No abstract

We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity which may represent an underlying continuous jet flow, but we also see some stationary and even apparently inward moving features which co-exist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure.The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the time scale of flux density variations is consistent with a steep power law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows, and contrary to the assumption of simple unified models, not all sources have intrinsic speeds close to c.Usually, the observed jet flow is in the general direction of an established jet. However, many jets show significant bends and twists, where the observed motions are non-radial, but are alingned with the local jet direction suggesting that the jet flow occurs along pre-existing bent channels. In a few cases we have observed a clear change in the direction of a feature as it flows along the jet. Radio jets which are also strong gamma-ray sources detected by EGRET appear to have significantly faster speeds than the non EGRET sources, consistent with the idea that gamma ray sources have larger Doppler factors than non gamma-ray sources. Sources at high redshift have systematically lower angular speeds than low redshift jets, consistent with standard cosmologies.

No abstract

We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam area at 3 GHz with 8 FWHM resolution and 1.0 µJy beam −1 rms noise near the pointing center. The P (D) distribution from the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S(µJy) < 10 range. At this level the brightness-weighted differential count S 2 n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are starforming galaxies; and ≈ 96% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by AGNs, and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR) / radio correlation and account for most of the FIR background at λ ≈ 160 µm. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough The confusion at centimeter wavelengths is low enough that neither the planned SKA nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is ≤ 0.01 µJy at ν = 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE 2 at 3.3 GHz, they cannot be located in or near galaxies and most are ≤ 0.03 µJy at 1.4 GHz.

We have examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array ( VLBA) at 15 GHz. With projected baselines out to 440 Mk, we are able to investigate source structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for shortwavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes.We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the submilliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron-emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 10 11 and 10 13 K, but they extend up to 5 ; 10 13 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.

We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the > 3σ level. Although most features within a jet typically have speeds within ∼ 40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc deprojected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope −1.4 ± 0.2. The parent jets of the brightest radio quasars have a space density of 261 ± 19 Gpc −3 and unbeamed 15 GHz luminosities above ∼ 10 24.5 W Hz −1 , consistent with FR II class radio galaxies.

We have used the VLBA at 15 GHz to image the structure of 132 strong compact AGN and quasars with a resolution better than one milliarcsecond and a dynamic range typically exceeding 1000 to 1. These observations were made as part of a program to investigate the sub-parsec structure of quasars and AGN and to study the changes in their structure with time. Many of the sources included in our study, particularly those located south of declination +35 degrees, have not been previously imaged with milliarcsecond resolution. Each of the sources has been observed at multiple epochs. In this paper we show images of each of the 132 sources which we have observed. For each source we present data at the epoch which had the best quality data. In a future paper we will discuss the kinematics derived from the observations at all epochs.Most of the sources we have observed show the canonical core-jet morphology with structure somewhat characteristic of the jets seen on arcsecond scales with the VLA and Westerbork. The milliarcsecond jets generally appear one-sided, but two-sided structure is often found in lower luminosity radio galaxies and in high luminosity quasars with gigahertz peaked spectra. In many cases there is significant curvature, sometimes up to 90 or more, particularly close to the core. In other cases the jets have a more gradual curvature. In some sources there are multiple bends or twists along the jet, suggestive of a three dimensional curved structure. Many of the jets may be described by a small number of apparently discrete components, but in other cases there appears to be a monotonically decreasing distribution of radio emission Usually the structure is unresolved along the direction perpendicular to the jet, but a few sources have broad plumes. Much of the visible parsec scale structure in compact radio sources can probably be explained as the projection of a relativistic beamed twisted jet, which appears bright at those positions where it approaches the viewer. In some low luminosity radio galaxies, the structure appears more symmetric at 2 cm than at longer wavelengths. The apparent long wavelength asymmetry in these sources is probably due to absorption by intervening ionized material.A few sources contain only a single component with any secondary feature at least a thousand times weaker. Peak rest frame brightness temperatures are typically of the order of 10 K with no [11][12] evidence for any excess over the limit of 10 K expected from inverse Compton cooling. We find no 12 obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.

No abstract