We review the evidence that spectral curvature in the extended emission of radio galaxies is caused by synchrotron losses, and that the spatial variation can be interpreted to yield ages and expansion speeds. One of the biggest worries has been the true value of the magnetic field, but X-ray detections of inverse-Compton radiation are beginning to suggest that “minimum energy” estimates are remarkably accurate. A critical test is to compare model and observed spectra over a broad frequency range; to date this has has only been done for Cygnus A, and the results proved controversial. Here we discuss several more cases and begin to draw some general conclusions.

Hotspots are usually well fitted by continuous injection models, as expected. In two cases the implied injection index is flatter than 0.5, too flat to be produced by standard Fermi acceleration in a non-relativistic shock. The bridge spectra are reasonably fitted by single-burst models, but in some objects the injection index is not constant across the lobes, showing instead a tendency to steepen in the inner bridge, where the break frequencies are lowest. The true spectral shape may be a more gradual curve than the standard models, possibly because of mixing of electron populations with different ages. Our results are limited by the inaccuracy of the absolute flux density scale, especially at low frequencies, and a fresh attack on the flux scale would be timely.

Low-radio-frequency observations played a remarkable role in the early days of radio astronomy; however, in the subsequent three or four decades their usefulness has largely been in terms of the finding-frequency of surveys. Recent technical innovation at the VLA has meant that spatially well-resolved imaging at low frequencies is now possible. Such imaging is essential to understanding the relationship between the hotspot and lobe emission in classical double radio sources, for example. We here present new images of 3C radio sources at 74 MHz and 330 MHz and discuss their implications.

There is some evidence from earlier studies that the two sources 0235 — 197 and 1203 + 043 exhibit low frequency (< 1 GHz) variability. This work shows that both sources have linear polarizations, if any, below the detection limits at 320 MHz, so we cannot explain the variability as being due to instrumental polarization effects as has been suggested for 3C159. Refractive scintillation may be the cause of the variability in 0235—197. The radio source 1203+043 lacks any bright compact component thereby ruling out a refractive scintillation mechanism for its variability. Consequently, it is possible that claims of variability in this source are spurious. However, the 320 MHz VLA observations show that 1203+043 has an ‘X'-shaped radio structure.

Using the WENSS survey, we have selected a complete sample of 26 Giant Radio Galaxies. We have investigated the properties of their radio structures using multifrequency radio observations. A spectral ageing analisys reveals that, among other things, that these sources are typically 50 — 100 Myr old and that their lobes have higher advance velocities than smaller sources of similar radio power. Further, we find pressure gradients in their radio lobes, suggesting that the lobes are still overpressured with respect to the environment. We find no evidence for a cosmological evolution of the radio lobe pressures with increasing redshift, at least up to z ∼ 0.3, other than that caused by our selection technique. A faint, higher redshift sample of giant sources is needed to constrain the pressure in their environments, the IGM. We are well underway with selecting such a sample, and the GMRT can play an important role in studying the radio properties of these sources.

We study the evolution of giant radio sources and show that they are less luminous than the smaller-sized sources, and are roughly consistent with the evolutionary scenarios where the smaller-sized sources evolve to giants undergoing radiative and expansion losses. Inverse-Compton radiation losses due to the microwave background radiation dominates over the synchrotron losses for giant sources, while for the smaller sources, the latter dominates. We show that giant radio galaxies and quasars are consistent with the unified scheme, and their large sizes are not due to stronger cores. We also present preliminary GMRT images of three giant sources.

An important aspects of the studies of giant radio sources is that they can constrain models of the cosmological evolution of radio sources (e.g. Condon 1984, ApJ, 287, 461; Dunlop & Peacock 1990, MNRAS, 247, 19) and their host galaxies, as well as the recent analytical models of time evolution of these sources (e.g. Kaiser, Dennett—Thorpe & Alexander 1997, MNRAS, 292, 723; Blundell et al. 1999, AJ, 117, 677). In order to achieve these goals, we undertook the following tasks:

We present some statistical results on a large sample of radio sources selected from the most important catalogs. Instrument selection effects on this sample are discussed for the first time. Our analysis suggests that the slope of the median spectral index becomes flatter with decreasing flux density. But the slope is rather small.

We suggest that most compact radio sources are young objects advancing outwards through a dense environment which is asymmetric on opposite sides of the nucleus. This gas might be intimately related to the supply of fuel for the young radio source.

Interplanetary scintillation observations of 56 compact steep-spectrum radio sources have been carried out at 102 MHz on the Large Phased Array. Observations have shown that 42 sources have low frequency cut-offs in their spectra. The physical conditions in the compact components of the quasars and radio galaxies studied are estimated.

The angular structure of extragalactic radio sources was studied at decameter wavelengths with resolution up to 1 arcsec using the URAN interferometer network. A extended low frequency halo with steep spectrum is detected for some of them and the known components of others become, as a rule, bigger at these wavelengths.

We describe a method to cross-identify and construct continuum spectra for radio sources from decametric surveys with comparatively large positional errors. The 1822 sources from the UTR-2 catalogue (10—25 MHz, antenna beam of ∼40') were cross-identified with entries from other radio catalogues at higher frequencies. Using the CATS database we found accurate positions for 2314 counterparts to UTR sources, as well as a best-fit radio spectrum for most of these. About 350 (or 19%) of the UTR sources appear to be blends of two or more sources.

We used radio source catalogues accessible from the CATS database to establish radio continuum spectra for decametric radio sources in the UTR-2 catalogue. In an attempt to find further candidates for high-redshift radio galaxies, we searched the FIRST and NVSS surveys for counterparts of a sample of UTR sources with ultra-steep radio spectra (USS, α ≤ −1.2, S ∼ vα). We derived accurate positions and sizes for 23 of these USS sources. The search for optical counterparts from the APM (object) and DSS (image) databases, as well as infrared and X—ray identifications of these UTR sources are in progress.

With the aim to extend a sample of the distant radio objects, about 100 steep spectrum FRII radio galaxies (SS FRII RG) from the RATAN—600 RC catalog were mapped by the VLA1 and identified with optical objects down to R = 24m − 25m with the 6 m telescope. The mean estimated redshift of SS FRII RGs from the RC list happened to be greater than 1.5. BV RI photometry was made, and by standard model fitting we have estimated “colour” redshift and the age of stellar population of the parent gE galaxy in 50 cases. Several objects were found in which active star formation began in the first billion of years after the Big Bang.

We present results of cross—identifications of the objects from two large catalogues, IRAS and the low radio frequency Texas catalogue at 365 MHz in the database CATS. The resulting list consists of 715 pairs of sources within a circle of radius of 60 arcsec. It has been cross—identified with the GB6 4.85 GHz, NVSS 1.4 GHz, FIRST 1.4 GHz, WENSS 325 MHz radio catalogues, ROSAT catalogues and different optical data lists, including APM catalogue. We selected 3 subsamples: objects matching in 3″radius, ultra steep spectrum (USS) radio sources, inverse spectrum radio sources, galactic sources. Several objects have radio spectral index α < −2, some of them in Galactic plane.

We present the nuclear structure of the radio galaxy 3C236 obtained by VLBI and MERLIN observations at both 1.663 and 5 GHz. Comparison with an HST WFPC-2 image of 3C236 suggests that the radio axis is at an angle of ∼ 30° to the plane of the sky, and 3C236 has an overall size of 4.5 Mpc and contains the longest jet known in the universe.

We review the evidence that detectable fluxes of X-rays are produced by inverse Compton scattering of nuclear photons with the relativistic electrons in the radio lobes of strong FRII radio galaxies within the FRII-RL quasar unification scheme. We report here on the possible detection of this effect in two steep spectrum RL quasars. This may have important implications on the physics and evolution of powerful radio galaxies.

In this work we explain the integrated spectrum measurements (Braude et.al.,1978—1995) in terms of the synchrotron theory and the the recent measurements of radio fine structure in different classes of sources (Carilli et.al.,1991). This provides a new method of investigating extragalactic radio sources

There is now abundant evidence for high latitude gas in the Milky Way and external edge-on galaxies. The halo gas is seen in various ISM tracers, including HI, Hα, and radio continuum, and shows considerable structure, including discrete features which span the disk-halo interface. While some correlation with underlying star formation activity has been observed, several energy discrepancies are outlined which suggest that conventional processes related to massive star formation are not sufficient to account for the observed emission. Indeed, new sensitive data are showing that high latitude gas may be a common characteristic of all galaxies, not just those with high star formation rates.

We present preliminary GMRT maps of M 82 and NGC 3079 at 327 MHz and 610 MHz.

Radio recombination lines (RRL) at 8 GHz and 15 GHz detected from four starburst galaxies are shown to arise in compact high density HII regions, which are undetectable below ∼4 GHz. Detection of an RRL at 1.4 GHz towards one galaxy and upper limits in the other three are consistent with the presence of an equal amount of low density diffuse gas. Continuum flux density measurements using the GMRT will be important in constraining the properties of the diffuse gas.