P. Thomasson

Compact steep-spectrum sources from the S4 sample

We present the results of 5-GHz observations with the VLA A-array of a sample of candidate compact steep-spectrum (CSS) sources selected from the S4 survey. We also estimate the symmetry parameters of high-luminosity CSS sources selected from different samples of radio sources, and compare these with the larger sources of similar luminosity to understand their evolution and the consistency of the CSS sources with the unified scheme for radio galaxies and quasars. The majority of CSS sources are likely to be young sources advancing outwards through a dense asymmetric environment. The radio properties of CSS sources are found to be consistent with the unified scheme, in which the axes of the quasars are observed close to the line of sight, while radio galaxies are observed close to the plane of the sky.

Signatures of restarted activity in core-dominated, triple radio sources selected from the FIRST survey

Signatures of the re-occurrence of activity in radio-loud AGNs, indicated either by the so-called double-double or X-shaped structures, have been observed in a number of radio sources. All such objects known to date have linear sizes of the order of a megaparsec. A number of the sources that are appreciably more compact than this, but that exhibit hints of a past phase of activity, were found in the VLA FIRST survey. Their structures show symmetric relic lobes straddling relatively bright, unresolved cores. Observations of the cores of 15 such structures with MERLIN at 5 GHz have shown that four of them are doubles or core-jets on the subarcsecond scale. Misalignments of ∆PA > 30 ◦ between the axis of the inner structure and the line connecting the fitted maxima of the arcminute-scale relic lobes are clearly visible in three of the four sources. From these results, we can infer that a rapid repositioning of the central engine in each of these three radio sources is the most plausible interpretation of the observed morphology and that a merger is most likely the original cause of such a repositioning. In the case of TXS 1033+026, the optical image extracted from the SDSS archives clearly suggests that two objects separated by only 2.7 kpc (projected onto the sky plane) are indeed merging. The inner parts of TXS 0818+214 and TXS 1312+563 could be interpreted as double-lobed, and consequently, these sources could be of the double-double type; but further multifrequency observations are necessary to provide support for such an interpretation.

3C 459: a highly asymmetric radio galaxy with a starburst

Multifrequency radio observations of the radio galaxy 3C 459 using MERLIN, VLA and the EVN and an optical Hubble Space Telescope (HST) image using the F702W filter are presented. The galaxy has a very asymmetric radio structure, a high infrared luminosity and a young stellar population. The eastern component of the double-lobed structure is brighter, much closer to the nucleus and is significantly less polarized than the western one. This is consistent with the jet on the eastern side interacting with dense gas, which could be due to a merged companion or dense cloud of gas. The HST image of the galaxy presented here exhibits filamentary structures and is compared with the MERLIN 5-GHz radio map. EVN observations of the prominent central component, which has a steep radio spectrum, show a strongly curved structure suggesting a bent or helical radio jet. The radio structure of 3C 459 is compared with other highly asymmetric, Fanaroff‐Riley II radio sources, which are also good candidates for studying jet‐cloud interactions. Such sources are usually of small linear size and it is possible that the jets are interacting with clouds of infalling gas that fuel the radio source.

FIRST-based survey of Compact Steep Spectrum sources IV. Multifrequency VLBA observations of very compact objects

Context. Evidence has been mounting recently that activity in some radio-loud AGNs (RLAGNs) can cease shortly after ignition and that perhaps even a majority of very compact sources may be short-lived phenomena because of a lack of stable fuelling from the black hole. Thus, they can fade out before having evolved to large, extended objects. Re-ignition of the activity in such objects is not rul ed out. Aims. With the aim of finding more examples of these objects and to in vestigate if they could be RLAGNs switched off at very early stages of their evolution, multifrequency VLBA observations of six sources with angular sizes significantly less than an arcse cond, yet having steep spectra, have been made. Methods. Observations were initially made at 1.65 GHz using the VLBA with the inclusion of Effelsberg telescope. The sources were then re-observed with the VLBA at 5, 8.4 and 15.4 GHz. All the observations were carried out in a snapshot mode with phase referencing. Results. One of the sources studied, 0809+404, is dominated by a compact component but also has diffuse, arcsecond-scale emission visible in VLA images. The VLBI observations of the “core” structure have revealed that this is also diffuse and fading away at higher frequencies. Thus, the inner component of 0809+404 could be a compact fading object. The remaining five sourc es presented here show either core-jet or edge-brightened double-lobed structures indicating that they are in an active phase. Conclusions. The above result is an indication that the activity of the hos t galaxy of 0809+404 may be intermittent. Previous observations obtained from the literature and those presented here indicate that activity had ceased once in the past, then restarted , and has recently switched off again.

CSSs in a sample of B2 radio sources of intermediate strength

We present radio observations of 19 candidate compact steep-spectrum (CSS) objects selected from a well-defined, complete sample of 52 B2 radio sources of intermediate strength. These observations were made with the VLA A-array at 4.835 GHz. The radio structures of the entire sample are summarised and the brightness asymmetries within the compact sources are compared with those of the more extended ones, as well as with those in the 3CRR sample and the CSSs from the B3-VLA sample. About 25 per cent of the CSS sources exhibit large brightness asymmetries, with a flux density ratio for the opposing lobes of>5, possibly due to interaction of the jets with infalling material. The corresponding percentage for the larger-sized objects is only about 5 per cent. We also investigate a possible dependence of the flux density asymmetry of the lobes on redshift, since this might be aected by more interactions and mergers in the past. No such dependence is found. A few individual objects of interest are discussed in the paper.

Merlin Astrometry of 11 Radio Stars

We report accurate positions in the International Celestial Reference Frame (ICRF) for 11 radio stars. Observations were made using the Multi-Element Radio Linked Interferometer Network at a radio frequency of 5 GHz. The positions are estimated to be accurate at the 5 mas level. Positions were obtained directly in the ICRF by phase referencing the radio stars to ICRF quasars whose positions are estimated to be accurate at the 0.25 mas level. We use our results together with results of previous observations to obtain proper-motion estimates for these stars. The average proper-motion uncertainties are 1.1 mas yr-1 in μα cos δ and 1.2 mas yr-1 in μδ, comparable to the Hipparcos values.

The Enigmatic Radio Source T Tauri S

We have observed the T Tau multiple star system using the Multielement Radio-linked Interferometer Network at a frequency of 5 GHz. The compact centimeter wavelength radio emission from T Tau S has been found to consist of at least two components separated by approximately 70 mas in right ascension at Julian epoch 2001.89. Using this data and that previously published, we show that the most intense, compact radio emission may not all be coincident in position but associated with the M-type star in the binary system T Tau S. This emission may be due to magnetic reconnection processes in the YSO system. The estimated masses of the stars using the Hipparcos parallax are 4.0 M☉ for Sa and 1.0 M☉ for Sb. However, using the currently accepted parallax for the Taurus-Auriga complex of 140 pc, the value of these masses is reduced by a factor of 2. This is consistent with T Tau Sa being a pre-main-sequence star.

MERLIN observations of Stephan's Quintet

We present MERLIN L-band images of the compact galaxy group, Stephans Quintet (SQ). The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact group, is seen to have a triple radio structure typical of many extra-galactic radio sources that have a flat spectrum core and two steep spectrum lobes with hot spots. The two lobes are asymmetrically distributed on opposite sides of the core along the minor axis of the galaxy. Ultraviolet (UV) emission revealed in a high-resolution channel (HRC)/ACS Hubble Space Telescope (HST) image is strongly aligned with the radio plasma and we interpret the intense star formation in the core and north lobe as an event induced by the collision of the north radio jet with over-dense ambient material. In addition, a remapping of archive Very Large Array (VLA) L-band observations reveals more extended emission along the major axis of the galaxy, which is aligned with the optical axis. Images formed from the combined MERLIN and archive VLA data reveal more detailed structure of the two lobes and hot spots.

The emission structure of formaldehyde megamasers

The formaldehyde megamaser emission has been mapped for the three host galaxies IC 860, IRAS 15107+0724 and Arp 220. Elongated emission components are found at the nuclear centres of all galaxies with an extent ranging between 30 and 100 pc. These components are superposed on the peaks of the nuclear continuum. Additional isolated emission components are found superposed in the outskirts of the radio continuum structure. The brightness temperatures of the detected features ranges from 0.6 to 13.4 x 10(4) K, which confirms their masering nature. The masering scenario is interpreted as amplification of the radio continuum by foreground molecular gas that is pumped by far-infrared radiation fields in these starburst environments of the host galaxies.

A survey of Low Luminosity Compact sources

Based on the FIRST and SDSS catalogues a flux density limited sample of weak Compact Steep Spectrum (CSS) sources with radio luminosity below 1026WHz–1 at 1.4 GHz has been constructed. Our previous multifrequency observations of CSS sources have shown that low luminosity small-scale objects can be strong candidates for compact faders. This finding supports the idea that some small-size radio sources are short-lived phenomena because of a lack of significant fuelling. They never ‘grow up’ to become FRI or FRII objects. This new sample marks the start of a systematic study of the radio properties and morphologies of the population of low luminosity compact (LLC) objects. An investigation of this new sample should also lead to a better understanding of compact faders. In this paper, the results of the first stage of the new project – the L-band MERLIN observations of 44 low luminosity CSS sources are presented (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)