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The jets of radio AGN provide one of the most important forms of active galactic nuclei (AGN) feedback, yet considerable uncertainties remain about how they are triggered. Since the molecular gas reservoirs of the host galaxies can supply key information about the dominant triggering mechanism(s), here we present Atacama Large Millimeter/sub-millimeter Array CO(1-0) observations of a complete sample of 29 powerful radio AGN (⁠|$P_{1.4\,{\rm GHz}} \gt 10^{25}$|  W Hz |$^{-1}$| and |$0.05 \lt z \lt 0.3$|⁠) with an angular resolution of about 2–3 arcsec (corresponding to 2–8 kpc). We detect molecular gas with masses in the range |$10^{8.9} \lt M_{{\rm H}_2} \lt 10^{10.2}$| M |$_\odot$| in the early-type host galaxies of ten targets, while for the other 19 sources, we derive upper limits. The detection rate of objects with such large molecular masses – |$34\pm 9$|  per cent – is higher than in the general population of non-active early-type galaxies (ETGs: |$\lt $| 10 per cent). The kinematics of the molecular gas are dominated in most cases by rotating disc-like structures, with diameters up to 25 kpc. Compared with the results for samples of quiescent ETG in the literature, we find a larger fraction of more massive, more extended and less settled molecular gas structures. In most of the CO-detected sources, the results are consistent with triggering of the AGN as the gas settles following a merger or close encounter with a gas-rich companion. However, in a minority of objects at the centres of rich clusters of galaxies, the accretion of gas cooling from the hot X-ray haloes is a plausible alternative to galaxy interactions as a triggering mechanism. [ABSTRACT FROM AUTHOR]

We present the first systematic search for UV signatures from radio source-driven active galactic nuclei (AGN) feedback in Compact Steep Spectrum (CSS) radio galaxies. Owing to their characteristic sub-galactic jets (1–20 kpc projected linear sizes), CSS hosts are excellent laboratories for probing galaxy scale feedback via jet-triggered star formation. The sample consists of seven powerful CSS galaxies, and two galaxies host to radio sources >20 kpc as the control, at low to intermediate redshifts (z < 0.6). Our new Hubble Space Telescope images show extended UV continuum emission in six out of seven CSS galaxies, with five CSS hosts exhibiting UV knots cospatial and aligned along the radio-jet axis. Young (≲ 10 Myr), massive (≳ 5 M ⊙) stellar populations are likely to be the dominant source of the blue excess emission in radio galaxies at these redshifts. Hence, the radio-aligned UV regions could be attributed to jet-induced starbursts. Lower near-UV star formation rates compared to other indicators suggest low scattered AGN light contribution to the observed UV. Dust attenuation of UV emission appears unlikely from high internal extinction correction estimates in most sources. Comparison with evolutionary synthesis models shows that our observations are consistent with recent (∼1−8 Myr old) star-forming activity likely triggered by current or an earlier episode of radio emission, or by a confined radio source that has frustrated growth, due to a dense environment. While follow-up spectroscopic and polarized light observations are needed to constrain the activity-related components in the observed UV, the detection of jet-induced star formation is a confirmation of an important prediction of the jet feedback paradigm. [ABSTRACT FROM AUTHOR]

The far-IR/sub-mm wavelength range contains a wealth of diagnostic information that is important for understanding the role of active galactic nuclei (AGN) in galaxy evolution. Here we present the results of Herschel PACS and SPIRE observations of a complete sample of 46 powerful 2 Jy radio AGN at intermediate redshifts (0.05 < z < 0.7), which represent the deepest pointed observations of a major sample of radio AGN undertaken by Herschel. In order to assess the importance of non-thermal synchrotron emission at far-IR wavelengths, we also present new APEX sub-mm and ALMA mm data. We find that the overall incidence of non-thermal contamination in the PACS bands (<200  μ m) is in the range 28–43 per cent; however, this rises to 30–72 per cent for wavelengths (>200  μ m) sampled by the SPIRE instrument. Non-thermal contamination is strongest in objects with compact CSS/GPS or extended FRI radio morphologies, and in those with type 1 optical spectra. Considering thermal dust emission, we find strong correlations between the 100 and 160  μ m monochromatic luminosities and AGN power indicators, providing further evidence that radiation from the AGN may be an important heating source for the far-IR emitting dust. Clearly, AGN contamination – whether by the direct emission from synchrotron-emitting lobes and cores, or via radiative heating of the cool dust – needs to be carefully considered when using the far-IR continuum to measure the star formation rates in the host galaxies of radio AGN. [ABSTRACT FROM AUTHOR]

We use deep Herschel observations of the complete 2Jy sample of powerful radio active galactic nuclei (AGNs) in the local Universe (0.05 <  z  < 0.7) to probe their cool interstellar medium (ISM) contents and star-forming properties, comparing them against other samples of nearby luminous AGNs and quiescent galaxies. This allows us to investigate triggering and feedback mechanisms. We find that the dust masses of the strong-line radio galaxies (SLRGs) in our sample are similar to those of radio-quiet quasars, and that their median dust mass (M dust = 2 × 107 M⊙) is enhanced by a factor of ∼200 compared to that of non-AGN ellipticals, but lower by a factor of ∼16 relative to that of local ultraluminous infrared galaxies (ULIRGs). Along with compelling evidence for merger signatures in optical images, the SLRGs in our sample also show relatively high star formation efficiencies, despite the fact that many of them fall below the main sequence for star-forming galaxies. Together, these results suggest that most of our SLRGs have been retriggered by late-time mergers that are relatively minor in terms of their gas contents. In comparison with the SLRGs, the radio AGNs with weak optical emission lines (weak-line radio galaxies – WLRGs) and edge-darkened radio jets (Fanaroff–Riley Class I radio sources – FRIs) have both lower cool ISM masses and star formation rates (by a factor of >30), consistent with being fuelled by a different mechanism (e.g. the direct accretion of hot gas). [ABSTRACT FROM AUTHOR]

Investigation of the triggering mechanisms of radio active galactic nuclei (radio AGN) is important for improving our general understanding of galaxy evolution. In the first paper in this series, detailed morphological analysis of high-excitation radio galaxies (HERGs) with intermediate radio powers suggested that the importance of triggering via galaxy mergers and interactions increases strongly with AGN radio power and weakly with optical emission-line luminosity. Here, we use an online classification interface to expand our morphological analysis to a much larger sample of 155 active galaxies (3CR radio galaxies, radio-intermediate HERGs, and Type 2 quasars) that covers a broad range in both 1.4 GHz radio power and [O  iii ] λ5007 emission-line luminosity. All active galaxy samples are found to exhibit excesses in their rates of morphological disturbance relative to 378 stellar-mass- and redshift-matched non-active control galaxies classified randomly and blindly alongside them. These excesses are highest for the 3CR HERGs (4.7σ) and Type 2 quasar hosts (3.9σ), supporting the idea that galaxy mergers provide the dominant triggering mechanism for these subgroups. When the full active galaxy sample is considered, there is clear evidence to suggest that the enhancement in the rate of disturbance relative to the controls increases strongly with [O  iii ] λ5007 emission-line luminosity but not with 1.4 GHz radio power. Evidence that the dominant AGN host types change from early-type galaxies at high radio powers to late-type galaxies at low radio powers is also found, suggesting that triggering by secular, disc-based processes holds more importance for lower-power radio AGN. [ABSTRACT FROM AUTHOR]

Active galactic nuclei (AGNs) with intermediate radio powers are capable of driving multiphase outflows in galaxy bulges, and are also more common than their high-radio-power counterparts. In-depth characterization of the typical host galaxies and likely triggering mechanisms for this population is therefore required in order to better understand the role of radio AGN feedback in galaxy evolution. Here, we use deep optical imaging data to study the detailed host morphologies of a complete sample of 30 local radio AGNs with high-excitation optical emission (HERG) spectra and intermediate radio powers [ z < 0.1; 22.5 < log(⁠|$L_{\rm 1.4GHz}$|⁠) < 24.0 W Hz-1]. The fraction of hosts with morphological signatures of mergers and interactions is greatly reduced compared to the 2Jy radio-powerful galaxies [log(⁠|$L_{\rm 1.4GHz}$|⁠) > 25.0 W Hz-1] with strong optical emission lines: 53 ± 9 per cent compared with 94 ± 4 per cent. In addition, the most radio-powerful half of the sample has a higher frequency of morphological disturbance than the least radio-powerful half (67 ± 12 per cent and 40 ± 13 per cent, respectively), including the eight most highly disturbed galaxies. This suggests that the importance of triggering nuclear activity in high-excitation radio galaxies (HERGs) through mergers and interactions reduces with radio power. Both visual inspection and detailed light profile modelling reveal a mixed population of early-type and late-type morphologies, contrary to the massive elliptical galaxy hosts of radio-powerful AGNs. The prevalence of late-type hosts could suggest that triggering via secular, disc-based processes has increased importance for HERGs with lower radio powers (e.g. disc instabilities and large-scale bars). [ABSTRACT FROM AUTHOR]

Young radio AGN are pivotal for our understanding of many of the still-debated aspects of AGN feedback. In this paper we present a study of the interstellar medium (ISM) in the compact, peaked-spectrum radio galaxy PKS B1934-63 using X-shooter observations. Most of the warm ionized gas resides within a circum-nuclear disk with a radius of about 200 pc that is likely to constitute the gas reservoir from which the central black hole feeds. On the other hand, we find a biconical outflow of warm ionized gas with an estimated radius of 59 ± 12 pc. This matches the radial extent of the radio source and suggests that the outflow is jet driven. Thanks to the superior wavelength coverage of the data, we can estimate the density of the warm ionized gas using the trans-auroral line technique, and we find that the outflowing gas has remarkably high density, up to log ne (cm−3) ≃ 5.5. The estimated mass outflow rate is low (Ṁ = 10−3 −10−1M⊙ yr−1), and the AGN feedback operates at relatively low efficiency (Ė/Lbol ∼ 10−4−10−3%). In addition, optical and near-IR line ratios show that the expansion of the radio source drives fast shocks (with velocities vs ≳ 500 km s−1) that ionize and accelerate the outflowing gas. At odds with the properties of other compact, peaked-spectrum radio sources hosting warm ionized gas outflows, we do not find signs of kinematically disturbed or outflowing gas in phases colder than the warm ionized gas. We argue that this is due to the young age of our source and thus to the recent nature of the AGN-ISM interaction, and suggest that cold gas forms within the outflowing material and the shock-ionized outflowing gas of PKS B1934-63 did not have enough time to cool down and accumulate in a colder phase. This scenario is also supported by the multi-phase outflows of other compact and young radio sources in the literature. [ABSTRACT FROM AUTHOR]

The 2 Jy sample is a survey of radio galaxies with flux densities above 2 Jy at 2.7 GHz. As part of our ongoing work on the southern subset of 2 Jy sources, in paper I of this series we analysed the X-ray cores of the complete 2 Jy sample with redshifts 0.05 < z < 0.7. For this work we focus on the X-ray emission associated with the extended structures (jets, lobes and environments) of the complete subset of 2 Jy sources with 0.05 < z < 0.2 that we have observed with Chandra. We find that hotspots and jet knots are ubiquitous in Fanaroff-Riley class II (FRII) sources, which also inhabit systematically poorer environments than the Fanaroff-Riley class I (FRI) sources in our sample. Spectral fits of the hotspots with good X-ray statistics invariably show properties consistent with synchrotron emission and we show that inverse-Compton mechanisms underpredict the X-ray emission we observe by 1-2 orders of magnitude. Inverse-Compton emission is detected from many of the lobes in our sample and we find that the lobes of the FRII sources show magnetic fields lower by up to an order of magnitude than expected from equipartition extrapolations. This is consistent with previous results, which show that most FRII sources have electron energy densities higher than minimum energy requirements. [ABSTRACT FROM AUTHOR]

We present an analysis of 2.05 μm Hubble Space Telescope polarimetric data for a sample of 13 nearby Fanaroff–Riley type II (FRII) 3CR radio sources (0.03 < z < 0.11) that are classified as narrow-line radio galaxies (NLRG) at optical wavelengths. We find that the compact cores of the NLRG in our sample are intrinsically highly polarized in the near-infrared (near-IR) (6 < P2.05 μm < 60 per cent), with the electric vector (E-vector) perpendicular to the radio axis in 54 per cent of the sources. The levels of extinction required to produce near-IR polarization by the dichroic extinction mechanism are consistent with the measured values recently reported in Ramírez et al., provided that this mechanism has its maximum efficiency. This consistency suggests that the nuclear polarization could be due to dichroic extinction. In this case, toroidal magnetic fields that are highly coherent would be required in the circumnuclear tori to align the elongated dust grains responsible for the dichroic extinction. However, it is not entirely possible to rule out other polarization mechanisms (e.g. scattering, synchrotron emission) with our observations at only one near-IR wavelength. Therefore, further polarimetry observations at mid-IR and radio wavelengths will be required to test whether all the near-IR polarization is due to dichroic extinction. [ABSTRACT FROM PUBLISHER]

The high-redshift radio galaxy MRC 1138-262 ('Spiderweb Galaxy'; z = 2.16) is one of the most massive systems in the early Universe and surrounded by a dense 'web' of proto-cluster galaxies. Using the Australia Telescope Compact Array, we detected CO(1-0) emission from cold molecular gas - the raw ingredient for star formation - across the Spiderweb Galaxy. We infer a molecular gas mass of MH2 = 6 x 1010 M&#664- (for MH2/L'CO = 0.8). While the bulk of the molecular gas coincides with the central radio galaxy, there are indications that a substantial fraction of this gas is associated with satellite galaxies or spread across the intergalactic medium on scales of tens of kpc. In addition, we tentatively detect CO(1-0) in the star-forming proto-cluster galaxy HAE 229, 250 kpc to the West. Our observations are consistent with the fact that the Spiderweb Galaxy is building up its stellar mass through a massive burst of widespread star formation. At maximum star formation efficiency, the molecular gas will be able to sustain the current star formation rate (SFR ≈ 1400 M&#664- yr-1, as traced by Seymour et al.) for about 40 Myr. This is similar to the estimated typical lifetime of a major starburst event in infrared luminous merger systems. [ABSTRACT FROM AUTHOR]

ABSTRACT We present optical, infrared (IR) and radio observations of the powerful Fanaroff-Riley type II (FR II) radio source PKS 0347+05 ( z = 0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double active galactic nucleus (AGN) system, comprising a weak-line radio galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at the same redshift. Our deep Gemini optical images show a highly disturbed morphology, with a warped dust lane crossing through the halo and nuclear regions of the radio galaxy host, tidal tails and a bridge connecting the radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our Gemini optical spectrum of the radio galaxy shows evidence for a reddened young stellar population of age ≤100 Myr. Further evidence for recent star formation activity in this source is provided by the detection of strong polycyclic aromatic hydrocarbon features in mid-IR Spitzer/IRS spectra. Together, these observations support a model in which both AGN have been triggered simultaneously in a major galaxy merger. However, despite the presence of a powerful FR II radio source, and the apparently plentiful supply of fuel provided by the merger, the nucleus of the radio galaxy shows only weak, low-ionization emission-line activity. We speculate that the fuel supply to nuclear regions of the radio galaxy has recently switched off (within the last ∼106 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FR II radio source. Based on this scenario, we derive a lower limit on the typical lifetimes of powerful, intermediate-redshift FR II radio sources of [ABSTRACT FROM AUTHOR]

ABSTRACT PKS B2152−699, which has radio power characteristic of sources that dominate radio feedback, is exceptional in showing a wide range of features associated with radio-galaxy/gas interactions. We present new deep radio (Australia Telescope Compact Array), X-ray ( Chandra) and ground-based optical observations, and test the energetics of the feedback model. We report the first high-resolution observations of the radio jet, finding that the inner jet extends ∼8.5 kpc (10° viewing angle) in the direction of an optical emission-line high-ionization cloud (HIC) before taking a zigzag path to a position offset from the HIC. Jet synchrotron radiation to X-ray energies is seen. The HIC is associated with cool, kT∼ 0.3 keV, X-ray gas of anomalously low metallicity. On larger scales, the radio galaxy displays all three X-ray features that together confirm supersonic expansion of the lobes into the external medium: gas cavities, inverse Compton emission showing excess internal lobe pressure, and high-contrast arms of temperature above the kT∼ 1 keV ambient medium. The well-formed southern lobe on the counterjet side is expanding with a Mach number between 2.2 and 3. The lobe energy appears to be more gently dissipated in the north. We estimate a cavity power ∼3 × 1043 erg s−1, which falls well below previously reported correlations with radio power. The total inferred time-averaged jet power, ∼4 × 1044 erg s−1, is dominated by the kinetic and thermal energy of shocked gas, and if used instead would bring the source into better agreement with the correlations. The southern hotspot is the more complex, with a spiral polarization structure. Its bright peak emits synchrotron X-rays. The fainter northern hotspot is particularly interesting, with X-rays offset in the direction of the incoming jet by ∼1 arcsec relative to the radio peak. Here modest (δ∼ 6) relativistic beaming and a steep radio spectrum cause the jet to be X-ray bright through inverse Compton scattering before it decelerates. With such beaming, a modest proton content or small departure from minimum energy in the jet will align estimates of the instantaneous and time-averaged jet power. The hotspots suggest acceleration of electrons to a maximum energy ∼1013 eV in the jet termination shocks. [ABSTRACT FROM AUTHOR]

Observations of ionized and neutral gas outflows in radio galaxies (RGs) suggest that active galactic nucleus (AGN) radio jet feedback has a galaxy-scale impact on the host interstellar medium, but it is still unclear how the molecular gas is affected. Thus, it is crucial to determine the physical conditions of the molecular gas in powerful RGs to understand how radio sources may regulate the star formation in their host galaxies. We present deep Spitzer Infrared Spectrograph (IRS) high-resolution spectroscopy of eight nearby RGs that show fast HI outflows. Strikingly, all of these HI-outflow RGs have bright H2 mid-IR lines that cannot be accounted for by UV or X-ray heating. This strongly suggests that the radio jet, which drives the HI outflow, is also responsible for the shock excitation of the warm H2 gas. In addition, the warm H2 gas does not share the kinematics of the ionized/neutral gas. The mid-IR-ionized gas lines (with FWHM up to 1250 km s-1 for [Ne II] 12.8μm) are systematically broader than the H2 lines, which are resolved by the IRS in ≈60% of the detected lines (with FWHM up to 900 km s-1). In five sources, 3C 236, 3C 293, 3C 459, 4C 12.50, and PKS 1549-79, the [Ne II] 12.8μm line, and to a lesser extent the [Ne III] 15.5μm and [Ne V] 14.3μm lines, clearly exhibits blueshifted wings (up to -900 km s-1 with respect to the systemic velocity) that match well the kinematics of the outflowing HI or ionized gas. The H2 lines do not show these broad wings, except tentative detections in 4C 12.50, 3C 459, and PKS 1549-79. This shows that, contrary to the HI gas, the H2 gas is inefficiently coupled to the AGN jet-driven outflow of ionized gas. While the dissipation of a small fraction (<10%) of the jet kinetic power can explain the turbulent heating of the molecular gas, our data show that the bulk of the warm molecular gas is not expelled from these galaxies. [ABSTRACT FROM AUTHOR]

We present deep Spitzer/Infrared Spectrograph (IRS) spectra for complete samples of 46 2 Jy radio galaxies (0.05 < z < 0.7) and 19 3CRR FRII radio galaxies (z < 0.1), and use the detection of polycyclic aromatic hydrocarbon (PAH) features to examine the incidence of contemporaneous star formation and radio-loud active galactic nucleus (AGN) activity. Our analysis reveals PAH features in only a minority (30%) of the objects with good IRS spectra. Using the wealth of complementary data available for the 2 Jy and 3CRR samples we make detailed comparisons between a range of star formation diagnostics: optical continuum spectroscopy, mid- to far-IR (MFIR) color, far-IR excess and PAH detection. There is good agreement between the various diagnostic techniques: most candidates identified to have star formation activity on the basis of PAH detection are also identified using at least two of the other techniques. We find that only 35% of the combined 2 Jy and 3CRR sample show evidence for recent star formation activity (RSFA) at optical and/or MFIR wavelengths. This result argues strongly against the idea of a close link between starburst and powerful radio-loud AGN activity, reinforcing the view that, although a large fraction of powerful radio galaxies may be triggered in galaxy interactions, only a minority are triggered at the peaks of star formation activity in major, gas-rich mergers. However, we find that compact radio sources (D < 15 kpc) show a significantly higher incidence of RSFA (>75%) than their more extended counterparts (≈ 15%-25%). We discuss this result in the context of a possible bias toward the selection of compact radio sources triggered in gas-rich environments. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the results of a comparison between the optical morphologies of a complete sample of 46 southern 2 Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7) and those of two control samples of quiescent early-type galaxies: 55 ellipticals at redshifts z ≤ 0.01 from the Observations of Bright Ellipticals at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2 < z < 0.7 in the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We find that a significant fraction of quiescent ellipticals at low and intermediate redshifts show evidence for disturbed morphologies at relatively high surface brightness levels, which are likely the result of past or on-going galaxy interactions. However, the morphological features detected in the galaxy hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2 mag brighter than those present in their quiescent counterparts. Indeed, if we consider the same surface brightness limits, the fraction of disturbed morphologies is considerably smaller in the quiescent population (53 per cent at z < 0.2 and 48 per cent at 0.2 ≤ z < 0.7) than in the PRGs (93 per cent at z < 0.2 and 95 per cent at 0.2 ≤ z < 0.7 considering strong-line radio galaxies only). This supports a scenario in which PRGs represent a fleeting active phase of a subset of the elliptical galaxies that have recently undergone mergers/interactions. However, we demonstrate that only a small proportion (≲20 per cent) of disturbed early-type galaxies are capable of hosting powerful radio sources. [ABSTRACT FROM AUTHOR]

BSTRACT [ABSTRACT FROM AUTHOR]

We report the clearest detection to date of dusty torus signatures in a weak-line radio galaxy (WLRG). The deep Spitzer InfraRed Spectrograph (IRS) rest-frame mid-infrared (MIR) spectrum of the WLRG PKS 42 () shows a clear spectral turnover at s suggestive of warm dust, as well as a 9.7-μm silicate absorption feature. In addition, the hard X-ray results, based on Chandra data, strongly support a picture in which PKS 42 has a torus and accretion disc more typical of strong-line radio galaxies (SLRGs). The MIR and X-ray spectra are markedly different from those of other WLRGs at similar redshifts, and here we show that the former can be successfully fitted with clumpy torus models with parameters characteristic of Type-2 AGN tori: close to edge-on () and relatively broad (), with an outer radius of 2 pc, , and AGN bolometric luminosity . The presence of a compact torus in PKS 42 provides evidence that this WLRG is fuelled by cold, rather than hot, gas accretion. We suggest that WLRGs are a diverse population, and PKS 42 may represent a type of radio galaxy in which the AGN activity has been recently re-triggered as a consequence of intermittent gas supply, or in which the covering factor of the narrow-line region (NLR) clouds is relatively low. [ABSTRACT FROM AUTHOR]

In this paper we discuss the results of a programme of spectral synthesis modelling of a sample of starburst radio galaxies in the context of scenarios for the triggering of the activity and the evolution of the host galaxies. New optical spectra are also presented for a subset of the objects discussed. The starburst radio galaxies - comprising 15-25 per cent of all powerful extragalactic radio sources - frequently show disturbed morphologies at optical wavelengths, and unusual radio structures, although their stellar masses are typical of radio galaxies as a class. In terms of the characteristic ages of their young stellar populations (YSPs), the objects can be divided into two groups: those with YSP ages Gyr, in which the radio source has been triggered quasi-simultaneously with the main starburst episode, and those with older YSP in which the radio source has been triggered or re-triggered a significant period after the starburst episode. Most of the former group are associated with a large mid- to far-IR (MFIR) continuum and [O ] emission-line luminosities ( L, W), while most of the latter have lower luminosities. Combining the information on the YSP with that on the optical morphologies of the host galaxies, we deduce that the majority of the starburst radio galaxies have been triggered in galaxy mergers in which at least one of the galaxies is gas rich. However, the triggering (or re-triggering) of the radio jets can occur immediately before, around or a significant period after the final coalescence of the merging nuclei, reflecting the complex gas infall histories of the merger events. Although 25 per cent of starburst radio galaxies are sufficiently bright at MFIR wavelengths to be classified as ultraluminous infrared galaxies (ULIRGs), we show that only the most massive ULIRGs are capable of evolving into radio galaxies. Finally, for a small subset of starburst radio galaxies in rich clusters of galaxies, cooling flows associated with the hot X-ray haloes offer a viable alternative to mergers as a trigger for the radio jet activity. Overall, our results provide further evidence that a powerful radio jet activity can be triggered via a variety of mechanisms, including different evolutionary stages of major galaxy mergers; clearly, radio-loud AGN activity is not solely associated with a particular stage of a unique type of gas accretion event. [ABSTRACT FROM AUTHOR]

ABSTRACT We present deep Gemini Multi-Object Spectrograph South (GMOS-S)/ Gemini optical broad-band images for a complete sample of 46 southern 2 Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7). Based on them, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). The high-quality observations presented here show for the first time that the overall majority of PRGs at intermediate redshifts (78-85 per cent) show peculiarities in their optical morphologies at relatively high levels of surface brightness (; ). The observed morphological peculiarities include tails, fans, bridges, shells, dust lanes, irregular features, amorphous haloes and multiple nuclei. While the results for many of the galaxies are consistent with them being observed at, or after, the time of coalescence of the nuclei in a galaxy merger, we find that more than one-third of the sample are observed in a pre-coalescence phase of the merger, or following a close encounter between galaxies that will not necessarily lead to a merger. By dividing the sample into Weak-Line Radio Galaxies (WLRGs; 11 objects) and Strong-Line Radio Galaxies (SLRGs; 35 objects) we find that only 27 per cent of the former show clear evidence for interactions in their optical morphologies, in contrast to the SLRGs, of which at least 94 per cent appear interacting. This is consistent with the idea that many WLRGs are fuelled/triggered by the Bondi accretion of hot gas. However, the evidence for interactions and dust features in a fraction of them indicates that the accretion of cold gas cannot always be ruled out. Of the 28 per cent of the sample that display evidence for significant starburst activity, we find that 92 per cent present disturbed morphologies, following the same general trend as the total and SLRG samples. By comparing our PRGs with various samples of quiescent ellipticals from the literature, we conclude that the percentage of morphological disturbance that we find here exceeds that found for quiescent ellipticals when similar surface brightnesses are considered. Overall, our study indicates that galaxy interactions are likely to play a key role in the triggering of active galactic nuclei (AGN)/jet activity. [ABSTRACT FROM AUTHOR]

An important aspect of solving the long-standing question as to what triggers various types of active galactic nuclei (AGN) involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a series of papers that systematically study the large-scale properties of cold neutral hydrogen (H i) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper, we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of H i gas (with sizes up to 190 kpc and masses up to ) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in H i mass with radio-source size likely indicates that either these compact radio sources are confined by large amounts of gas in the central region or that their fuelling is inefficient and different from the fuelling process of classical FR I radio sources. To first order, the overall H i properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies. [ABSTRACT FROM AUTHOR]

This paper is the second in a series in which we present intermediate-resolution, wide-wavelength coverage spectra for a complete sample of 14 compact radio sources, taken with the aim of investigating the impact of the nuclear activity on the circumnuclear interstellar medium (ISM) in the early stages of radio source evolution. In the first paper (Holt, Tadhunter & Morganti), we presented the kinematic results from the nuclear emission-line modelling and reported fast outflows in the circumnuclear gas. In this paper, we use the line fluxes to investigate the physical conditions and dominant ionization mechanisms of the emission-line gas. We find evidence for large electron densities and high reddening in the nuclear regions, particularly in the broader, blueshifted components. These results are consistent with the idea that the young, recently triggered radio sources still reside in dense and dusty cocoons deposited by the recent activity triggering event (merger/interaction). In addition, we find that the quiescent nuclear and extended narrow components are consistent with active galactic nucleus (AGN) photoionization, split between simple-slab AGN photoionization and mixed-medium photoionization models. For the nuclear broader and shifted components, the results are less clear. Whilst there are suggestions that the broader components may be closer to shock plus precursor models on the diagnostic diagrams, and that the electron temperatures and densities are high, we are unable to unambiguously distinguish the dominant ionization mechanism using the optical emission-line ratios. This is surprising given the strong evidence for jet–cloud interactions (broad emission lines, large outflow velocities and strong radio-optical alignments), which favours the idea that the warm gas has been accelerated in shocks driven by the radio lobes expanding through a dense cocoon of gas deposited during the triggering event. [ABSTRACT FROM AUTHOR]

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O iii] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H i-rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy. [ABSTRACT FROM AUTHOR]

We present an investigation into the absorber in front of the radio galaxy MRC 2025−218, using integral field spectroscopy obtained at the Very Large Telescope, and long-slit spectroscopy obtained at the Keck II telescope. The properties of MRC 2025−218 are particularly conducive to study the nature of the absorbing gas, i.e. this galaxy shows bright and spatially extended Lyα emission, along with bright continuum emission from the active nucleus. Lyα absorption is detected across , has a covering factor of ∼1, and shows remarkably little variation in its properties across its entire spatial extent. This absorber is kinematically detached from the extended emission line region (EELR). Its properties suggest that the absorber is outside of the EELR. We derive lower limits to the H i, H ii and H column densities for this absorber of and , respectively. Moreover, the relatively bright emission from the active nucleus has allowed us to measure a number of metal absorption lines: C i, C ii, C iv, N v, O i, Si ii, Si iv, Al ii and Al iii. The column density ratios are most naturally explained using photoionization by a hard continuum, with an ionization parameter . Shocks or photoionization by young stars cannot reproduce satisfactorily the measured column ratios. Using the ratio between the Si ii* and Si ii column densities, we derive a lower limit of for the electron density of the absorber. The data do not allow useful constraints to be placed on the metallicity of the absorber. We consider two possibilities for the nature of this absorber: the cosmological infall of gas, and an outflow driven by supernovae or the radio jets. We find it plausible that the absorber around 2025−218 is in outflow. We also find good agreement between the observed properties of the H i absorber and the properties of the H i absorption expected from the cosmological infall model of Barkana & Loeb. [ABSTRACT FROM AUTHOR]

We present the detection of an enormous disc of cool neutral hydrogen (H i) gas surrounding the S0 galaxy NGC 612, which hosts one of the nearest powerful radio sources (PKS 0131−36). Using the Australia Telescope Compact Array, we detect of H i emission-line gas that is distributed in a 140-kpc wide disc-like structure along the optical disc and dust lane of NGC 612. The bulk of the gas in the disc appears to be settled in regular rotation with a total velocity range of 850 km s−1, although asymmetries in this disc indicate that perturbations are being exerted on part of the gas, possibly by a number of nearby companions. The H i disc in NGC 612 suggests that the total mass enclosed by the system is , implying that this early-type galaxy contains a massive dark matter halo. We also discuss an earlier study by Holt et al. that revealed the presence of a prominent young stellar population at various locations throughout the disc of NGC 612, indicating that this is a rare example of an extended radio source that is hosted by a galaxy with a large-scale star-forming disc. In addition, we map a faint H i bridge along a distance of 400 kpc in between NGC 612 and the gas-rich barred galaxy NGC 619, indicating that likely an interaction between both systems occurred. From the unusual amounts of H i gas and young stars in this early-type galaxy, in combination with the detection of a faint optical shell and the system's high infrared luminosity, we argue that either ongoing or past galaxy interactions or a major merger event are a likely mechanism for the triggering of the radio source in NGC 612. This paper is part of an ongoing study to map the large-scale neutral hydrogen properties of nearby radio galaxies and it presents the first example of large-scale H i detected around a powerful Fanaroff–Riley type II (FR-II) radio galaxy. The H i properties of the FR-II radio galaxy NGC 612 are very similar to those found for low-power compact radio sources, but different from those of extended Fanaroff–Riley type I (FR-I) sources. [ABSTRACT FROM AUTHOR]

We present European Southern Observatory Very Large Telescope (ESO VLT) and William Herschel Telescope (WHT) spectroscopic observations of two powerful radio galaxies at intermediate redshifts (3C 213.1 and 3C 459), obtained with the aim of establishing the nature and evolutionary status of the host galaxies. Spectral synthesis modelling has been used to demonstrate that young stellar populations (YSP) make a major contribution to the integrated optical light in both sources, contributing at least 37 and 80 per cent of the integrated B-band light in 3C 213.1 and 3C 459, respectively. While in the case of 3C 213.1 the YSP has an intermediate age (0.4–0.8 Gyr) and comprises 3–30 per cent of the total stellar mass, adequate fits to the spectra of 3C 459 require a combination of younger (<0.1 Gyr) and intermediate (0.2–1.2 Gyr) age YSP components that together comprise 5–100 per cent of the total stellar mass. Both the optical and far-IR properties of 3C 459 are consistent with its status as an ultraluminous infrared galaxy (ULIRG), and a scenario in which the observed radio source has been triggered close to the peak of star formation activity in a major gas-rich merger. On the other hand, in 3C 213.1 it is likely that the radio source has been triggered (or re-triggered) a significant period after the starburst peak, and the far-IR luminosity of the source is substantially lower. Following correction for the contributions of the YSP, the absolute magnitudes of the host galaxies of both sources are relatively modest (≤2 L*), thus demonstrating that powerful radio activity is not always associated with the most massive and luminous elliptical galaxies. These results serve to emphasize that the radio source population is diverse, in terms of both the host galaxy properties and the triggering mechanism(s) for the radio source activity. [ABSTRACT FROM AUTHOR]

We present high-quality, wide spectral coverage long-slit optical spectra for 12 powerful radio sources at low and intermediate redshifts that show evidence for a substantial ultraviolet (UV) excess. These data were taken using the William Herschel Telescope and the ESO Very Large Telescope with the aim of determining the detailed properties of the young stellar populations (YSPs) in the host galaxies as part of a larger project to investigate evolutionary scenarios for the active galactic nucleus (AGN) host galaxies. The results of our spectral synthesis model fits to the spectra highlight the importance of taking into account AGN-related components (emission lines, nebular continuum, scattered light) and reddening of the stellar populations in studies of this type. It is also clear that careful examination of the fits to the spectra, as well consideration of auxiliary polarimetric and imaging data, are required to avoid degeneracies in the model solutions. In three out of the 12 sources in our sample we find evidence for broad permitted line components, and a combination of AGN-related continuum components and an old (12.5 Gyr) stellar population provides an adequate fit to the data. However, for the remaining nine sources we find strong evidence for YSPs. In contrast to some recent studies that suggest relatively old post-starburst ages for the YSPs in radio galaxies (0.3–2.5 Gyr), we deduce a wide range of ages for the YSPs in our sample objects (0.02–1.5 Gyr), with ∼50 per cent of the sample showing evidence for young YSP ages (≲0.1 Gyr) in their nuclear regions. The nuclear YSPs are often significantly reddened [0.2 < E( B− V) < 1.4] and make up a substantial fraction (∼1–35 per cent) of the total stellar mass in the regions sampled by the spectroscopic slits. Moreover, in all the cases in which we have sufficient spatial resolution we find that the UV excess is extended across the full measurable extent of the galaxy (typically 5–30 kpc), suggesting galaxy-wide starbursts. The implications for photometric and spectroscopic studies of active galaxies are discussed. [ABSTRACT FROM AUTHOR]

Using long-slit emission-line spectra we detect a fast outflow of ionized gas, with velocities up to 1000 km s−1, in the nearby powerful radio galaxy . The fast outflow is located about 1 kpc east of the nucleus, in a region of enhanced radio emission due to the presence of a distorted radio jet. We present results that indicate that this fast outflow is caused by a jet–ISM interaction. The kinematics of the outflowing ionized gas are very similar to those of a fast outflow of neutral hydrogen gas in this galaxy, suggesting that both outflows are the result of the same driving mechanism. While the mass of the outflowing ionized gas is about , the total H i mass involved in the neutral outflow is about 100 times higher . This shows that, despite the high energies that must be involved in driving the outflow, most of the gas remains, or again becomes, neutral. Other outflows of ionized gas, although not as pronounced as in the region of the enhanced radio emission, are also seen in various other regions along the axis of the inner radio jets. The regular kinematics of the emission-line gas along the major axis of the host galaxy reveals a rotating ionized gas disc 30 kpc in extent. [ABSTRACT FROM AUTHOR]

We present high-quality long-slit spectra for three nearby powerful radio galaxies– 3C 293, 3C 305 and PKS 1345+12. These were taken with the aim of characterizing the young stellar populations (YSP), and thereby investigating the evolution of the host galaxies, as well as the events that triggered the activity. Isochrone spectral synthesis modelling of the wide wavelength coverage spectra of nuclear and off-nuclear continuum-emitting regions have been used to estimate the ages, masses and luminosities of the YSP component, taking full account of reddening effects and potential contamination by activity-related components. We find that the YSP make a substantial contribution to the continuum flux in the off-nuclear regions on a radial scale of 1–20 kpc in all three objects. Moreover, in two objects we find evidence for reddened post-starburst stellar populations in the near-nuclear regions of the host galaxies. The YSP are relatively old (0.1–2 Gyr), massiveand make up a large proportion (∼1–50 per cent) of the total stellar mass in the regions of the galaxies sampled by the observations. Overall, these results are consistent with the idea that the nuclear activity of active galactic nuclei in some radio galaxies is triggered by major gas-rich mergers. Therefore, these radio galaxies form part of the subset of early-type galaxies that is evolving most rapidly in the local Universe. Intriguingly, the results also suggest that the radio jets are triggered relatively late in the merger sequence, and that there is an evolutionary link between radio galaxies and luminous/ultraluminous infrared galaxies. [ABSTRACT FROM AUTHOR]

We present spectroscopic observations of a complete subsample of 13 low-luminosity radio galaxies selected from the 2-Jy sample of Tadhunter et al. The underlying continuum in these sources was carefully modelled in order to make a much-needed comparison between the emission-line and continuum properties of Fanaroff–Riley type Is (FRIs) and those of other classes of radio sources. We find that five galaxies in the sample show a measurable ultraviolet (UV) excess: two of these sources are BL Lacs, but in the remaining three galaxies we argue that the most likely contributor to the UV excess is a young stellar component. Therefore, excluding the BL Lacs, we find that ∼30 per cent of the sample show evidence for young stars, which is similar to the results obtained for higher luminosity samples. We compare our results with far-infrared measurements in order to investigate the far-infrared–starburst link. The nature of the optical–radio correlations is investigated in light of these new available data and, in contrast to previous studies, we find that the FRI sources follow the correlations with similar slopes to those found for the Fanaroff–Riley type IIs. Finally, we compare the luminosities of the emission lines in the FRI and BL Lac sources and find a significant difference between the [O iii] line luminosities of the two groups. Our results are discussed in the context of the unified schemes for low-powered radio sources. [ABSTRACT FROM AUTHOR]

ABSTRACT We present intermediate-resolution spectra for the powerful radio galaxy Cygnus A taken with the ISIS spectrograph on the William Herschel Telescope. The spectra show complex emission-line profiles across the entire extent of the narrow-line region (NLR) along the radio axis (PA 105°), with line splittings (ΔV∼ 300 km s[sup -1]) at radial distances of 1–2 kpc to both the north-east and south-west, and a combination of broad (500 < FWHM < 800 km s[sup -1]) and narrow (FWHM < 300 km s[sup -1]) components within a radial distance of 1 kpc of the nucleus. The wide spectral coverage of the data allows us for the first time to measure the reddening, physical conditions and ionization of the kinematic subcomponents individually. Only the broad component detected in the near-nuclear regions shows evidence for significant reddening over and above that due to the Galaxy. The level of reddening of the nuclear broad component is consistent with the idea that the extinction associated with the kiloparsec-scale dust lane induces a mild (≤factor 2) anisotropy in the total emission-line flux, although there is no evidence for a compact, high-density inner narrow-line region in this source. Following correction for reddening, we find a remarkable degree of uniformity in the physical conditions and ionization state between the various spatial and kinematic subcomponents, suggesting a common ionization mechanism for all the subcomponents, despite the differences in kinematics. We use diagnostic diagrams to demonstrate that photoionization by the active galactic nucleus is the dominant ionization mechanism. However, a combination of matter- and radiation-bounded photoionized components is required to explain the strength of the [Fevii]λ6087 line and the high electron temperature measured from the [Oiii] (5007 + 4959)/4363 diagnostic ratio. A major outstanding issue for our understanding of the NLR gas in this and other radio galaxies is how the uniformity in the ionization and physical conditions can be maintained in a mixed photoionized medium across a range of spatial scales and kinematic subcomponents. [ABSTRACT FROM AUTHOR]

Integral-field spectroscopic observations are presented for three powerful radio galaxies, namely 3C 277.3 (Coma A; z = 0.0857), 3C 171 (z = 0.2384) and 3C 265 (z = 0.811), which are known to be undergoing jet-cloud interactions. The morphology, kinematics and ionization of the gas in the emission-line structure of these sources are mapped and analysed. One-dimensional spectra are also extracted and integrated over the different emission-line regions in each galaxy. In two of the galaxies (3C 277.3 and 3C 171) the radio sizes are of similar extent to the emission-line structure. For these, enhanced emission-line regions are found associated with the radio structures, in addition to complex kinematics and low ionization states close to the radio hotspots, indicating that jet-induced shocks disturb and ionize the gas. Interestingly, the bright — presumably shock-ionized — emission-line region coincident with the radio jet knot in 3C 277.3 shows quiescent kinematics and high ionization state. Possible explanations for this puzzling result are proposed. The images of 3C 171 and 3C 265 indicate that the lateral expansion of the cocoon has a significant effect on the kinematics and ionization of the gas, showing for the first time that the effects of the radio source are felt far from the jet axis. In addition, the presence of a stellar-photoionized H II region is detected in the extended emission-line nebula of the radio galaxy 3C 277.3. [ABSTRACT FROM AUTHOR]

We present William Herschel Telescope spectropolarimetry observations of a complete RA-limited sample of nine low-redshift (0.05

We present deep, narrow-band and continuum images of the powerful high-redshift radio galaxy 3C 265 (z=0.811) , taken with the TAURUS Tunable Filter on the William Herschel Telescope, together with detailed long-slit spectroscopic observations along the axis defined by the UV/optical emission elongation. The deep images reveal the existence of cones in the ionization structure of 3C 265 within ∼7 arcsec (58 kpc) of the nucleus, where the emission-line structure is not observed to be closely aligned with the radio axis. This indicates that anisotropic illumination from the central active nucleus dominates on a small scale. In contrast, at larger distances (&gsim;10 arcsec; 80 kpc) from the nucleus, low-ionization emission gas is closely aligned with the radio axis, suggesting that jet–cloud interactions may become the dominant mechanism in the line-emitting gas on a larger scale. Moreover, the presence of a high-velocity cloud at 2.5 arcsec from the nucleus, close to the radio axis, indicates that even close to the nucleus (∼20 kpc) jet-induced shocks have an important kinematic effect. However, spectroscopic analysis of this region reveals that the ionization state of the high-velocity gas is similar to or higher than that of the surroundings, which is opposite to what we would expect for a cloud that has been compressed and accelerated by jet-induced shocks. Our images show that, while on a large scale the low-ionization emission-line structures are aligned with the radio axis, on a smaller scale, where AGN-photoionization dominates, the highest surface-brightness structure is aligned with the closest companion galaxy (misaligned with the radio axis). This suggest that much of the emission-line structure reflects the intrinsic gas distribution, rather than the ionization pattern imprinted by the radio jets or by illumination from the central AGN. Overall, our results underline the need for a variety of mechanisms to explain the properties of the extended emission-line gas in the haloes of radio galaxies. [ABSTRACT FROM AUTHOR]

We present accurate measurements of the physical conditions in five powerful radio galaxies, as derived from deep, long-slit spectroscopic observations. All five objects show prominent extended line emission, and have X-ray luminosities similar to those of isolated elliptical galaxies. The data are high enough quality that the electron density and temperature can be measured at several positions across the emission-line nebulae. We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [O III](4959+5007)/4363 line ratio are in the range 10 000 < T[sub e] < 20 000 K, whilst [S II](6716/6731) densities fall between 100-500cm[sup -3]. Using these values, we find pressures within the line-emitting clouds a factor of 10-100 times higher than expected for pressure balance with the hot X-ray haloes of the host galaxies. Previous studies of sources that show significant evidence of jet-cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects. We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus. [ABSTRACT FROM AUTHOR]

We present spectroscopic and polarimetric observations of a complete, optically unbiased sample of 2-Jy radio galaxies at intermediate redshifts (0.15 < z < 0.7). These data - which cover the nuclear regions of the target galaxies — allow us to quantify for the first time the various components that contribute to the UV excess in the population of powerful, intermediate-redshift radio galaxies. We find that, contrary to the results of previous surveys — which have tended to be biased towards the most luminous and spectacular objects in any redshift range — the contribution of scattered quasar light to the UV excess is relatively minor in most of the objects in our sample. Only seven objects (32 per cent of the complete sample) show significant polarization in the rest-frame UV, and none of the objects in our sample is polarized in the near-UV at the P > 10 per cent level. Careful measurement and modelling of our spectra have allowed us to quantify the contributions of other components to the UV excess. We show that nebular continuum (present in all objects at the 3—40 per cent level), direct active galactic nucleus (AGN) light (significant in 40 per cent of objects) and young stellar populations (significant in 15—50 per cent of objects) all make important contributions to the UV continuum in the population of powerful radio galaxies. These results serve to emphasize the multicomponent nature of the UV continuum in radio galaxies. The results also point to an interesting link between the optical/UV and far-IR properties of our sample objects, in the sense that the objects with the clearest evidence for optical/UV starburst activity are also the most luminous at far-IR wavelengths. This supports the idea that the cooler dust components in radio galaxies are heated by starbursts rather than by AGN. [ABSTRACT FROM AUTHOR]

Focuses on the spectroscopic observations of the emission-line outflows of the radio galaxy PKS1549-79. Evolution of the radio-sources; Analysis of the high-ionization forbidden lines; Discussion of the kinematics of the emission lines.

We use deep Herschel Space Observatory observations of a 90 per cent complete sample of 32 intermediate-redshift 2Jy radio galaxies (0.05 < z < 0.7) with strong emission lines to estimate the dust masses of their host galaxies and thereby investigate the triggering mechanisms for their quasar-like AGN. The dust masses derived for the radio galaxies (7.2 × 105 < Md < 2.6 × 108 M⊙) are intermediate between those of quiescent elliptical galaxies on the one hand, and ultraluminous infrared galaxies (ULIRGs) on the other. Consistent with simple models for the co-evolution of supermassive black holes and their host galaxies, these results suggest that most radio galaxies represent the late time re-triggering of AGN activity via mergers between the host giant elliptical galaxies and companion galaxies with relatively low gas masses. However, a minority of the radio galaxies in our sample (∼20 per cent) have high, ULIRG-like dust masses, along with evidence for prodigious star formation activity. The latter objects are more likely to have been triggered in major, gas-rich mergers that represent a rapid growth phase for both their host galaxies and their supermassive black holes. [ABSTRACT FROM PUBLISHER]

It remains uncertain which continuum and emission line diagnostics best indicate the bolometric powers of active galactic nuclei (AGNs), especially given the attenuation caused by the circumnuclear material and the possible contamination by components related to star formation. Here we use mid-IR spectra along with multiwavelength data to investigate the merit of various diagnostics of AGN radiative power, including the mid-IR [Ne III] λ25.89 μm and [O IV] λ25.89 μm fine-structure lines, the optical [O III] λ5007 forbidden line, and mid-IR 24 μm, 5 GHz radio, and X-ray continuum emission, for complete samples of 46 2Jy radio galaxies (0.05 < z < 0.7) and 17 3CRR FRII radio galaxies (z < 0.1). We find that the mid-IR [O IV] line is the most reliable indicator of AGN power for powerful radio-loud AGNs. By assuming that the [O IV] is emitted isotropically, and comparing the [O III] and 24 μm luminosities of the broad- and narrow-line AGNs in our samples at fixed [O IV] luminosity, we show that the [O III] and 24 μm emission are both mildly attenuated in the narrow-line compared to the broad-line objects by a factor of ≈2. However, despite this attenuation, the [O III] and 24 μm luminosities are better AGN power indicators for our sample than either the 5 GHz radio or the X-ray continuum luminosities. We also detect the mid-IR 9.7 μm silicate feature in the spectra of many objects but not ubiquitously: at least 40% of the sample shows no clear evidence for these features. We conclude that, for the majority of powerful radio galaxies, the mid-IR lines are powered by AGN photoionization. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of VIMOS-Integral Field Unit (IFU) and FORS1 spectroscopy of two radio galaxies: PKS1932-46 and PKS2250-41. The two sources display highly extended emission line regions, with varied ionization states and gas kinematics. Both sources appear to lie in rich environments, and may be undergoing interactions with companion bodies. The more distant emitting material generally displays simple, narrow emission line profiles, often at significant velocity offsets from the system rest-frame, and may be formed from the tidal debris of a previous interaction. [Copyright &y& Elsevier]

We present the first results of a study aimed to detect large H I structures in radio galaxies. In two of the three cases presented (Coma A and B2 0648+27), the detection of a large amount of H I distributed over several tens of kpc suggests a major merger as the origin of the observed system and allows us to infer when this merger must have occurred. The situation is less clear for the third object (3C 433). We propose an evolutionary sequence for one of the radio galaxies studied. The implications of this study of neutral hydrogen in nearby radio galaxies for high-z objects and ‘normal’ (i.e. radio-quiet) elliptical galaxies are also discussed. [Copyright &y& Elsevier]