R. C. Vermeulen

SUB-MILLIARCSECOND IMAGING OF QUASARS AND ACTIVE GALACTIC NUCLEI. III. KINEMATICS OF PARSEC-SCALE RADIO JETS

We report the results of a 15 GHz (2 cm) multiepoch Very Long Baseline Array (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 that can represent an underlying continuous jet flow, but we also see some stationary and even apparently inward-moving features that coexist 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 timescale 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 nonradial but are aligned with the local jet direction, suggesting that the jet flow occurs along preexisting 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 that 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. Subject headings: galaxies: active — galaxies: jets — quasars: general — radio continuum: galaxies On-line material: machine-readable tables

MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS. V. MULTI-EPOCH VLBA IMAGES

We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination –20°, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond.

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei

We have used the Very Long Baseline Array at 15 GHz to image the structure of 132 strong compact active galactic nuclei (AGNs) and quasars with a resolution better than 1 mas and a dynamic range typically exceeding 1000:1. These observations were made as part of a program to investigate the subparsec structure of quasars and AGNs 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?, 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 that we have observed. For each source we present data at the epoch that 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 Very Large Array and Westerbork telescopes. 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 1000 times weaker. Peak rest-frame brightness temperatures are typically of the order of 1011?1012 K, with no evidence for any excess over the limit of 1012 K expected from inverse Compton cooling. We find no obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure

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 Mλ, 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 short-wavelength 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 sub-milliarcsecond 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 1011 and 1013 K, but they extend up to 5 × 1013 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.

When Is BL Lac Not a BL Lac

We have found broad Hα and Hβ line emission in spectra of BL Lacertae taken on 1995 May 21 and June 1: on those occasions BL Lac did not meet the defining criteria for the class named after it. The Hα line luminosity, ~2 × 1041 h-2 erg s-1 for H0 = 100 h km s-1 Mpc-1, is significantly above the detection threshold in older published spectra which do not show the line. Several possible explanations for the increase in broad line luminosity are discussed. The continuum was at the faint end of its commonly observed range: V ~ 16.0 on 1995 May 21. The polarization, measured on 1995 June 1, was also near the low end of the typical range in BL Lac: ~5.0% in PA ~ 22° near 7500 A, rising to ~6.1% in PA ~ 15° near 4900 A. Absorption features in the spectrum show the presence of the associated galaxy, but the light is still dominated by an underlying smooth continuum, the intrinsic shape of which is well approximated between 8000 and 4000 A by Fν ∝ ν-1.7, indicating a synchrotron origin. We confirm the narrow emission line redshift, z = 0.0686 ± 0.0004.

Observations of H i absorbing gas in compact radio sources at cosmological redshifts

We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z = 0.85. Searches for H i 21cm absorption were made with the Westerbork Synthesis Radio Telescope at UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from � = 0.16 to � � 0.001. There is a substantial variety of line profiles, including Gaussians of less than 10kms 1 , to more typically 150kms 1 , as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred kms 1 . Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1×10 19 and 3.3×10 21 (Tsp/100 K)(1/f) cm 2 . There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the H i velocity is substantially negative (up to v = 1420kms 1 ) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.

Spectropolarimetry of Two Broad Absorption Line Quasars with the W. M. Keck Telescope

We have observed the broad absorption line quasars PHL 5200 and 01052265 with spectropolarimetry at the Keck telescope. In PHL 5200 the continuum is consistent with a power law with constant polarization,p 55.1%. A drop in polarization in the red is explained with dilution by unpolarized Fe II emission. In both objects the permitted emission lines are unpolarized. In PHL 5200 the semiforbidden line C III] l1909 is weakly polarized; weattributethistoresonancescattering.Thepolarizationrisesinthebroadabsorptiontroughs,toapeakof12% in both objects. The high values ofpare restricted to a narrow velocity range that is well inside the absorption troughs. In each object, the polarization position angle is constant, except that in PHL 5200 there are marginally significant rotations in the C IVtrough and in the C III] emission line. We describe two simple geometries that can explain some of these features. In thefirst, there are scattered and direct rays; in the troughs the direct ray (unpolarized) is largely absorbed, and we mainly see the highly polarized scattered ray. In the other, the high trough polarization is caused by resonance scattering. Subject headings: polarization—quasars: absorption lines—quasars: emission lines— quasars: individual (PHL 5200, 01052265)

The second Caltech-Jodrell Bank VLBI survey. 1: Observations of 91 of 193 sources

We define the sample for the second Caltech-Jodrell Bank very long base interferometry (VLBI) survey. This is a sample of 193 flat- or gigahertz-peaked-spectrum sources selected at 4850 MHz. This paper presents images of 91 sources with a resolution of approximately 1 mas, obtained using VLBI observations at 4992 MHz with a global array. The remaining images and the integrated radio spectra will be presented in a forthcoming paper by Henstock et al.

The presence and distribution of HI absorbing gas in sub-galactic sized radio sources

We consider the incidence of Hi absorption in intrinsically small sub-galactic sized extragalactic sources selected from sources classified as Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources. We find that the smaller sources ( 0.5 kpc). Both a spherical and an axi- symmetric gas distribution, with a radial power law density profile, can be used to explain this anti-correlation between projected linear size and Hi column density. Since most detections occur in objects classified as galaxies, we argue that if the unified schemes apply to GPS/CSSs a disk distribution for the Hi is more likely. The most favoured explanation for the compact sizes of the GPS/CSSs is that they are young sources evolving in a power law density medium. For the GPSs with measured expansion velocities, our derived densities are within an order of magnitude of those estimated from ram-pressure confinement of the lobes assuming equipartition. Our results therefore support the youth model.

The Second Caltech--Jodrell Bank VLBI Survey. II. Observations of 102 of 193 Sources

This is the second of two papers presenting the Second Caltech-Jodrell Bank VLBI survey (CJ2). The CJ2 sample consists of 193 flat- and gigahertz-peaked-spectrum sources selected at 4850 MHz. In this paper we present images of the remaining 102 sources with a resolution of ~1 mas, obtained using VLBi snapshot observations at 4992 MHz with a global array. We also present integrated radio spectra for the entire CJ2 sample.