R. T. Schilizzi

Observations of H i absorbing gas in compact radio sources at cosmological redshifts

We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z = 0.85. Searches for H i 21cm absorption were made with the Westerbork Synthesis Radio Telescope at UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from � = 0.16 to � � 0.001. There is a substantial variety of line profiles, including Gaussians of less than 10kms 1 , to more typically 150kms 1 , as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred kms 1 . Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1×10 19 and 3.3×10 21 (Tsp/100 K)(1/f) cm 2 . There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the H i velocity is substantially negative (up to v = 1420kms 1 ) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.

VLBI astrometry of circumstellar OH masers: Proper motions and parallaxes of four AGB stars

The main-line OH masers around 4 AGB stars have been observed with the NRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5 yrs. Using a phase referencing technique, the position of the most compact maser spot of each star was monitored with respect to two extragalactic reference sources. For U Her and W Hya, we observe the most blue-shifted maser spot, while for R Cas and S CrB we only detect a compact red-shifted maser spot. We managed to determine an accurate proper motion and parallax for U Her, R Cas and S CrB, while additional motion of the compact blue-shifted maser of W Hya is shown to possibly be related to the stellar pulsation. The motion and radio position are compared with the stellar trajectory and absolute optical position determined by the Hipparcos satellite. For U Her and W Hya, the most blue-shifted maser is consistent with the amplified stellar image. The new distances are compared with several published P−L relations, and in this respect the VLBI distances seem an improvement upon the Hipparcos distances.

A new sample of faint Gigahertz Peaked Spectrum radio sources

The Westerbork Northern Sky Survey (WENSS) has been used to select a sample of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two orders of magnitude lower than bright GPS sources investigated in earlier studies. Sources with inverted spectra at frequencies above 325 MHz have been observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to select genuine GPS sources. This has resulted in a sample of 47 GPS sources with peak frequencies ranging from similar to 500 MHz to > 15 GHz, and peak flux densities ranging from similar to 40 to similar to 900 mJy. Counts of GPS sources in our sample as a function of flux density have been compared with counts of large scale sources from WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The counts can be made similar if the number of large scale sources at 2 GHz is divided by 250, and their flux densities increase by a factor of 10. On the scenario that all GPS sources evolve into large scale radio sources, these results show that the lifetime of a typical GPS source is similar to 250 times shorter than a typical large scale radio source, and that the source luminosity must decrease by a factor of similar to 10 in evolving from GPS to large scale radio source. However, we note that the redshift distributions of GPS and large scale radio sources are different and that this hampers a direct and straightforward interpretation of the source counts. Further modeling of radio source evolution combined with cosmological evolution of the radio luminosity function for large sources is required.

Optical spectroscopy of faint gigahertz peaked-spectrum sources

We present spectroscopic observations of a sample of faint gigahertz peaked-spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS), Redshifts have been determined for 19 (40 per cent) of the objects. The optical spectra of the GPS sources identified with low-redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by active galactic nucleus-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep-spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z similar to 2-3, It is unlikely that a significant population of low-redshift GPS quasars is missed as a result of selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift-luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio spectral properties, and hypothesize that GPS quasars are a subclass of flat-spectrum quasars.

VLBI, MERLIN and HST observations of the giant radio galaxy 3C 236

We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C 236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of similar to5 degrees can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C 236 by de Koff et al. (2000), shows that the normal to the dust disk in the nucleus is similar to 30 degrees from the plane of the sky and within 12 degrees of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of similar to 30 degrees to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc (H-0 = 75 kms(-1) Mpc(-1), q(0) = 0.5) for 3C 236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first similar to 400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at less than or equal to 35 degrees to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness. At the resolution of WSRT data at 327 MHz, the jet to the south-east is apparently continuous over a distance of 2.5 Mpc, making this the largest jet known in the universe. It is likely, however, that activity in the nucleus of 3C 236 is episodic but with a shorter duty cycle than in the double-double sources studied by Schoenmakers et al. (2000) and Kaiser et al. (2000).We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of about 5 degrees can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C236 by de Koff et al. (2000), shows that the normal to the dust disk in the nucleus is ~30 degrees from the plane of the sky and within 12 degrees of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of ~30 degrees to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc (H_o = 75 km/s/Mpc, q_o = 0.5) for 3C236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first ~400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at<= 35 degrees to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness.

Polarimetry of GPS and CSS Sources

The role of radio polarimetry in the understanding of GPS and CSS sources is explored. After an initial discussion of what can be learned from polarimetry, the expectations of a simple evolutionary sequence of GPS/CSO to CSS to FR i/FR ii sources are explored. Observational results are then compared with the expectations. Conclusions include: the GPS category is likely not a single homogeneous class of objects; Faraday depth effects are very strong inside the inner 3 kpc of CSS and CSO sources; in at least 3C 138 the Faraday screen has very fine scale (subparsec) structure; and there is evidence for increased ionisation near bends in some CSS jets probably due to jet-ISM interaction. New results on 3C 138, 3C 43, and 3C 454 are given.

A Parkes half-Jansky sample of GHz peaked spectrum galaxies

This paper describes the selection of a new southern/equatorial sample of gigahertz peaked spectrum (GPS) radio galaxies, and subsequent optical charge-coupled device imaging and spectroscopic observations using the ESO 3.6-m telescope. The sample consists of 49 sources with −40 ◦ 20 ◦ and S2.7 GHz >0.5 Jy, selected from the Parkes PKSCAT90 survey. Approximately 80 per cent of the sources are optically identified, and approximately half of the identifications have available redshifts. The R-band Hubble diagram and evolution of the host galaxies of GPS sources are reviewed.

Fundamental galaxy parameters for radio-loud active galactic nuclei and the black hole-radio power connection

We have determined the central velocity dispersions and surface brightness profiles for a sample of powerful radio galaxies in the redshift range 0.06 < z < 0.31, which were selected on the basis of their young radio source. The optical hosts follow the fundamental plane of elliptical galaxies, showing that young radio sources reside in normal ellipticals, as do other types of radio galaxies into which these objects are believed to evolve. As young radio sources are relatively straightforward to select and the contributions of the active galactic nuclei (AGN) light to the optical spectra are minimal, these objects can readily be used to study the evolution of the fundamental plane of elliptical galaxies out to z = 1, independent of optical selection effects. The black hole masses, M b h , of the objects in our sample have been determined using the tight empirical relation of M b h with central velocity dispersion, σ e , and for literature samples of classical radio galaxies and optically selected ellipticals. Only the optically selected inactive galaxies are found to exhibit a correlation between M b h and radio luminosity. In contrast, the radio powers of the AGN in the samples do not correlate with M b h at all, with objects at a given black hole mass ranging over seven orders of magnitude in radio power. It is unlikely that beaming of the radio emission is responsible for the range in radio powers. We have been able to tie in the local population of powerful radio sources with its parent population of inactive elliptical galaxies: the local black hole mass function has been determined using the elliptical galaxy luminosity function and the Faber-Jackson and M b h -σ relations. This was combined with the fraction of radio-loud black holes as a function of M b h , as determined from the optically selected galaxy sample, to derive the local volume-density of radio galaxies and the distribution of their black hole masses. These are shown to be consistent with the local radio luminosity function and the distribution of black hole masses as found in the radio-selected samples, and confirms that elliptical galaxies comprise the large majority of the radio-loud population of active galaxies.

Constraining the Evolution of Young Radio-Loud AGN

GPS and CSS radio sources are the objects of choice to investigate the evolution of young radio- loud AGN. Previous investigations, mainly based on number counts and source size distributions, indicate that GPS/CSS sources decrease significantly in radio power when evolving into old, extended objects. We suggest this is preceded by a period of increase in radio luminosity, which lasts as long as the radio source is confined within the core-radius of its host galaxy. We have selected a sample of nearby compact radio sources, unbiased by radio spectrum, to determine their luminosity function, size distribution, dynamical ages, and emission line properties in a complete and homogeneous way. First results indicate that the large majority of objects (>80%) exhibit classical GPS/CSS radio spectra, and show structures consistent with them being compact double or compact symmetric objects. This sample provides an ideal basis to further test and constrain possible evolution scenarios, and to investigate the relation between radio spectra and morphologies, orientation and Doppler boosting in samples of young radio-loud AGN, in an unbiased way.

Faint Gigahertz Peaked Spectrum sources and the evolution of young radio sources

Abstract GPS sources are the objects of choice to study the initial evolution of extragalactic radio sources, since it is most likely that they are the young counterparts of large scale radio sources. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. The difference in redshift distribution between young and old radio sources must be due to a difference in slope of their luminosity functions, and we argue that this slope is strongly affected by the luminosity evolution of the individual sources. A luminosity evolution scenario is proposed in which GPS sources increase in luminosity and large scale radio sources decrease in luminosity with time. It is shown that such a scenario agrees with the local luminosity function of GPS galaxies.