arXiv subjects
Publications and source records attributed to R. C. Vermeulen.
The idea that the radio jets in AGN contain material in relativistic motion is supported by many lines of observational evidence, including morphology, brightness temperature estimated with interferometers and with intrinsic variations, interstellar scintillations, X-rays, and superluminal motion. These are largely independent, and taken together make an irrefutable case for relativistic motion.
We investigate the relation between the Doppler factor determined from variations in total flux at 22 and 37 GHz, and the apparent transverse velocity determined from VLBA observations at 2 cm. The data are consistent with the relativistic beaming theory for compact radio sources, in that the distribution of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close to the "equipartition value" calculated by Readhead (1994). We deduce the distribution of Lorentz factors for a group of 48 sources; the values range up to about gamma=40.
We report a change in the trajectory of a well studied jet component of the quasar 3C279. The component changes in apparent projected speed and direction, and we find it to be moving with a Lorentz factor gamma >~ 15 at an initial angle of <~ 1 degree to the line of sight. The new trajectory of the component has approximately the same speed and direction as an earlier superluminal feature, originally seen in the early 1970s. The new direction for the component is also much better aligned with larger scale VLBA and VLA structure out to 0.1 arcseconds. We suggest that the trajectory change is a collimation event occurring at >~ 1 kiloparsec (deprojected) along the jet. While the change in trajectory on the sky appears to be 26 degrees the intrinsic change is <~ 1 degree. We estimate the Doppler factor prior to the change in direction to be D >~ 28 and after the change to be D >~ 23. Comparison to independent constraints on the Doppler factor suggest that the energy in the radiating particles cannot greatly exceed the energy in the magnetic field unless the volume filling factor is very much less than one.
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) have larger HI column densities than the larger 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.
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 HI 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 tau=0.16 to tau<=0.001. There is a substantial variety of line profiles, including Gaussians of less than 10km/s, to more typically 150km/s, as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred km/s. Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(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 HI velocity is substantially negative (up to v=-1420km/s) 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.
In the Caltech-Jodrell Survey of bright flat-spectrum radio sources, 3-5 epochs have now been observed for nearly all 293 sources; a uniquely large sample. The derivation of component motions has not yet been completed; it is complicated by, for example, variability and jet curvature. Nevertheless, some basic results are clear. The average apparent velocity in CJF quasars is near 3c, while for radio galaxies and BL Lacs it is around 1c. The distribution of velocities is broad, indicating a broad distribution of jet Lorentz factors, with many low values, and/or a decoupling of the Lorentz factors between the cores and the moving jet components, due to bending, speed changes, or due to pattern motions (shocks). A hint has emerged at this workshop that the average apparent velocities might be lower at lower radio frequencies; this will need careful verification.
The bright radio structure of the LINER elliptical galaxy NGC1052 is dominated by bi-symmetric jets on parsec scales. Features move outward on both sides of the core at v_{app}~0.26c. We have established the occurrence of free-free absorption, and suggest the presence of a patchy, geometrically region oriented roughly orthogonal to the jets; components have a wide range of spectral shapes and brightness temperatures. We distinguish three velocity systems in HI absorption. The ``high velocity'' system is the most prominent of these; it is redshifted from systemic by about 150 km/s. In HI VLBI it is seen towards both jets, but appears to be restricted to a shell 1 to 2 pc away from the core. The central hole might be largely ionised, and could be connected to the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH main line absorption over at least the full ~250 km/s velocity range seen in HI. In the ``high velocity'' system, the profiles of the OH main lines and HI are similar, which suggests co-location of molecular and atomic gas. The OH satellite lines are also detected in the ``high velocity'' system: 1612 MHz in absorption and 1720 MHz in emission, with complementary strength. But we have no satisfactory model to explain all properties; the connection to H_2O masing gas at the same velocity but apparently a different location is also unclear.
Combining our own observations with data from the literature, we consider the incidence of HI absorption in Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources. Here we present our preliminary results, where we find that the smaller GPS sources (<1 kpc) on average have larger HI column densities than the larger CSS sources (>1 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 galaxy classified objects, we argue that if the unified schemes apply to the GPS/CSSs, a disk distribution for the HI is more likely.
(Abridged) We discuss multiple VLBI continuum and spectral line observations and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at approximately 0.26c in bi-symmetric jets, most likely oriented near the plane of the sky. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium. Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption in the inner parsec, probably together with synchrotron self-absorption. There is apparently a geometrically thick but patchy structure oriented roughly orthogonal to the jets. The western jet is receding: it is covered more deeply and extensively. HI spectral line VLBI reveals atomic gas in front of both jets. There appear to be three velocity systems. The deepest, at "high velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2 pc away from the core, within which this gas might be largely ionised. WSRT spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity system the OH profiles are similar to HI, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H2O masing gas seen elsewhere. We have also detected both 18cm OH satellite lines in the high velocity system. They have conjugate profiles: 1612 MHz is in absorption, and 1720 MHz in emission.
High resolution radio observations remain the most direct way to study the formation and evolution of radio jets associated with the accretion onto massive black holes. We report preliminary results of our seven year VLBA observational program to understand the nature of relativistic beaming in blazars and the surrounding environment of massive black holes. Most blazars show an apparent outward flow away from an active core. However, in a few sources the motion appears inward, most likely the result of projection of a curved trajectory which bends back toward along the line of sight. The apparent motion of jet features is not always oriented along the direction separating the feature from the core, and in a few cases we have observed a clear change in the direction and velocity of a feature as it flows along the jet. In other sources, the motion appears to follow a simple ballistic trajectory. We find no simple relation between the time scales of flux density changes and apparent component velocities.
We present the results of a multi-epoch VLBA study of the powerful radio source 4C +12.50 (PKS 1345+125) at a wavelength of 2 cm. This compact radio source is associated with a hidden quasar whose host galaxy shows signs of a recent merger. It has been classified as a compact symmetric source (CSO) due to its small overall size (~220 pc) and twin-jet morphology, although it also has faint extended emission that may be a relic of previous activity. We report the detection of exceedingly high linear fractional polarization in isolated features of the southern jet (up to 60%), which is highly unusual for a CSO. Given the large amount of gas present in the host galaxy, we would expect significant Faraday depolarization across the whole source, unless the depolarizing gas is fairly clumpy. The southern jet also contains two features that are moving outward from the core at apparent speeds of v/c = 1.0 +/- 0.3 and 1.2 +/- 0.3. These represent the first positive detections of superluminal motion in a CSO, and taken together with the jet to counter-jet flux ratio, suggest an intrinsic flow speed of v/c = 0.84 +/- 0.12. The apparent ridgeline of the jet and counter-jet are consistent with a conical helix of wavelength 280 pc that is the result of Kelvin-Helmholtz instabilities driven by a slow precession of the jet nozzle. A fit to the data implies that the nozzle is precessing around a cone with half-angle 23 degrees, whose axis lies at an angle of 82 degrees to the line of sight. We suggest that the "S"-shaped jet morphologies commonly seen in recent AGN outflows such as 4C +12.50 may simply reflect the fact that their black hole spin axes are still precessing, and have not had sufficient time to align with their accretion disks.
This is a paper on young jet material in a frustratingly complex environment. NGC 1052 has a compact, flat- or GHz-peaked-spectrum radio nucleus consisting of bi-symmetric jets, oriented close to the plane of the sky. Many features on both sides move away at v_app~0.26c (H_0=65 km/s/Mpc). VLBI at seven frequencies shows a wide range of spectral shapes and brightness temperatures; there is clearly free-free absorption, probably together with synchrotron self-absorption, on both sides of the core. The absorbing structure is likely to be geometrically thick and oriented roughly orthogonal to the jets, but it is patchy. HI VLBI shows atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems, at least two of which are local to the AGN environment. The "high velocity system", 125 to 200 km/s redward of systemic, seems restricted to a shell 1 to 2 pc away from the core. Closer to the centre, this gas might be largely ionised; it could cause the free-free absorption. WSRT spectroscopy shows 1667 and 1665 MHz OH absorption over a wide velocity range. OH and HI profile similarity suggests co-location of molecular and atomic ``high velocity'' gas; the connection to H2O masing gas is unclear. Further, at ``high velocity'' we detected the OH 1612 MHz satellite line in absorption and the 1720 MHz line in emission, with complementary strengths.
We summarize the results of VLBA and global VLBI observations of SS433 between 1995 and 2000. With these observations we resolve the inner jet of the source and identify an absorption region (~25 AU), the "central radio gap". The radio gap is caused by free-free absorption of the jet radio emission by a flattened outflow from the binary system. Radio emission is detected on 100 AU scales perpendicular to the normal jets ("equatorial emission region"; "equatorial outflow"). At some epochs the emission is smooth but compact features are frequently detected. We suggest that equatorial outflows may be common in microquasars.
We present VLBI imaging results for SS433 from the period 1995-2000. An equatorial emission region is detected and confirmed in 7 experiments at 1.6 and 5 GHz. The structure of the region changes with time on scales of weeks to months. The nature of emission is unknown, but it is certainly non-thermal, maybe optically thin synchrotron. The spectrum however might be attenuated by a thermal population of electrons. There were previous indications for an equatorial outflow from the system. We do detect outward motion of a radio component, and estimate a speed of 1200/sin(i) km/s. Theoretical work and IR observations suggest that the mass-loss rate in SS433 is much higher in this region than in the jets. We suggest that the high-mass microquasars might all have equatorial outflows; in fact, it is the dominant form of mass-loss in these systems.
We are investigating the kinematics of jets in active galactic nuclei on parsec scales by studying a representative population of sources. This study is being carried out using the Very Long Baseline Array at 15 GHz, with more than 800 images taken since 1994. In this contribution we present an overview of the diversity of kinematics for a complete sample of sources.
We report further results from our imaging survey at 15GHz (lambda=2 cm) with the Very Long Baseline Array. This paper presents single epoch images for 39 sources, bringing the total number of objects in the sample to 171. Our sample is representative of a complete unbiased sample and it will be used for statistical analysis of source properties. We compare the observed brightness temperatures derived from our VLBA observations to those derived from total intensity variations at 22 and 37 GHz. These are consistent with intrinsic brightness temperatures in the range 10^10 to 10^12 K. We also present three new spectroscopic redshift values: z=0.517+/-0.001 for 0026+346, z=1.591+/-0.003 for 0727-115, and z=0.2016+/-0.0004 for 1155+251. Images from this VLBA 2 cm survey are available on the Internet under http://www.cv.nrao.edu/2cmsurvey.
Westerbork Radio Synthesis Telescope observations of the redshifted 21cm line absorber against the z_em = 0.871 double lobed quasar 3C196 show that the intervening absorber is an L approximately equal L* spiral galaxy (3C196-G1) and that the absorbing layer of cold gas extends to radii of at least 30 h_50^-1 kpc. The new data solve several long standing puzzles about this system by (1) discovering a second 21cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spatially ``resolving'' the two absorption features to isolate the absorption along the two lines of sight to the opposing radio lobes. These findings resolve the disagreement in redshift between the UV metal and 21cm lines, and as well as demonstrating that the neutral layer does absorb both lobes of the background radio source. Simple kinematic models with an inclined, rotating gas disk match the observed 21cm profile and are also compatible with both the redshift and velocity spread of the absorption measured in UV resonance lines along a third, independent line of sight to the quasar nucleus and with the lack of 21cm absorption in as earlier VLBI experiment that was sensitive to opacity against the hot spot in the northern lobe. The inferred rotation speed and luminosity for the galaxy are compatible with the z approximately 0 Tully-Fisher Relation. This system illustrates well how 21cm absorption against extended background radio sources is a powerful tool in determining the nature of the damped Lyman-alpha class of QSO absorption line system.
Using Very Long Baseline Interferometry we have searched a sample of 300 compact radio sources for examples of multiple imaging produced by gravitational lensing; no multiple images were found with separations in the angular range 1.5--50 milliarcsec. This null result allows us to place a limit on the cosmological abundance of intergalactic supermassive compact objects in the mass range $\sim 10^{6}$ to $\sim 10^{8}$M$_{\odot}$; such objects cannot make up more than $\sim 1%$ of the closure density (95% confidence). A uniformly distributed population of supermassive black holes forming soon after the Big Bang do not, therefore, contribute significantly to the dark matter content of the Universe.