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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]

The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected type 2 quasars – the QSOFEED sample (⁠|$L_{\rm [O\, \small {III}]}\gt 10^{8.5}\, \mathrm{L}_{\odot }$|⁠ ; z < 0.14). Based on visual inspection by eight classifiers, we find clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with 65 |$^{+6}_{-7}$|  per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared with only 22 |$^{+5}_{-4}$|  per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies – a 5σ difference. The type 2 quasar hosts are a factor of 3.0 |$^{+0.5}_{-0.8}$| more likely to be morphologically disturbed than their matched non-AGN counterparts, similar to our previous results for powerful 3CR radio AGN of comparable [O  iii ] emission-line luminosity and redshift. In contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the pre-coalescence phase (61 |$^{+8}_{-9}$|  per cent). We argue that much of the apparent ambiguity that surrounds observational results in this field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological surface brightness dimming. [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]

We present optical imaging and long slit spectroscopic observations of nine luminous type 2 active galactic nuclei (AGN) within the redshift range 0.3

We present new HST/ACS medium- and narrow-band images and optical Isaac Newton Telescope long-slit spectra of the merging system Mrk273. The HST observations sample the [Oш]λλ4959,5007 emission from the galaxy and the nearby continuum. These data were taken as a part of a larger study of ultra-luminous infrared galaxies (ULIRGs) with the aim of investigating the importance of the warm, AGN induced outflows in such objects. The HST images show that the morphologies of the extended continuum and the ionised gas emission from the galaxy are decoupled, extending almost perpendicular to each other. In particular, we detect for the first time a spectacular structure of ionised gas in the form of filaments and clumps that extend ∼23 kpc to the east of the nuclear region. The quiescent ionised gas kinematics at these locations suggests that these filaments are tidal debris left over from a secondary merger event that are illuminated by an AGN in the nuclear regions. The images also reveal a complex morphology in the nuclear region of the galaxy for both the continuum and the [Oш] emission. Consistent with this complexity, we find a wide diversity of emission line profiles in these regions. Kinematic disturbance in the form of broad (FWHM > 500 km s-1) and/or strongly shifted (ǀΔVǀ > 150 km s-1 ) emission line components is found at almost all locations in the nuclear regions, but confined to a radius of ∼4 kpc to the east and west of the northern nucleus. In most cases, we are able to fit the profiles of all the emission lines of different ionisation with a kinematic model using two or three Gaussian components. From these fits, we derive diagnostic line ratios that are used to investigate the ionisation mechanisms at the different locations in the galaxy. We show that these line ratios are generally consistent with photoionisation by an AGN as the main ionisation mechanism. Finally, the highest surface brightness [Oш] emission is found in a compact region that is coincident with the so-called SE nuclear component. The compactness, kinematics, and emission line ratios of this component suggest that it is a separate nucleus with its own AGN. At this stage, further observations are required to confirm the dual (or multiple?) AGN nature of Mrk273. [ABSTRACT FROM AUTHOR]

We present deep, intermediate resolution, long-slit Gemini GMOS-S optical spectra of the Sloan Digital Sky Survey (SDSS) type II quasar J002531−104022, which is a highly disturbed system currently undergoing a major merger event. We use these data to model the ages and reddenings of the stellar populations in three distinct spatial regions and find a remarkable uniformity in the properties of the young stellar populations (YSPs) that dominate the optical spectra. The YSPs are all found to have relatively young ages (tysp < 40 Myr), strongly implying that the latest episode of star formation and quasar activity have been triggered quasi-simultaneously. The lack of reddening deduced from both continuum modelling and the measured Balmer decrements (E(B − V) < 0.3) suggests that starburst and/or active galactic nuclei-induced outflows have already effectively removed a substantial proportion of the gas and dust from the central region. These findings starkly contrast with model predictions which suggest an offset of a few 100 Myr between the peak of merger-induced star formation and the emergence of the optically visible quasar activity. Based on our stellar population fits, we also show that the total stellar mass is in the range (4–17) × 1010 M⊙, lower than typically found for quasar host galaxies. [ABSTRACT FROM AUTHOR]

We present an optical spectroscopic study of a 90 per cent complete sample of nearby ULIRGs (z < 0.175) with optical Seyfert nuclei, with the aim of investigating the nature of the near-nuclear (r ≲ 3.5 kpc) warm gas outflows. A high proportion (94 per cent) of our sample show disturbed emission line kinematics in the form of broad (FWHM > 500 km s−1) and/or strongly blueshifted (ΔV < −150 km s−1) emission line components. This proportion is significantly higher than found in a comparison sample of nearby ultraluminous infrared galaxies (ULIRGs) that lack optical Seyfert nuclei (19 per cent). We also find evidence that the emission line kinematics of the Sy-ULIRGs are more highly disturbed than those of samples of non-ULIRG Seyferts and Palomar–Green quasars in the sense that, on average, their [O iii] λλ5007, 4959 emission lines are broader and more asymmetric.The Sy-ULIRG sample encompasses a wide diversity of emission line profiles. In most individual objects, we are able to fit the profiles of all the emission lines of different ionization with a kinematic model derived from the strong [O iii] λλ4959, 5007 lines, using between two and five Gaussian components. From these fits, we derive diagnostic line ratios that are used to investigate the ionization mechanisms for the different kinematic components. We show that, in general, the line ratios are consistent with gas of supersolar abundance photoionized by a combination of AGN and starburst activity, with an increasing contribution from the AGN with increasing FWHM of the individual kinematic components, and the AGN contribution dominating for the broadest components. However, shock ionization cannot be ruled out in some cases. Our derived upper limits on the mass outflows rates and kinetic powers of the emission line outflows show that they can be as energetically significant as the neutral and molecular outflows in ULIRGs – consistent with the requirements of the hydrodynamic simulations that include AGN feedback. However, the uncertainties are large, and more accurate estimates of the radii, densities and reddening of the outflows are required to put these results on a firmer footing. [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 We present deep Gemini Multi-Object Spectrograph-South optical broad-band images for a complete sample of 20 Sloan Digital Sky Survey selected type II quasars taken from Zakamska et al., with redshifts in the range 0.3 < z < 0.41 and [O iii]λ5007 emission-line luminosities [ABSTRACT FROM AUTHOR]

ABSTRACT We present a compilation of Hubble Space Telescope ( HST) images of 58 luminous Sloan Digital Sky Survey (SDSS) type 2 active galactic nuclei (AGNs) at . Of these, 42 are type 2 quasars, which is a good representation of all optically selected SDSS type 2 quasars in this z range. We find that the majority of the host galaxies are ellipticals (30/42 or 71 per cent). This is consistent with studies of radio-loud and radio-quiet type 1 quasars, which show that their host galaxies are, in general, ellipticals. A significant fraction of type 2 quasars (≥25/42 or ≥59 per cent) show clear signatures of morphological disturbance. In most cases, these are clearly identified with merger/interaction processes. We discuss this in the context of related works on type 2 quasars and powerful radio galaxies. We study in detail the particular case of the radio-quiet type 2 quasar SDSS J143027.66-005614.8 at z= 0.318 based on imaging and spectroscopic data from the Very Large Telescope, HST and SDSS. The system shows highly complex morphology, similar to that found in many ultraluminous infrared galaxies, which suggests that it is in the late pre-coalescence stage of a major galaxy merger. The optical continuum spectrum is dominated by a young stellar population of age <80 Myr, probably formed as a consequence of the merger-induced starburst. Ionized gas is confirmed up to a maximum total extension of r∼ 13 kpc from the quasar, although gas is also possibly detected at up to r∼ 32 kpc. The ionizing mechanism, AGN versus stellar photoionization, varies depending on the spatial location. There is a nuclear ( r≤ few kpc) ionized outflow, which is blueshifted by ∼520 km s−1 relative to the systemic redshift and has FWHM ∼ 1600 km s−1. Several × 105 M⊙ at most are expected to participate in the outflow. We discuss the global properties of the object in the context of theoretical and observational studies of galaxy mergers/interactions and their role in the triggering of the nuclear and star formation activities in the most luminous active galaxies. [ABSTRACT FROM AUTHOR]

We present the results of a program of K- and -band imaging of a sample of 2 Jy radio galaxies with redshifts , for which the host galaxy morphologies and structural parameters (effective radius, Sérsic index and unresolved nuclear point source contribution) have been determined usinggalfit. Two-thirds of our sample are best modelled as being hosted by massive elliptical galaxies with Sérsic indices of , with the remainder being better suited either by a mixture of morphological components (usually a bulge plus a small, less luminous, disc component) or by more discy galaxy models with . Our measured galaxy sizes are generally in very good agreement with other imaging programs, both space- and ground-based. We also determine a slightly higher average nuclear point source contribution than similar Hubble Space Telescope based programs. This is due to our inability to separate the active galactic nuclei emission from compact circum-nuclear stellar emission, but does not bias our modelling of the remainder of the host galaxies and our results remain robust. We also observe that roughly half of the objects in our sample are either undergoing major or minor merger activity or are clearly morphologically disturbed. [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]

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 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 the results of a multiwavelength study of the z= 0.31 radio source PKS2250−41. Integral field unit and long-slit spectroscopy obtained using VIMOS and FORS1 on the VLT, and archival Hubble Space Telescope optical imaging observations are used to study the morphology, kinematics and ionization state of the extended emission-line region (EELR) surrounding this source, and also a companion galaxy at a similar redshift. Near-infrared imaging observations obtained using the New Technology Telescope are used to analyse the underlying galaxy morphologies. The EELR displays a complex variety of different gas kinematics and ionization states, consistent with a mixture of radio source shocks and active galactic nucleus (AGN) photoionization. The radio galaxy is likely to lie within a group environment, and is plausibly undergoing interactions with one or more other objects. The disc-like galaxy to the north-east of the radio source lies at a similar redshift to the radio galaxy itself, and has its major axis position angle aligned with the filamentary continuum and line emission extending outwards from the radio galaxy. This filamentary structure is most plausibly interpreted as a tidal structure associated with an interaction involving the radio source host galaxy and the aligned companion galaxy to the north-east; this encounter may have potentially triggered the current epoch of radio source activity. Overall, PKS2250−41 displays some of the best evidence that radio source activity can be triggered in this manner. While the environment and recent interactions of a radio galaxy can have some bearing on its subsequent evolution, our data also highlight the varied means by which the radio source can effect changes in adjacent objects. Our IFU and long-slit spectroscopy confirm the presence of radio source shocks within the western radio lobe, and, together with our continuum observations, add further weight to the presence of a faint continuum source coincident with the secondary hotspot in the western radio lobe. On the basis of our multiwavelength observations of this object, we suggest that the radio source has indeed triggered recent star formation within this faint companion. [ABSTRACT FROM AUTHOR]

We present the results of a multiwavelength study of the radio source PKS1932−46. Integral field unit spectroscopy using the Visible Multi-object Spectrograph (VIMOS) on the Very Large Telescope (VLT) is used to study the morphology, kinematics and ionization state of the extended emission-line region (EELR) surrounding this source, and also a companion galaxy at a similar redshift. Near- and far-infrared imaging observations obtained using the New Technology Telescope and Spitzer are used to analyse the underlying galaxy morphologies and the nature of the active galactic nucleus (AGN). The host galaxy is identified as an elliptical. Combining Spitzer mid-infrared (mid-IR) with X-ray, optical and near-IR imaging observations of this source, we conclude that its AGN is underluminous for a radio source of this type, despite its status as a broad-line object. However, given its relatively large [O iii] luminosity it is likely that the AGN was substantially more luminous in the recent past ( ago). The EELR is remarkably extensive and complex, reminiscent of the systems observed around sources at higher redshifts/radio powers, and the gas is predominantly ionized by a mixture of AGN photoionization and emission from young stars. We confirm the presence of a series of star-forming knots extending north–south from the host galaxy, with more prodigious star formation occurring in the merging companion galaxy to the north-east, which has sufficient luminosity at mid- to far-IR (MFIR) wavelengths to be classified as a luminous infrared galaxy (LIRG). The most plausible explanation of our observations is that PKS1932−46 is a member of an interacting galaxy group, and that the impressive EELR is populated by star-forming, tidal debris. We suggest that the AGN itself may currently be fuelled by material associated either with the current interaction or with a previous merger event. Surprisingly, it is the companion object, rather than the radio source host galaxy, which is undergoing the bulk of the star formation activity within the group. [ABSTRACT FROM AUTHOR]

We use deep optical, infrared and radio observations to explore the symbiosis between nuclear activity and galaxy evolution in the southern compact radio source PKS 1549−79 . The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. The merger hypothesis is further supported by the detection of a young stellar population (YSP), which, on the basis of spectral synthesis modelling of our deep Very Large Telescope (VLT) optical spectra, was formed 50–250 Myr ago and makes up a significant fraction of the total stellar mass (1–30 per cent). Despite the core-jet structure of the radio source, which is consistent with the idea that the jet is pointing close to our line of sight, our H i 21-cm observations reveal significant H i absorption associated with both the core and the jet. Moreover, the luminous, quasar-like active galactic nucleus (AGN) is highly extinguished at optical wavelengths and show many properties in common with narrow-line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines [full width at half-maximum (FWHM) ∼ 1940 km s−1], highly blueshifted [O iii]λλ5007,4959 lines and evidence that the putative supermassive black hole is accreting at a high Eddington ratio . The results suggest that accretion at high Eddington ratio does not prevent the formation of powerful relativistic jets. Together, the observations lend strong support to the predictions of some recent numerical simulations of galaxy mergers in which the black hole grows rapidly through merger-induced accretion following the coalescence of the nuclei of two merging galaxies, and the major growth phase is largely hidden at optical wavelengths by the natal gas and dust. Although the models also predict that AGN-driven outflows will eventually remove the gas from the bulge of the host galaxy, the visible warm outflow in PKS 1549−79 is not currently capable of doing so. However, much of the outflow may be hidden by the material obscuring the quasar and/or tied up in hotter or cooler phases of the interstellar medium. By combining our estimates of the reddening of the quasar with the H i column derived from the 21-cm radio observations, we have also made the first direct estimate of the H i spin temperature in the vicinity of a luminous AGN: . [ABSTRACT FROM AUTHOR]

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]

ABSTRACT We present a compilation of Hubble Space Telescope ( HST) images of 58 luminous Sloan Digital Sky Survey (SDSS) type 2 active galactic nuclei (AGNs) at . Of these, 42 are type 2 quasars, which is a good representation of all optically selected SDSS type 2 quasars in this z range. We find that the majority of the host galaxies are ellipticals (30/42 or 71 per cent). This is consistent with studies of radio-loud and radio-quiet type 1 quasars, which show that their host galaxies are, in general, ellipticals. A significant fraction of type 2 quasars (≥25/42 or ≥59 per cent) show clear signatures of morphological disturbance. In most cases, these are clearly identified with merger/interaction processes. We discuss this in the context of related works on type 2 quasars and powerful radio galaxies. We study in detail the particular case of the radio-quiet type 2 quasar SDSS J143027.66-005614.8 at z= 0.318 based on imaging and spectroscopic data from the Very Large Telescope, HST and SDSS. The system shows highly complex morphology, similar to that found in many ultraluminous infrared galaxies, which suggests that it is in the late pre-coalescence stage of a major galaxy merger. The optical continuum spectrum is dominated by a young stellar population of age <80 Myr, probably formed as a consequence of the merger-induced starburst. Ionized gas is confirmed up to a maximum total extension of r∼ 13 kpc from the quasar, although gas is also possibly detected at up to r∼ 32 kpc. The ionizing mechanism, AGN versus stellar photoionization, varies depending on the spatial location. There is a nuclear ( r≤ few kpc) ionized outflow, which is blueshifted by ∼520 km s−1 relative to the systemic redshift and has FWHM ∼ 1600 km s−1. Several × 105 M⊙ at most are expected to participate in the outflow. We discuss the global properties of the object in the context of theoretical and observational studies of galaxy mergers/interactions and their role in the triggering of the nuclear and star formation activities in the most luminous active galaxies. [ABSTRACT FROM AUTHOR]