R. Morganti

The nature of the optical-radio correlations for powerful radio galaxies

The nature of the optical–radio correlations for powerful radio galaxies is investigated using spectroscopic observations of a complete sample of southern 2-Jy radio sources. In line with previous work, we find that significant correlations exist between the luminosities of the [Oxa0III]λxa05007, [Oxa0II]λxa03727 and Hβ emission lines and the radio luminosity. However, our observations are not easily reconciled with the idea that these correlations are caused by the increase in the power of the photoionizing quasar as the jet power increases, with average ISM properties not changing appreciably with redshift or radio power: not only do we find that the scatter in the L[Oxa0iii] versus Lradio correlation is significantly larger than in L[Oxa0ii] versus Lradio and LHβ versus Lradio correlations, but the ionization state deduced from the emission lines does not increase with radio power as predicted by the simple, constant ISM, photoionization model. We conclude that (a) there exists a considerable range in the quasar ionizing luminosity at a given redshift, and (b) the mean density of the emission-line clouds is larger in the high-redshift/high-power radio sources. The latter density enhancement may be either a consequence of the increased importance of jet–cloud interactions or, alternatively, the result of a higher pressure in the confining hot ISM, in the high-redshift objects. xa0Apart from the general scatter in the correlations, we identify a distinct group of objects with [Oxa0III]λxa05007 luminosities which are more than an order of magnitude lower than in the general population radio galaxies at similar redshift. These weak-line radio galaxies (WLRGs) are likely to be sources in which the central ionizing quasars are particularly feeble. xa0Deep spectra show that many of the sources in our sample are broad-line radio galaxies (BLRGs). The fact that the BLRGs are observed out to the redshift limit of the survey, overlapping in redshift with the quasars, argues against the idea that BLRGs are simply the low-radio-power counterparts of high-power, high-redshift quasars. Either there exists a considerable range in the intrinsic luminosities of the broad-line AGN for a given redshift or radio power, or the BLRGs represent partially obscured quasars. The degree of scatter present in the L[Oiii] versus Lradio correlation supports the former possibility.

Centaurus A: multiple outbursts or bursting bubble?

We present new radio observations of the brighter region of the northern lobe (the northern middle lobe, NML) of Centaurus A obtained at 20xa0cm with the Australia Telescope Compact Array. The angular resolutions are ∼50 and ∼130xa0arcsec, therefore much higher than for the previously available radio images of this region. The most interesting feature detected in our images is a large-scale jet that connects the inner radio lobe and the NML, and which is imaged for the first time. The NML itself appears as diffuse emission with a relatively bright ridge on the eastern side. n n n nThe radio morphology of Centaurus A and, in particular, its NML could be the result of a precessing jet that has undergone a strong interaction with the environment at least on the northern side. The very big drop in intensity between the inner jet and the large-scale jet can be explained with a sequence of bursts of activity at different epochs in the life of the source. Alternatively (or additionally) a ‘bursting bubble’ model is proposed to explain this big drop in intensity, which could also explain the good collimation of the large-scale jet. In this model, the plasma accumulated in the inner lobe would be able to ‘burst’ out only through one nozzle, which would be the region where the large-scale jet forms. The location of the nozzle would represent a region where the pressure gradient is more favourable. n n n nFrom the comparison between the radio emission and the regions of ionized gas discovered by Graham & Price (the so-called optical filaments) we find that the inner optical filament (∼8xa0kpc from the centre) falls about 2xa0arcmin (∼2xa0kpc) away from the large-scale radio jet. Thus, the inner filament does not seem to have experienced a direct interaction with the radio plasma. The complex velocity field observed in this filament could therefore be the result of strong instabilities produced by the ‘bursting bubble’. The outer filaments appear to be, in projection, closer to and aligned with the radio emission in the transition region between the jet and the lobe, suggesting a direct interaction with the radio jet. However, also in this case a more complicated interaction than assumed so far has to be occurring because of the relative position of the ionized and neutral gas regions compared with the radio jet as well as the kinematics of the ionized gas.

The soft X-ray properties of a complete sample of radio sources

We present the soft X-ray (0.1--2.4 keV) properties of a complete sample of 88 southern radio sources derived from the Wall & Peacock 2-Jy sample. It comprises 68 radio galaxies, 18 quasars and 2 BL Lac objects. Whereas both BL Lac objects and all but one quasar are detected in the {it ROSAT} All-Sky Survey, the fraction of detected radio galaxies is only

Radio jet interactions in the radio galaxy PKS 2152–699

sim

Radio Continuum and Emission Line Morphologies of Southern Seyfert Galaxies

60 per cent. For the undetected sources upper limits to the X-ray flux are given. We confirm the correlation of the soft X-ray luminosity (lx) with the core radio luminosity (lc) for galaxies as well as for quasars using partial correlation analysis, whereas the corresponding correlations between lx and lt are probably spurious due to sample selection effects. We also find strong correlations between lx and lc for both Fanaroff-Riley type I (FR~I) and type II galaxies. The broad-line radio galaxies (BLRGs) and the quasars are at the top end of the X-ray luminosity distribution and the detection rate of these objects generally is higher than that of the narrow- or weak-lined radio galaxies. This indicates the presence of an anisotropic X-ray component in BLRGs and quasars, as predicted by unified schemes for radio sources.

2-JY Sample of Southern Radio Sources

We present radio observations of the radio galaxy PKS 2152–699 obtained with the Australia Telescope Compact Array. The much higher resolution and signal-to-noise ratio of the new radio maps reveal the presence of a bright radio component about 10 arcsec north-east of the nucleus. This lies close to the highly ionized cloud previously studied in the optical and here shown in a broad-band red snapshot image with the HST PC 2. It suggests that PKS 2152–699 may be a jet/cloud interaction similar to 3C 277.3. This could cause the change in the position angle (of ∼xa020°) of the radio emission from the inner to the outer regions. On the large scale, the source has Fanaroff & Riley type II morphology although the presence of the two hotspots in the centres of the lobes is unusual. The northern lobe shows a particularly relaxed structure while the southern one has an edge-brightened, arc-like structure.

A Powerful Jet/Cloud Interaction in the Radio Galaxy PKS 2250-41

We have collected emission line ([Oiii] 5007 and Halpha+[Nii]) and radio continuum images for a volume limited (cz <= 3600 km/s) sample of southern Seyfert galaxies. The optical data were obtained at ESO using the 2.2 m, NTT and 3.6 m telescopes, and have a typical resolution of 1 and noise level of ~1e-16 ergs cm-2 s-1 arcsec-2. The radio continuum observations were obtained with the VLA (6 cm) and ATCA (3 cm) and have a resolution of ~1 matching that of the optical images. All, but one of the observed sources were detected above the noise limit of ~0.15 mJy. nIn the radio, 30% of the sources show linear structure, 25% are only slightly resolved or diffused, and 45% remain unresolved at the ~1 resolution. As in previous work, a correlation is found between the size of the radio structure and the radio power. The radio sources in Seyfert 2 galaxies have, on average, larger linear size than their type 1 counterparts, but there is no significant difference in radio power between types 1 and 2. No significant difference is found in the spectral indices of the two Seyfert types. nExtended emission is common in Seyfert galaxies -- essentially all objects observed show extended Halpha+[Nii] and nearly 50% show extended [Oiii] emission. At least 40% (18 out of 50) of the galaxies show high excitation extended emission line region (EELR) well outlined in the excitation map. The EELR morphology vary from linear to conical to S-shaped and even X-shaped. Almost exclusively all elongated EELR are in Seyfert 2 galaxies. The orientation of the EELR appears to be random relative to either the major (or minor) axis or relative to the non-axisymmetric structures, such as bars or ovals, when present, and there is a hint of relation to the morphological type.

The nebular contribution to the extended UV continua of powerful radio galaxies

We have collected multi-waveband (radio, optical and X-ray) data for a complete sample of southern radio sources. The sample includes 88 objects selected from the Wall & Peacock (1985) catalogue that is complete down to S 2.7GHz = 2 Jy, δ < 10° and the Z < 0.7. This database (Tadhunter et al. (1993), Morganti et al. (1993), and Siebert et al. these Proceedings) provides an important tool for investigating the nature of anisotropics and orientation effects in AGN and the physical causes of the correlation between their emission at different frequencies.

A study of cores in a complete sample of radio sources

Extended emission line regions aligned with the radio axis are a common feature of powerful radio galaxies and there is much interest in the origin of the extended gas and excitation mechanism. One model that can produce this alignment is photoionization by anisotropic nuclear continuum radiation. However, strong evidence exists, especially in high redshift radio galaxies, for powerful interactions between the relativistic radio jets and the ISM/IGM. Here we present the results of our study of the southern radio galaxy PKS 2250-41 (z = 0.308). This object is the most spectacular found in a sample of southern radio sources studied by Tadhunter et al. (1993) and it displays particularly clear evidence for such an interaction (Tadhunter et al. 1994; Dickson et al. 1995).