are unlikely to be significantly affected by radio power selection effects. Hence, the observed redshift dependences of these parameters are likely to reflect real redshift evolution. A previous study by Wellman, Daly, & Wan indicates that the powerful FR II sources in this sample lie in cluster-like gaseous environments. The fact that the ambient gas density of the FR II sources is not affected by radio power selection means that these sources can be used to probe the evolution of their environments. Specifically, the observed decrease of the ambient gas density with redshift is likely to reflect real redshift evolution rather than being caused by radio power selection effects. It is interesting to note that the current data suggest that the Mach number of lobe advance is independent of redshift, while the lobe propagation velocity increases with redshift, as summarized in the final table in this paper. Radio power selection effects are likely to be quite weak, if present at all, in the study of the nonthermal pressure inside a classical double source, and the study of the characteristic size of a source. The two-parameter fitting and partial rank correlation analysis applied to the ambient gas temperature and luminosity in directed kinetic energy of the jet yield inconclusive results, so these direct analyses do not clearly indicate whether the redshift evolution of each of these two parameters is due to radio power selection effects. However, each parameter is estimated using other parameters such as the Mach number of lobe advance, the lobe propagation velocity, and the nonthermal pressure inside the radio lobe, which do not suffer significant radio power selection effects.

Very-long-baseline radio interferometry surveys of the compact structure in active galactic nuclei.

Very-long-baseline radio interferometry (VLBI) imaging surveys have been undertaken since the late 1970s. The sample sizes were initially limited to a few tens of objects but the snapshot technique has now allowed samples containing almost 200 sources to be studied. The overwhelming majority of powerful compact sources are asymmetric corejects of one form or another, most of which exhibit apparent superluminal motion. However 5-10% of powerful flat-spectrum sources are 100-parsec (pc)-scale compact symmetric objects; these appear to form a continuum with the 1-kpc-scale double-lobed compact steep-spectrum sources, which make up 15-20% of lower frequency samples. It is likely that these sub-galactic-size symmetric sources are the precursors to the large-scale classical double sources. There is a surprising peak around 90 degrees in the histogram of misalignments between the dominant source axes on parsec and kiloparsec scales; this seems to be associated with sources exhibiting a high degree of relativistic beaming. VLBI snapshot surveys have great cosmological potential via measurements of both proper motion and angular size vs. redshift as well as searches for gravitational "millilensing."

On the Age of GPS Radio Sources

The GHz-Peaked-Spectrum (GPS) radio sources are galaxies and quasars with compact structures on scales of tens to hundreds of parsecs and spectrum peaking around 1 GHz. Phillips and Mutel (1982) suggested that the double GPS sources (also called compact doubles, CD) are an early stage of the evolution of the classical double sources of Fanaroff-Riley class II type. We have presented a model (Carvalho, 1985) in which we show that the properties of the CD sources are consistent with such a scenario. Their ages would be of the order of 104 yr.

Surface photometry of radio galaxies – II. Cluster sources

CCD surface photometry of 52 radio galaxies in the R-band is reported. The project was designed to study the parent galaxies of radio sources in rich clusters of galaxies and to compare the results with our earlier study of FR I and FR II sources. A radio versus optical luminosity plot shows the same division between FR I and FRII radio sources much more clearly because of the larger sample. A good correlation between radio and optical luminosity is evident for the FR I sources alone but, as reported earlier, FR II sources do not fit this correlation. The results from the larger data set suggest that the FR I/FR II break may be a function of both radio and optical luminosity. The parent galaxies of all the radio sources studied seem to form a one-parameter family with no differences in galaxies inside and outside clusters except that optically brighter galaxies seem to occur more often in rich clusters. The fainter optical galaxies near L* fit r1/4 laws well, while more luminous galaxies deviate toward D-like structures. This change in structure seems to be smoothly correlated with increasing luminosity. There is also a trend for the more luminous galaxies to be more elongated. Classical Double sources in our sample are associated with the lowest optical luminosities near L*. Narrow Angle Tail sources and Twin Jets are associated with intermediate optical luminosities, while the most radio-loud Twin Jet sources are found in the most luminous optical galaxies. More distorted sources such as Narrow Angle Tail sources and Bent Twin Jets or Wide Angle Tail sources clearly occur more often in rich clusters; however, they seem to fit into the correlation between radio and optical luminosity independent of their structure.

The 1 MPC giant radio galaxy IC 711 - 3 KM Westerbork observations at 92 CM

New Westerbork obsevations at 92 cm of the galaxy IC 711 show a radio trail that extends 1 Mpc long, much farther out than previously observed at shorter wavelengths. These new observations confirm IC 711 as the longest head-tail galaxy known, and move IC 711 to the fifth rank among galaxies with the largest radio extension from an optical galaxy nucleus (after the classical double sources 3C 236, 3C 326, HB 13, and MSH 05-22). 20 references.

Superluminal Behaviour of the Double-Lobed Radio Galaxy 3C111

Due to sensitivity problems only about a dozen of the powerful double-lobed radio galaxies have so far been mapped with VLBI. Even less is known about the time dependence of the small scale structure in these objects (Preuss and Alef, 1987). We have recently reported the first results of our monitoring program of classical double sources. We observed strong changes of the pc-scale structure in 3C111 (Götz et al., 1987) and 3C390.3 (Alef et al., 1987) implying “superluminal behaviour” in both sources. This is the first time that such a phenomenon has been found in lobe-dominated radio galaxies. Both objects are Broad Line Radio Galaxies of type N, and their radio emission from m to cm wavelengths is dominated by their outer lobes which are ∼300 kpc apart (H0 = 50 km/s/Mpc).

Compact radio sources in the 3C catalog

view Abstract Citations (118) References (90) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Compact radio sources in the 3C catalog. Pearson, T. J. ; Perley, R. A. ; Readhead, A. C. S. Abstract VLA maps with a resolution of 0.4 arcsec have been made at 6 cm wavelength for 36 compact sources from the 3C catalog. Most of the sources are steep spectrum compact sources exhibiting a variety of structures. While several appear to be small classical double sources, others are noted to be more complex. An attempt is made to divide the sources into groups by morphology, in terms of complexity, one-sided asymmetry, double sources, and unresolved sources. Publication: The Astronomical Journal Pub Date: May 1985 DOI: 10.1086/113782 Bibcode: 1985AJ.....90..738P Keywords: Astronomical Catalogs; Astronomical Maps; Extragalactic Radio Sources; Galactic Structure; Interplanetary Medium; Calibrating; Flux Density; Red Shift; Very Long Base Interferometry; Void Ratio; Astronomy full text sources ADS | data products SIMBAD (36) NED (33)

The bridges of classical double radio sources

New observations with the VLA of a complete sample of 39 3C sources with angular sizes larger than 45 arcsec are discussed. Nearly all of the classical double sources at redshifts less than 0.5 have bridges linking the central galaxy with the outer edge of the source, and our maps of these objects show that two-thirds of the bridges are significantly distorted near the central galaxy. Such distortions may be predicted from the jet model for double radio sources if the jet is much lighter than the confining medium, since the dynamics of the bridge region are then dominated by backflow of material that has passed through the hotspots. There is a correlation between axial ratio and luminosity which may be physically related to the compactness relation of Jenkins & McEllin.

Bent Jets in Radio Quasars

How is energy transported out from the central engine in quasars and radio galaxies to the distant radio lobes? This problem has been around since the early discovery of classical double radio sources, and is still not answered in detail. The idea of relativistic beams was first suggested by Martin Rees as a means of transporting plasma out of the nucleus (Rees, 1971, Blandford and Rees, 1974). This idea gained support first from the discovery of hot spots in the radio lobes of these large classical double sources, and later by observations of the beams themselves in radio galaxies. As more jets were observed, it became obvious that they were often curved, serpentine, or even sharply bent. This behavior has been modeled as precession of the central nozzle (Bridle et al., 1976, Ekers et al., 1978), as nuclear refraction (Henriksen et al., 1981), as a growing plasma instability (Hardee, 1981) and as various combinations of the above. At the present time, it seems safest to conclude that there are some examples of each of these processes known.

Radio jets and bridges in the classical double sources 3C388 and 0816 + 526

We report on 4885-MHz full synthesis high-resolution (1″) observations [using the NRAO Very Large Array (VLA)] of the ‘classical’ double radio sources 0816 + 526 and 3C388. Classical double sources are characterized by strong emission peaks (lobes) at the extremities of a linear structure. Although at lower resolution these sources both appear to have simple double-lobed morphologies, the VLA maps reveal several important new features. 0816 + 516 was found to have very sharp leading edges indicative of strong deceleration, possibly as a result of encountering a dense extragalactic background gas. The transverse boundaries of the components are also quite steep but still consistent with constant emissivity-filled cylinders rather than limb-brightening. The morphology of 3C388 is dramatically different from 0816 + 526, since it includes a bright radio jet. Unlike previously observed radio jets which are usually found between the nucleus of the radio galaxy and its extended lobes, the jet in 3C388 is embedded inside the southwestern lobe and a prominent gap in the radio emission exists between the radio nucleus and the beginning of the jet. These observations clearly illustrate that a variety of source substructures, which must be dealt with in theoretical models, exist within radio galaxies which were previously classified simply as classical doubles.

Classical double sources in the directions of rich clusters of galaxies

view Abstract Citations (15) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Classical double sources in the directions of rich clusters of galaxies. Burns, J. O. ; Owen, F. N. ; Rudnick, L. Abstract Interferometric observations of two classical double radio sources (0816+526 and 1232+414) located in the directions of rich Abell clusters of galaxies are reported. Total-intensity and polarized-intensity maps of the two sources obtained at 2695 and 8085 MHz are presented, and the integrated source properties are summarized. The spectral indices, degrees of polarization, and rotation measures along the source major axes are analyzed. The observed and derived parameters are examined in the context of current theoretical models, particularly the single- and double-ejection plasmoid models. For the single-ejection model, the ram pressure necessary to confine the double source components with 'equipartition' velocity is found to produce a low value for the density of the intergalactic medium, which may be reasonable if the sources are not members of the coincident clusters. It is noted that a continous resupply of particles seems most consistent with the derived and observed physical source parameters if the sources are cluster members. Publication: The Astronomical Journal Pub Date: April 1978 DOI: 10.1086/112208 Bibcode: 1978AJ.....83..312B Keywords: Galactic Clusters; Galactic Radiation; Intergalactic Media; Microwave Emission; Radio Sources (Astronomy); Astronomical Maps; Brightness; Galactic Structure; Polarized Electromagnetic Radiation; Turbulence Effects; Astrophysics; Clusters of Galaxies:Radio Sources; Polarization:Radio Sources full text sources ADS |