High resolution imaging of parsec-scale jets: the impact of the VSOP observations

Resolution improvements achieved with the ongoing space VLBI mission VSOP provide an excellent opportunity for studying compact, parsec-scale jets in active galactic nuclei. Visibilities on space baselines contain structural information that cannot be recovered by super-resolving ground-array data at the same frequency. We illustrate this by discussing the results of recent VSOP observations of parsec-scale jets in 0836+710 and 3C273.     

VSOP Space VLBI observations of Pearson-Readhead survey sources

The VSOP Space VLBI mission uses the HALCA spacecraft, launched from Japan in February 1997, in conjunction with ground radio observatories around the world to create a high resolution radio-wavelength imaging facility. We are using this unique facility to observe a sub-sample of Pearson-Readhead Survey sources at 4.8 GHz to determine core brightness temperatures and pc-scale jet properties. We will highlight one of the sources that has been observed using a combination of the HALCA spacecraft and the EVN, 1642+690, and describe the preliminary brightness temperature distribution for the sub-sample, based on all data analyzed to date.     

VSOP and ground-based VLBI observations of Markarian 421

We have produced 22 VLBI images of the TeV blazar Markarian 421 at 11 epochs, including a Space VLBI observation with the HALCA satellite. We measure the speeds of the three innermost jet components to be 0.64±0.33, 0.48±0.09, and 0.06±0.09c (H₀=65 km s⁻¹ Mpc⁻¹). Interpretation of these subluminal speeds in terms of the high Doppler factor demanded by the TeV observations is discussed.     

VLBI observations of high-opacity HI gas in NGC 5793

We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×10²² cm⁻² assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm⁻³). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H₂O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO.     

VSOP monitoring of the Quasar 1928+738

We present the results from a campaign to monitor the relatively low redshift (z=0.3) circumpolar superluminal quasar 1928+738 with VSOP during the first Announcement of Opportunity period. The four epochs of data show that there have been substantial structural changes in this source near the core on the time-scale of a few months.     

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. 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     and/or peak flux densities     was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with     was observed at 1.6 GHz using a 14-station global VLBI array. In this way, 44 of the 47 sources (94 per cent) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify three, 11, seven and two 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.     

VLBI observations of supernova remnants in Messier 82

We have used the European VLBI Network (EVN) at 18 cm to study five of the more compact radio sources in the starburst galaxy M82. The angular resolution of the observations is 15 mas, corresponding to 0.2 pc at the distance of M82. The observations reveal shells ranging in diameter from 40 to 90 mas (0.6 to 1.4 pc), although the strongest source (41.95+575) is only marginally resolved by these measurements (∼20×10 mas²).
We have found clear evidence for expansion in one of the shell sources (43.31+592) by re-analysing, in wide-field mode, EVN data taken in 1986. Between 1986 and 1997 this source has increased its diameter by 13.6±2 mas, corresponding to an average expansion velocity of 9850±1500 km s⁻¹. If we assume that the remnant is in free expansion, this is consistent with a supernova event in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact source (41.95+575) which was known to be present in the 1960s. 41.95+575 shows no clear evidence for expansion (<4000 km s⁻¹), consistent with a greater age; this is further evidence of its anomalous status. Comparison of the EVN images with earlier MERLIN data is also consistent with expansion in at least two more of the sources. We discuss the flux density variability of the compact sources in M82 and conclude that, with the exception of 41.95+575 and two transient sources, there is little evidence for significant changes in flux density of most of the remnants since the early 1980s.     

Intraday radio variability in active galactic nuclei

Rapid flux density variations on timescales of the order of a day or less (Intraday Variability, IDV) in the radio regime are a common phenomenon within the blazar class. Observations with the Effelsberg 100-m telescope of the MPIfR showed that the variations occur not only in total intensity, but also in the polarized intensity and in polarization angle. Here we present an overview of our IDV-observations and discuss briefly some models which may explain the effect.     

IRAS F02044+0957: An interacting system

A list of 750 objects has been compiled using the Astrophysical CATalogs Support System (CATS) database, by cross-identifying sources in the IRAS catalogues and the catalogue of the Texas survey at 365 MHz. We have carried out a search for optical counterparts of those objects, where the difference in positions between the two catalogues and the APM is less than 3. One of these sources, IRAS F02044+0957, was observed with the RATAN-600 radio telescope at four frequencies in April 1999. Optical spectroscopy of the components of the system was made with the 2.1-m telescope of the Guillermo Haro Observatory. The radio and optical spectra, the NVSS radio map and the optical and infrared images allow us to conclude that the steep spectrum (=–0-94 ± 0-02) radio source IRAS F02044+0957 is a pair of interacting galaxies, a LINER and a HII galaxy, at z=0.093.Published in Astrofizika, Vol. 48, No. 1, pp. 113–124 (February 2005).     

Evidence for parsec-scale absorption from VSOP observations of the BL Lacertae object 0735+178

VLBI total intensity and linear polarization images of the BL Lacertae object 0735+178 have been obtained at     using the     VLA as a ground array together with the HALCA orbiting antenna, and at     about a month later using the VLBA. The most surprising result to come from a comparison of these two images, which have nearly equivalent resolution, is that the path of the jet appears to be appreciably different at the two wavelengths. We have interpreted this as evidence for free–free absorption to the east of the core, near a bend of about 90° in the 2-cm image where the emission is very weak in the 6-cm image. Overall, the polarization structures seen in corresponding regions at the two wavelengths are in good agreement. We find tentative evidence for enhanced rotation measures near two sharp bends in the VLBI jet.     

Phase-reference mapping of a weak-cored double-lobed source in the 1636+473 system using MERLIN and Global VLBI

We present maps and astrometric results from first epoch, simultaneous MERLIN and Global VLBI observations of the very weak core of a double-lobed radio source, using as reference the bright, compact, flat-spectrum quasar 1636+473 A, 20 arcsec away. The observations were made in May 1995 at 5 GHz. The phase-referenced VLBI map of the weak core, of resolution 1.5 mas, contains a high percentage of the core flux density seen in the MERLIN map. Further hybrid mapping iterations reveal a faint, one-sided core extension on the same side as the MERLIN jet. We explore the effect of both temporal and other coherence losses on the phase-reference map. We relate our results to the structural asymmetry in weak AGN cores within the context of unification models.     

1144+35: A giant radio galaxy with superluminal motion

We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. On the parsec scale, we see a complex jet component moving away from the center of activity at 2.7h₅₀⁻¹ c. We detect a faint parsec-scale counter-jet and derive a jet velocity of 0.95c and an angle to the line of sight of 25°, consistent with an intrinsically symmetric ejection. These findings lend credence to the claim that even the jets of low-power radio galaxies start out relativistically.     

An HI Survey of Clusters in the Local Universe:

We outline the project of multifrequency observation of five clusters of galaxies spanning a redshift between 0.05 and 0.2. The core of the project is an HI survey of clusters accomplished with the VLA in its C configuration, and complemented with GMRT data. The 21 cm imaging is being combined with optical spectroscopy and deep NIR imaging with the aim to obtain a database on galaxy evolution in the nearby universe. We choose a sample of clusters with different degrees of dynamical evolution, some containing an important population of starburst or/and post–starburst galaxies, and a hot intracluster medium with emission in X–ray. This revised version was published online in July 2006 with corrections to the Cover Date.     

Total flux density radio observations as a tool for understanding AGN behaviour

A wealth of information on the properties of jets in Active Galactic Nuclei (AGNs) can be derived from total flux density observations at high radio frequencies. This includes, for example, the Doppler factor, the Lorentz factor, and the viewing angle of the jet. We have earlier calculated these parameters for a sample of ～80 sources of different AGN types using almost 20 years of 22 and 37 GHz data from Metsähovi Radio Observatory. We have now gathered data for an additional ten years, and studied the long term characteristic variability time scales of a large sample of AGNs using the first order structure function, the discrete autocorrelation function and the Lomb-Scargle periodogram. Some of the results will be presented in this paper. We also stress the importance of long term observations of AGNs, the main reason for this being misinterpretations of source properties due to insufficient time coverage. Only a few observing epochs will too easily lead to incorrect conclusions about variability, continuum spectra, and the general detectability of the source, not to mention the exclusion of interesting objects from further studies. This is particularly important when considering, for example, the Planck satellite for which the quality of the main mission product, the accurate cosmic microwave background anisotropy maps, depends heavily on the elimination of foreground sources such as AGNs.     

The VLBI Polarization Properties of the Compact Steep-spectrum Source 1150+497

Using the Very Long Baseline Array of the USA, we have made the double-band full-polarization observations towards the compact steep-spectrum source 1150+497, and obtained the distributions of the polarization flux intensity and polarization vector of this source at the 5 GHz and 8 GHz wave bands. The results show that the polarization of the source is mostly concentrated in the core region. Besides, by fitting the polarization angles at three frequencies, the milliarcsecond-scale rotation measure of the source 1150+497 is derived to be 66 rad/m2 for the first time. After removing the ratation measure, the distribution of intrinsic magnetic fields of the source on milliarcsecond scale is obtained. It demonstrates that the direction of the magnetic vector is consistent with the direction of its jet.     

High resolution imaging of the Hubble Deep and Flanking Fields

42 hours of A-array VLA data and 18 days MERLIN data at 1.4 GHz have been combined to image a 10 arcminute field centred on the Hubble Deep Field (HDF). This area encloses both the Hubble Deep and Flanking Fields. A complete sample of 87 sources have been detected with flux densities above 40 μJy. All these have been imaged with the MERLIN+VLA combination to produce images with 0.2, 0.3 and 0.5 arcsecond resolution. These are the most sensitive 1.4 GHz images yet made with rms noise levels of 3.3 μJy/beam in the 0.2 arcsecond images. About 70% of the microJy sources are found to be starburst type systems associated with major disk galaxies in the redshift range 0.4–1. Some 20% are found to be low-luminosity AGN systems identified with field ellipticals at redshifts close to 1. The remaining 10% are associated with optically faint systems close to or beyond the HDF limit; many of these may be dust-shrouded starbursts at high redshift. We propose to extend this study to include VLBI data of comparable sensitivity to investigate the compact radio structures found in the microJy source population.     

A possible active galactic nucleus in M82?

MERLIN and VLA observations of the galaxy M82 have detected a jet-like feature emanating from one of the compact sources, 44.01+59.6. The proximity of this source to the dynamical centre of M82 led us to suspect that it could be a weak active nucleus rather than an SNR. We imaged this source using the EVN at 15 mas and although it shows a compact shell-like structure which could be consistent with an SNR, we note that the EVN image shows a bright region within the source which is elongated along the jet direction. There are distinct similarities between this source and the Sgr A complex at the centre of our own Galaxy which contains the compact AGN, Sgr A^*.     

The youngest lobe-dominated radio sources

We present an analysis of multi-epoch global VLBI observations of the Compact Symmetric Objects: 2352+495 and 0710+439 at 5 GHz. Analysis of data spread over almost two decades shows strong evidence for an increase in separation of the outer components of both sources at a rate of 0.2h⁻¹c (for q=0.5 and H=100h km s⁻¹Mpc⁻¹). Dividing the overall sizes of the sources by their separation rates implies that these Compact Symmetric Objects have a kinematic age 10⁴ years. These results (and those for other CSOs) strongly argue that CSOs are indeed very young sources that probably evolve into much larger classical doubles.     

Overview and current status of the VSOP mission

An overview and status report of the VLBI Space Observatory Programme is presented.