D. Dicken

THE ORIGIN OF THE INFRARED EMISSION IN RADIO GALAXIES. II. ANALYSIS OF MID- TO FAR-INFRARED SPITZER OBSERVATIONS OF THE 2JY SAMPLE

We present an analysis of deep mid- to far-infrared (MFIR) Spitzer photometric observations of the southern 2Jy sample of powerful radio sources (0.05 < z < 0.7), conducting a statistical investigation of the links between radio jet, active galactic nucleus (AGN), starburst activity and MFIR properties. This is part of an ongoing extensive study of powerful radio galaxies that benefits from both complete optical emission line information and a uniquely high detection rate in the far-infrared (far-IR). We find tight correlations between the MFIR and [OIII] lambda 5007 emission luminosities, which are significantly better than those between MFIR and extended radio luminosities, or between radio and [OIII] luminosities. Since [OIII] is a known indicator of intrinsic AGN power, these correlations confirm AGN illumination of the circumnuclear dust as the primary heating mechanism for the dust producing thermal MFIR emission at both 24 and 70 mu m. We demonstrate that AGN heating is energetically feasible, and identify the narrow-line region clouds as the most likely location of the cool, far-IR emitting dust. Starbursts make a major contribution to the heating of the cool dust in only 15%-28% of our targets. We also investigate the orientation dependence of the continuum properties, finding that the broad-and narrow-line objects in our sample with strong emission lines have similar distributions of MFIR luminosities and colors. Therefore our results are entirely consistent with the orientation-based unified schemes for powerful radio galaxies. However, the weak line radio galaxies form a separate class of objects with intrinsically low-luminosity AGNs in which both the optical emission lines and the MFIR continuum are weak.

The origin of the infrared emission in radio galaxies. I. New mid- to far-infrared and radio observations of the 2 Jy sample

As part of a large study to investigate the nature of the longer wavelength continuum emission of radio-loud AGNs, we present new mid- to far-infrared (MFIR) and high-frequency radio observations for a complete sample of 2 Jy powerful southern radio galaxies at intermediate redshifts (0.05 < z < 0.7). Utilizing the sensitivity of the Spitzer Space Telescope, we have made deep MIPS observations at wavelengths of 24, 70, and 160 mu m, detecting 100% of our sample at 24 mu m and 90% at 70 mu m. This high detection rate at MFIR wavelengths is unparalleled in samples of intermediate-redshift radio galaxies. Complementing these results, we also present new high-frequency observations (15-24 GHz) from the Australia Telescope Compact Array and the Very Large Array, which are used to investigate the potential contamination of the MFIR continuum by nonthermal synchrotron emission. With the latter observations we detect compact cores in 59% of our complete sample and deduce that nonthermal contamination of the MFIR continuum is significant in a maximum of 30% of our total sample. MFIR fluxes, radio fluxes, and spectral energy distributions for the complete sample are presented here, while in a second paper we will analyze these data and discuss the implications for our understanding of the heating mechanism for the warm/cool dust, star formation in the host galaxies, and the unified schemes for powerful radio sources.

The environments of luminous radio galaxies and type-2 quasars

We present the results of a comparison between the environments of (1) a complete sample of 46 southern 2-Jy radio galaxies at intermediate redshifts (0.05 <z< 0.7), (2) a complete sample of 20 radio-quiet type-2 quasars (0.3 ≤ z ≤ 0.41), and (3) a control sample of 107 quiescent early-type galaxies at 0.2 ≤ z< 0.7 in the Extended Groth Strip. The environments have been quantified using angular clustering amplitudes (Bgq) derived from deep optical imaging data. Based on these comparisons, we discuss the role of the environment in the triggering of powerful radio-loud and radio-quiet quasars. When we compare the Bgq distributions of the type-2 quasars and quiescent early-type galaxies, we find no significant difference between them. This is consistent with the radio-quiet quasar phase being a short-lived but ubiquitous stage in the formation of all massive early-type galaxies. On the other hand, powerful radio galaxies are in denser environments than the quiescent population, and this difference between distributions of Bgq is significant at the 3σ level. This result supports a physical origin of radio loudness, with high-density gas environments favouring the transformation of active galactic nucleus (AGN) power into radio luminosity, or alternatively, affecting the properties of the supermassive black holes themselves. Finally, focusing on the radio-loud sources only, we find that the clustering of weak-line radio galaxies (WLRGs) is higher than the strong-line radio galaxies (SLRGs), constituting a 3σ result. 82 per cent of the 2-Jy WLRGs are in clusters, according to our definition (Bgq 400), versus only 31 per cent of the SLRGs.

The Heating Mechanism for the Warm/Cool Dust in Powerful, Radio-loud Active Galactic Nuclei

The uncertainty surrounding the nature of the heating mechanism for the dust that emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits our understanding of the links between active galactic nuclei (AGNs) and galaxy evolution, as well as our ability to interpret the prodigious infrared and submillimeter emission of some of the most distant galaxies in the universe. Here we report deep Spitzer observations of a complete sample of powerful, intermediate-redshift (0.05 < z < 0.7) radio galaxies and quasars. We show that AGN power, as traced by [O III] λ5007 emission, is strongly correlated with both the mid-IR (24 μm) and the far-IR (70 μm) luminosities, but with increased scatter in the 70 μm correlation. A major cause of this increased scatter is a group of objects that falls above the main correlation and shows signs of prodigious recent star formation activity at optical wavelengths, along with relatively cool MFIR colors. These results provide evidence that illumination by the AGNs is the primary heating mechanism for the dust emitting at both 24 and 70 μm, with starbursts dominating the heating of the cool dust in only 20%-30% of objects. This implies that powerful AGNs are not always accompanied by the type of luminous starbursts that are characteristic of the peak of activity in major gas-rich mergers.

PKS2250-41: a case study for triggering ⋆

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. n n n nThe 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. n n n nWhile 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.

PKS1932−46: a radio source in an interacting group?★

We present the results of a multiwavelength study of the z= 0.23 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). n n n nThe host galaxy is identified as an ∼M★ 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 [Oxa0iii] luminosity it is likely that the AGN was substantially more luminous in the recent past (≲104 yr ago). n n n nThe 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). n n n nThe 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.

PKS 0347+05: a radio-loud/radio-quiet double active galactic nucleus system triggered in a major galaxy merger

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double 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 PAH 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 FRII 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 ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of tsim 5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.

PKS 0347+05

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double 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 PAH 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 FRII 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 ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of tsim 5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.

PKS0347+05: a radio-loud/radio-quiet double AGN system triggered in a major galaxy merger

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double 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 PAH 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 FRII 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 ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of tsim 5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.

PKS 0347+05: a radio-loud/radio-quiet double active galactic nucleus system triggered in a major galaxy merger: The PKS 0347+05 double-AGN system

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double 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 PAH 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 FRII 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 ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of tsim 5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.