K. A. Wills

Starbursts and the triggering of the activity in nearby powerful radio galaxies

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), massive (109 < MYSP < 2 × 1010 M⊙) and 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.

The origin of the UV excess in powerful radio galaxies: spectroscopy and polarimetry of a complete sample of intermediate-redshift radio galaxies

We present spectroscopic and polarimetric observations of a complete, optically unbiased sample of 2-Jy radio galaxies at intermediate redshifts (0.15 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.

Spectroscopy of the near-nuclear regions of Cygnus A: estimating the mass of the supermassive black hole

We use a combination of high spatial resolution optical and near-infrared spectroscopic data to make a detailed study of the kinematics of the narrow-line region (NLR) gas in the near-nuclear regions of the powerful, FRIIradio galaxy Cygnus A (z = 0.0560), with the overall goal of placing limits on the mass of any supermassive black hole in the core. Our K-band infrared observations (0.75-arcsec seeing) - taken with NIRSPEC on the Keck II telescope - show a smooth rotation pattern across the nucleus in the Paa and H 2 emission lines along a slit position (PA180°) close to perpendicular to the radio axis, however, there is no evidence for such rotation along the radio axis (PA105°). Higher spatial resolution observations of the [O III] λ5007 emission line -- taken with STIS on the Hubble Space Telescope (HST) - confirm the general rotation pattern of the gas in the direction perpendicular to the radio axis, and provide evidence for steep velocity gradients within a radius of 0.1 arcsec of the core - corresponding to the high surface brightness structure visible in high-resolution narrow-band images. The [O IH] line remains broad throughout the core region (FWHM ∼ 300-900 km s - 1 ), but the spatial distribution of [O III] provides no evidence for an unresolved inner narrow-line region (INLR). Assuming that the ionized gas is circularly rotating in a thin disc and that the large linewidths are due to activity-induced turbulence, the circular velocities measured from both the Keck and HST data lead to an estimate of the mass of the supermassive black hole of 2.5 ′ 0.7 x 10 9 M O .. For the host galaxy properties of Cygnus A, this mass is consistent with the global correlations between black hole mass and host galaxy properties deduced for non-active galaxies. Therefore, despite the extreme power of its radio source and the quasar-like luminosity of its active galactic nucleus (AGN), the black hole in Cygnus A is not unusually massive considering the luminosity of its host galaxy. Indeed, the estimated mass of the black hole in Cygnus A is similar to that inferred for the supermassive black hole in the FRI radio galaxy M87, despite the fact that the AGN and radio jets of Cygnus A are two to three orders of magnitude more powerful. Overall, these results are consistent with the idea that the properties and powers of the AGN in radio galaxies are determined as much by the mass accretion rates as by the absolute masses of their supermassive black holes. As well as providing evidence for a supermassive black hole in the core of Cygnus A, our data also demonstrate that nuclear activity has an important effect on the kinematics of the circumnuclear gas on a subkiloparsec scale. Most notably, the velocity offsets measured in the two outer HST/STIS slit positions are consistent with the presence of an activity-induced outflow in the NW cone.

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.

Emission lines and optical continuum in low-luminosity radio galaxies

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.

Timescales of merger, starburst and AGN activity in radio galaxy B2 0648+27

We use neutral hydrogen (HI) and optical spectroscopic observations to compare the timescales of a merger event, starburst episode and radio-AGN activity in the radio galaxy B2 0648+27. We detect a large ring-like structure of HI in emission around the early-type host galaxy of B2 0648+27 (M-HI = 8.5 x 10(9) M-circle dot, diameter = 190 kpc). We interpret this as the result of a major merger that occurred greater than or similar to 1.5 Gyr ago. From modelling optical long-slit spectra we find that a young stellar population of 0.3 Gyr, indicative of a past starburst event, dominates the stellar light throughout the galaxy. The off-set in time between the merger event and the starburst activity in B2 0648+27 suggests that the starburst was triggered in an advanced stage of the merger, which can be explained if the gas-rich progenitor galaxies contained a bulge. Although the exact age of the radio source remains uncertain, there appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity. We also observe an outflow of emission-line gas in this system, which is likely related to superwinds driven by the stars that formed during the starburst event. We argue that the radio galaxy B2 0648+27 is a link in the evolutionary sequence between Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-type galaxies.

The unfriendly ISM in the radio galaxy 4C 12.50 (PKS 1345+12)

The radio source 4C12.50 has often been suggested to be a prime candidate for the link between ultraluminous infrared galaxies and young radio galaxies. A VLBI study of the neutral hydrogen in the nuclear regions of this object shows that most of the gas detected close to the systemic velocity is associated with an off-nuclear cloud (~50 to 100 pc from the radio core) with a column density of ~10^22 T_spin/100 K) cm^(-2) and an HI mass of a few times 10^5 to 10^6 M_sun. We consider a number of possibilities to explain the results. In particular, we discus the possibility that this cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud -- at the edge of the northern radio jet/lobe -- suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet. The velocity profile of the gas is relatively broad (~150

Global very long-baseline interferometry observations of compact radio sources in M82

km/s) and we interpret this as kinematical evidence for interaction of the radio plasma with the cloud. We also consider the model where the cloud is part of a broader circumnuclear structure. Only a limited region of this structure would have sufficient background radio brightness and large enough column depth in neutral gas to obtain detectable HI absorption against the counterjet. The VLBI study of the neutral hydrogen in 4C12.50 suggests that HI detected near the systemic velocity (as it is often the case in radio galaxies) may not necessarily be connected with a circumnuclear disk or torus (as is very often assumed) but instead could be a tracer of the large-scale medium that surrounds the active nucleus and that may influence the growth of the young radio source.

Large-scale gas disk around the radio galaxy Coma A ?

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at λ18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 ± 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR. The latest global observations have also provided high resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being due to the confinement of a supernova by a high density circumstellar medium. It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

Near-infrared spectroscopy of PKS 1549-79: a protoquasar revealed?

We present WSRT and VLA radio observations of the neutral hydrogen in the radio galaxy Coma A. We detect extended H I absorption against both radio lobes of Coma A, at distances of about 30 kpc from the centre. Coma A is the rst radio galaxy in which H I is seen in absorption at such large distances from the nucleus. The match between the velocities of the neutral hydrogen and those of the extended ionized gas suggests that they are part of the same disk-like structure of at least 60 kpc in diameter. Most likely, this gas disk is partly ionised by the bulk motion of the radio lobes expanding into it. The gas mass of this disk is at least 10 9 M.T he relatively regular structure of the gas disk suggests that a merger occurred involving at least one large gas-rich galaxy, at least a few times 10 8 yr ago.