K. J. Inskip

The co-evolution of the obscured quasar PKS 1549−79 and its host galaxy: evidence for a high accretion rate and warm outflow

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 (z = 0.1523). 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 HI 21-cm observations reveal significant HI absorption associated with both the core and the jet. Moreover, the luminous, quasar-like active galactic nucleus (AGN) (MV 6.4) 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) similar to 1940 km s(-1)], highly blueshifted [O III]lambda lambda 5007,4959 lines (Delta V similar to 680 km s(-1)) and evidence that the putative supermassive black hole is accreting at a high Eddington ratio (0.3 3000 K.

The properties of the young stellar populations in powerful radio galaxies at low and intermediate redshifts

We present high-quality, wide spectral coverage long-slit optical spectra for 12 powerful radio sources at low and intermediate redshifts (z < 0.7) 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.

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.

Deep spectroscopy of z 1 6C radio galaxies - II. Breaking the redshift-radio power degeneracy

The results of a spectroscopic analysis of 3CR and 6C radio galaxies at redshift z � 1 are contrasted with the properties of lower redshift radio gala xies, chosen to be matched in radio luminosity to the 6C sources studied at z � 1, thus enabling the redshift-radio power de- generacy to be broken. Partial rank correlations and principal component analysis have been used to determine which of redshift and radio power are the critical parameters underlying the observed variation of the ionization state and kinematics of the emission line gas. (OII)/His shown to be a useful ionization mechanism diagnostic. Statistical analysis of the data shows that the ionization state of the emission line gas is strongly correlated with radio power, once the effects of other parameters are removed. No dependence of ionization state on cosmic epoch is observed, implying that the ionization state of the emission line gas is solely a function of the AGN properties rather than the host galaxy and/or environment. Statistical analysis of the kinematic properties of the emi ssion line gas shows that these are strongly correlated independently with both redshift a nd radio power. The correlation with redshift is the stronger of the two, suggesting that host gal axy composition or environment may play a role in producing the less extreme gas kinematics observed in the emission line regions of low redshift galaxies. For both the ionization and kinematic properties of the galaxies, the independent corre- lations observed with radio size are stronger than with eith er radio power or redshift. Radio source age is clearly a determining factor for the kinematics and ionization state of the ex- tended emission line regions.

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. 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.

Starburst radio galaxies: general properties, evolutionary histories and triggering: Starburst radio galaxies

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 tYSP≤ 0.1 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 III] emission-line luminosities (LIR > 1011 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.

HST and UKIRT imaging observations of z∼ 1 6C radio galaxies – I. The data

The results of Hubble Space Telescope and UKIRT imaging observations are presented for a sample of 11 6C radio galaxies with redshifts 0.85 < z < 1.5. The observations of the 6C sources reveal a variety of different features, similar to those observed around the higher power 3CR sources of similar redshifts. However, the extent and luminosity of the aligned emission appear less extreme in the case of the 6C radio galaxies. For both samples, the aligned emission clearly cannot be explained by a single emission mechanism; however, line emission and related nebular continuum emission often provide a significant contribution to the aligned emission.

Deviations from passive evolution - Star formation and the UV excess in z 1 radio galaxies

ABSTRACT Galaxy colours are determined for two samples of 6C and 3CR radio sources at z ∼ 1,differing by a factor of ∼ 6 in radio power. Corrections are made for emission line contam-ination and the presence of any nuclear point source, and the data analysed as a function ofboth redshift and the radio source properties. The galaxy colours are remarkably similar forthe two populations, and the UV excess evolves with radio source size similarly in both sam-ples, depsite the fact that the alignment effect is more extensive for the more powerful 3CRradio galaxies. These results seem to suggest that the alignment effect at these redshifts doesnot scale strongly with radio power, and is instead more closely dependent on galaxy mass(which is statistically comparable for the two samples). However, it is likely that the presenceof relatively young (∼< several 10 8 years old) stellar populations has considerably contami-nated the K−band flux of these systems, particularly in the case of the mor e powerful 3CRsources, which are ∼ 0.5mag more luminous than the predictions of passive evolution modelsat z ∼ 1. The higher luminosity of the 3CR alignment effect is balanced by emission at longerwavelengths, thereby leading to comparable colours for the two samples.Key words: galaxies: active – galaxies: photometry – galaxies: evolut ion

Deviations from passive evolution — star formation and the ultraviolet excess in z∼ 1 radio galaxies

ABSTRACT Galaxy colours are determined for two samples of 6C and 3CR radio sources at z ∼ 1,differing by a factor of ∼ 6 in radio power. Corrections are made for emission line contam-ination and the presence of any nuclear point source, and the data analysed as a function ofboth redshift and the radio source properties. The galaxy colours are remarkably similar forthe two populations, and the UV excess evolves with radio source size similarly in both sam-ples, depsite the fact that the alignment effect is more extensive for the more powerful 3CRradio galaxies. These results seem to suggest that the alignment effect at these redshifts doesnot scale strongly with radio power, and is instead more closely dependent on galaxy mass(which is statistically comparable for the two samples). However, it is likely that the presenceof relatively young (∼< several 10 8 years old) stellar populations has considerably contami-nated the K−band flux of these systems, particularly in the case of the mor e powerful 3CRsources, which are ∼ 0.5mag more luminous than the predictions of passive evolution modelsat z ∼ 1. The higher luminosity of the 3CR alignment effect is balanced by emission at longerwavelengths, thereby leading to comparable colours for the two samples.Key words: galaxies: active – galaxies: photometry – galaxies: evolut ion

Deep spectroscopy of 9C J1503+4528: a very young CSS radio source at z=0.521

9C J1503+4528 is a very young CSS radio galaxy, with an age of order 10^4 years. This source is therefore an ideal laboratory for the study ofthe intrinsic host galaxy/IGM properties, interactions between the radio source and surrounding ISM, links between star formation and AGN activity and the radio source triggering mechanism. Here we present the results of a spectroscopic analysis of this source, considering each of these aspects of radio source physics. We find that shock ionization by the young radio source is important in the central regions of the galaxy on scales similar to that of the radio source itself, whilst evidence for an AGN ionization cone is observed at greater distances. Line and continuum features require the presence of a young stellar population, the best-fit model for which implies an age of 5x10^6 years, significantly older than the radio source. Most interestingly, the relative sizes of radio source and extended emission line region suggest that both AGN and radio source are triggered at approximately the same time. If both the triggering of the radio source activity and the formation of the young stellar population had the same underlying cause, this source provides a sequence for the events surrounding the triggering process. We propose that the AGN activity in 9C J1503+4528 was causedby a relatively minor interaction, and that a super-massive black hole powering the radio jets must have been in place before the AGN was triggered.