We are using the VSOP space VLBI mission to observe a complete sample of Pearson-Readhead survey sources at 4.8 GHz to determine core brightness temperatures and pc-scale jet properties. To date we have imaged 27 of the 31 objects in our sample. Our preliminary results show that the majority of objects contain strong core components that remain unresolved on baselines of 30,000 km. The brightness temperatures of several cores significantly exceed 1012 K, which is indicative of highly relativistically beamed emission. We also find that core brightness temperature is correlated with intraday variability in compact AGNs.

We briefly describe the radio observations that form part of co-ordinated Chandra, HALCA+VLBA, and VLBA-only observations of the superluminal gamma-ray loud quasar 0836+710. The radio observations were at 6 frequencies (1.6, 5, 8, 15, 22, and 43 GHz) with the two lowest frequency observations being undertaken with the VLBA co-observing with the HALCA spacecraft (the space element of the Japanese VSOP mission). Combining the radio and X-ray data will allow constraints to be placed on the jet Doppler factor.

We present results of the first space VLBI observations of PKS 1921-293. An inner jet component about 1.5 mas north of the core is revealed for the first time. The compact core is partially resolved, but still has a brightness temperature (at the source rest frame) of 3.0×1012 K. A spectral index map made by combining the 1.6 GHz VSOP image with the 5.0 GHz VLBA+Y image at the first epoch is also presented.

The quasar 3C345 is one of the best examples of extra-galactic jet with superluminal components moving on helical paths. To get better constraints on the jet physics, we have imaged the source at the highest possible resolution, using mm-VLBI and space-VLBI observations. We observed 3C 345 at 5 different epochs so far (1 epoch with 3-mm ground VLBI, and 4 epochs with space VLBI at 6cm and 18 cm). These measurements expand significantly our ongoing monitoring of 3C 345. In this contribution, we discuss the properties of the jet and its moving components in the immediate vicinity of the core (< 3 mas).

BL Lacertae objects are active galactic nuclei with weak, sometimes undetectable, optical line emission and strong variability in total intensity and linear polarization over a broad range of wavelengths from ultraviolet to radio. It is believed that synchrotron radiation is the dominant emission mechanism virtually throughout the spectrum. Their strong linear polarization makes BL Lac objects prime targets for space VLBI polarization observations.

The results of polarimetric λλ6,4 cm VLBA observations of the inner 5pc of the jet and core of Vir A (3C274, J1230+1223) are reported. At λ4 cm, the core and the inner parsec are unpolarized and there is evidence for ordered magnetic fields and large RM gradients at larger distances.

Two Compact Steep-spectrum Sources (CSSs), 0548+165 and 1524-136, have been observed in dual polarization with the VLBA at 4.9 and 8.4 GHz. Rotation measures (RMs) up to 104 rad m−2 were derived for the jets of both sources. These high values possibly arise due to the external medium interacting with the radio jets, with measured depolarization being due to small-scale inhomogeneities in the medium.

BL Lacertae objects are active galactic nuclei with weak or undetectable line emission and strong variability in total intensity and linear polarization over a wide wavelength range from optical to radio. The radio emission and much of the optical emission is believed to be synchrotron radiation. Sources in the complete sample of BL Lac objects defined by Kühr and Schmidt (1990) have: 5 GHz fluxes of at least 1 Jy, radio spectral index α ≥ −0.5 (Sv ˜ v+α), rest frame equivalent width of the strongest emission lines less than 5 Å, and optical counterparts on the Sky Survey plates with brightness greater than 20m.

We performed a multifrequency and multiepoch study of the quasar PKS 1510-089 with ground and space VLBI, in order to study its nuclear properties and test current models for the production of the emission at X- and γ-ray energies. A preliminary analysis of our images suggests that the lower limit to the Lorentz factor γ at the distance of ∼ 12 pc from the central engine is γmin = 5. We used Ho = 100 km s−1 Mpc−1.

In order to investigate the genesis of powerful radio jet, we have mapped the central 10 pc region of the nearby radio galaxy NGC 6251 with a 0.2 pc resolution using VLBI at two radio frequencies, 5 GHz and 15 GHz, we have found the sub-parsec-scale counterjet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model.

“Number - flux density” distributions for total and correlated flux densities at various interferometer spacings in 355 extragalactic radio sources are analysed. Qualitative conclusions on the population of milliarcsecond components in these sources are presented.

We present statistical results on VLBI cores from large VLBI samples at C, L and S/X bands. Most of the results are consistent with the inhomogeneous jet model. The magnetic field distribution and particle density in the inhomogeneous jet model were estimated. We also found that the flux densities and luminosities of the cores at 5 GHz were correlated with those at optical band in the rest-frame.

A strong correlation is present between the extended radio and broad-line emission for a radio-loud quasar sample. The statistic behaviour on the core emission of lobe-dominated quasars is rather different from that of core-dominated quasars. The extended radio luminosity is a good tracer for jet power, while the core luminosity can only be a jet power tracer for core-dominated quasars.

On the basis of an intrinsically symmetric relativistic jet model, analytical equations between the mean jet speed and the mean jet flux asymmetry, and between their variances are derived in the context of an orientation-based unification scheme. The mean jet speeds of classical double radio sources are estimated on kiloparsec scales by using the asymmetry of jet-counterjet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is ∼ (0.54 ± 0.03)c, while for FRII low-redshift radio galaxies and intermediate-redshift quasars the values found are ∼ (0.4 ± 0.06)c and ≳ 0.6c respectively.

This review summarizes some aspects of the central kiloparsec scale structure of galaxies, and in particular spiral galaxies, elliptical galaxies and merger remnants. The focus is on results from optical and near-IR imaging and spectroscopy, with emphasis on recent work with the Hubble Space Telescope.

The archetypical, nearby (z=0.37) quasar 3C 48 is an unusual CSS radio source with excess far-IR emission, whose one-sided radio jet is aligned with the extended ionized emission and a putative second nucleus. Because of its high AGN luminosity and proximity, 3C 48 is a good candidate to search for kinematic signatures of the radio jet-gas coupling and/or of a recent interaction. The radio morphology and our ground-based integral field spectroscopy suggest that the jet is interacting with its immediate environment. Using STIS aboard HST in several slit positions within the central 1”, we map the kinematics and physical conditions of the extended emission line gas and their relations to near-nuclear star forming regions found in existing HST images.

Using archival HST/WFPC2 optical data and near-infrared VLT/ISAAC spectroscopy, the properties of the Lyman-break galaxy cB58 are analyzed. The z = 0.37 galaxy cluster MS1512+36 lenses it into strongly magnified arcs with ample substructure. The data enables an accurate reconstruction of the source, yielding milliarcseconds resolution; an unparalleled view of the central kpc of the z = 2.72 starburst galaxy.

Fine features of the dust distribution in the central region of NGC 4151 have been studied from high resolution images obtained by the Subaru Telescope. The two dust lanes lying along a circum-nuclear ellipse of 40“ ×20” have been recognized to consist of a number of fine filaments and knots. In the inner region, there has been found dust distributed in a bipolar cone region extending to 5” from the nucleus. This cone is exactly complementary to the well-known bipolar ionized gaseous region. A dense knot is seen at 2” SE from the nucleus, but no torus-like feature is recognized. The masses of neutral gas in the outer lanes and in the inner region are estimated as 106M⊙ and 105M⊙ respectively. These observational results are compared with results of numerical simulations of gas dynamics.

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We classify all the sources into three categories with multiple, double and single nucleus/nuclei, mainly based on a quantitative criterion of I-band luminosity. The resultant fractions of multiple, double and single nucleus/nuclei ULIRGs are 18%, 39% and 43%, respectively. This supports the multiple merger scenario as a possible origin of ULIRGs, in addition to the commonly-accepted pair merger model. Further statistical studies indicate that the fraction of AGN increases from multiple (18%) to double (39%) and then to single (43%) nucleus/nuclei ULIRGs. For the single nucleus category, there is a high luminosity tail in the luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This supports the statement that active galactic nuclei tend to appear at final merging stage. For multiple and double mergers, we also find a considerably high fraction of very close nucleus pairs (e.g., 2/3 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferentially host ULIRGs.

Selected with the primary objective of studying the differences between BL Lacs and their closest relatives, a sample of 55 optically bright, low luminosity core dominated radio sources has been studied. The spectroscopy carried out for this sample, however, yielded not only BL Lacs but also a completely unexpected number of sources with strong emission lines (much like those found in Sy galaxies), and almost 50% of galaxies with weak (EW ≤ 30Å), or absent emission lines. Such optical diversity is at odds with the traditional unification schemes for low luminosity AGN and it has prompted an optical imaging campaign. High resolution optical images obtained at NOT are discussed, in particular the central colours and profiles of the different types of sources found in the sample.