Using large radio telescopes at decametre wavelengths

With the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below ), an extensive program of radio observations was carried out, in 1999–2002, by using digital spectral and waveform analysers (DSP) of new generation, connected to several of the largest, decametre radio telescopes in the world (i.e., the UTR-2 and URANs arrays in Ukraine, and the Nançay Decametre Array in France).

We report and briefly discuss some new findings, dealing with decametre radiation from Jupiter and the Solar Corona: namely the discovery of new kinds of hyper fine structures in spectrograms of the active Sun, and a new characterisation of Jupiter's “millisecond” radiation, whose waveform samples, with time resolution down to 40 ns, and correlated measurements, by using far distant antennas (3000 km), have been obtained. In addition, scattering effects, caused by the terrestrial ionosphere and the interplanetary medium, could be disentangled through high time resolution and wide-band analyses of solar, planetary and strong galactic radio sources. Consequences for decametre wavelength imaging at high spatial resolution (VLBI) are outlined. Furthermore, in spite of the very unfavourable electromagnetic environment in this frequency range, a substantial increase in the quality of the observations was shown to be provided by using new generation spectrometers, based on sophisticated digital techniques. Indeed, the available, high dynamic range of such devices greatly decreases the effects of artificial and natural radio interference. We give several examples of successful signal detection in the case of much weaker radio sources than Solar System ones, down to the intensity level.

In summary, we conclude that searching for sensitivity improvement at the decametre wavelength is scientifically quite justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). In this task, one must be careful of some specifics of this wavelength range—somewhat unusual in “classical” radio astronomy—i.e., very high level and density of radio interference (telecommunications) and the variable terrestrial ionosphere.     

Pulsating radio emission at decametre wavelengths from the sun

Observations on the pulsation pattern in the time profile of short duration solar radio bursts at decametre wavelengths are presented. The pulsations are found to be present predominantly in the saturation phase of the burst. A tentative physical model based on the non-linear development of the waves interacting in a turbulent medium is invoked to explain the origin of the pulsations.     

GMRT detection of a New Wide-Angle Tail (WAT) radio source associated with the galaxy PGC 1519010

We report the serendipitous detection of a Wide-Angle Tail (WAT) radio galaxy at 240 and 610 MHz, using the Giant Metrewave Radio Telescope (GMRT). This WAT is hosted by a cD galaxy PGC 1519010 whose photometric redshift given in the SDSS DR6 catalogue is close to the spectroscopic redshifts (0.105, 0.106 and 0.107) of three galaxies found within 4′ of the cD. Using the SDSS DR6, we have identified a total of 37 galaxies within 15′ of the cD, whose photometric redshifts are between 0.08 and 0.14. This strongly suggests that the cD is associated with a group of galaxies whose conspicuous feature is a north-south chain of galaxies (filament) extending to at least 2.6 Mpc. The ROSAT all-sky survey shows a faint, diffuse X-ray source in this direction, which probably marks the hot intracluster gas in the potential well of this group. We combine the radio structural information for this WAT with the galaxy clustering in that region to check its overall consistency with the models of WAT formation. The bending of the jet before and after its disruption forming the radio plume, are found to be correlated in this WAT, as seen from the contrasting morphological patterns on the two sides of the core. Probable constraints imposed by this on the models of WAT formation are pointed out. We also briefly report on the other interesting radio sources found in the proximity of the WAT. These include a highly asymmetric double radio source and an ultra-steep spectrum radio source for which no optical counterpart is detected in the SDSS.     

Particle reacceleration in the Coma cluster: radio properties and hard X-ray emission

The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions.
We propose a two-phase model in which the relativistic electrons injected in the Coma cluster by some processes (starbursts, AGNs, shocks, turbulence) during a first phase in the past are systematically reaccelerated during a second phase for a relatively long time (∼1 Gyr) up to the present time. We show that for reacceleration time-scales of ∼0.1 Gyr this hypothesis can well account for the radio properties of Coma C. For the same range of parameters which explain Coma C we have calculated the expected fluxes from the inverse Compton scattering of the Cosmic Microwave Background (CMB) photons, finding that the hard X-ray tail discovered by BeppoSAX may be accounted for by the stronger reacceleration allowed by the model.
The possibility of extending the main model assumptions and findings to the case of the other radio haloes is also discussed, the basic predictions being consistent with the observations.     

Search for H I emission from superdisk candidates associated with radio galaxies

Giant gaseous layers(termed “superdisks”) have been hypothesized in the past to account for the strip-like radio emission gap(or straight-edged central brightness depression) observed between twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common. If they are entirely or partly composed of neutral gas, then it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive H I signal in any of the four sources, resulting in tight upper limits on the H I number density in the postulated superdisks,estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are 10^-4-10^-3 atoms per cm3 and 10^19-10^20 atoms per cm^2, respectively. No positive H I signal is detected even after combining all the four available H I images(with inverse variance weighting). This clearly rules out an H I dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open.     

The asymmetry of the Coma cluster of galaxies

A detailed reinvestigation of the Coma cluster basing on enlarged observational material of number density distributions and radial velocities of galaxies as well as radio and x-ray data is performed and supports our previous conclusion that this cluster possesses a substructure. This substructure is in the south-western region of the cluster and produces the impression of an elongated cluster structure in the direction of the bridge between Coma and A 1367. A 1656 seems to be a cD-like cluster with NGC 4874 as the only dominating giant central galaxy.     

Radio emission at 1.4 GHz from luminous compact galaxies

We analyse the properties of the 1.4 GHz continuum emission in 52 star‐forming compact luminous galaxies (LCGs). The fluxes of the 1.4 GHz thermal (free‐free) component are derived from the extinction‐ and aperture‐corrected fluxes of the Hα emission line. The fraction of the thermal 1.4 GHz emission is shown tobe in the range of 2–52 % with a median value of about 15–17 %, and its distribution is close to a log‐normal one. Both the thermal and non‐thermal radio continuum luminosities are shown to be proportional to the masses of the young stellar population of the LCGs. We discuss the approximation of the ratio of the 1.4 GHz luminosity‐to‐the mass of the young stellar population as a function of starburst age. The derived approximation is shown to be similar but less clearly defined compared to ones obtained previously for LCGs from their Hα and UV continuum luminosities. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of clusters of galaxies. III. An U band photometry in the central part of the Coma cluster

A photographic photometry in U was performed for 288 galaxies in the central field of the Coma cluster of galaxies.     

Compact radio cores and the relation between the radio and optical axes of elliptical galaxies

We have reinvestigated the reported tendency for the extended radio structures associated with bright elliptical galaxies to be oriented preferentially along the optical minor axes. It is found that such a tendency exists only for those galaxies in which the compact radio cores coincident with their nuclei are quite prominent. If the galaxies are divided into two groups according to whether their cores account for less than or greater than 10 per cent of the total flux density at 2.7 GHz, the angle Φ (between the radio axis and the optical minor axis) appears to be uniformly distributed between 0‡ and 90‡ for the former, but is nearly always < 30‡ for the latter group. One possible explanation is that the radio emission from compact cores suffers thermal absorption by ionized gas that is distributed differently in the two groups.     

The cluster environments of the z ∼ 1 3CR radio galaxies

An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6< z <1.8 is presented, based primarily upon K -band images down to K ∼20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K -band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular cross-correlation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters.
These data are complemented by J -band images also from UKIRT, and by optical images from the Hubble Space Telescope . The fields of the lower redshift ( z ≲0.9) radio galaxies in the sample generally show well-defined near-infrared colour–magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000 Å break show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or on-going star formation. At higher redshifts the colour–magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very red infrared colours ( J − K ≳1.75) expected of old cluster galaxies at these redshifts.
Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z ∼1 radio galaxies lie in (proto)cluster environments.     

On the question of radio emission of spiral galaxies in groups of galaxies

It has been shown that the radio emission properties of spiral galaxies, if the other conditions are the same, are determined rather by the presence of the close neighbours than by space density of galaxies around them. The rate of occurence of radio sources and their radio luminosities among the spiral members of groups of galaxies depend on the projected seperation between them and their nearest neighbour. The shorter this seperation the higher the probability of radio emission.