Powerful Radio Galaxies Research Articles

Extended inverse-Compton emission from distant, powerful radio galaxies

We present Chandra observations of two relatively high redshift FRII radio galaxies, 3C 432 and 3C 191 (z=1.785 and z=1.956 respectively), both of which show extended X-ray emission along the axis of the radio jet or lobe. This X-ray emission is most likely to be due to inverse-Compton scattering of Cosmic Microwave Background (CMB) photons. Under this assumption we estimate the minimum energy contained in the particles responsible. This can be extrapolated to determine a rough estimate of the total energy. We also present new, deep radio observations of 3C 294, which confirm some association between radio and X-ray emission along the NE-SW radio axis and also that radio emission is not detected over the rest of the extent of the diffuse X-ray emission. This, together with the offset between the peaks of the X-ray and radio emissions may indicate that the jet axis in this source is precessing.

A near-infrared view of the 3CR: properties of hosts and nuclei†

AbstractThe 3CR catalogue provides a statistical sampling of the most powerful radio galaxies out to z ∼ 0.3. Over the decade and a half of Hubble observations we have amassed a major multi-wavelength dataset on these sources, discovering amongst other things, new jets, hotspots, dust disks, and faint point-like nuclei. We present here the results of our latest snapshot survey, a near-complete sampling of the 3CR host galaxies at z < 0.3 in the near-IR (H-band). This un-extinguished view of the host galaxies has provided us with an accurate measure of the stellar/spheroid masses of the sources, and an unbiased view of their morphologies. We show that they exhibit an identical Kormendy relation to nearby QSO's and the massive Elliptical population, but are distinct from the Brightest Cluster Members, and mergers. We find that while a few sources exhibit signs of a recent or impending major merger, many more sources have remnants consistent with a gas-rich minor merger in their recent history. We detect unresolved nuclear sources in most (∼80%) of FRI, with their IR luminosities correlating linearly with radio core power. This implies that the IR nuclei are synchrotron radiation produced at the base of the relativistic jet, and confirms that no infrared (thermal) radiation in excess to synchrotron is present in FRIs, unlike in other classes of AGN.

The Stellar Mass Distribution of Powerful Radio Galaxies Across 1 < z < 5.2

AbstractWe present the results of a comprehensive Spitzer survey of 70 radio galaxies across 1 < z < 5.2. Using IRAC (3.6–8.0μm), IRS (16 μm) and MIPS (24–160 μm) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame 1.6 μm (H-band). We find that the fraction of emitted light at rest-frame 1.6 μm from stars is >80% for over half the high redshift radio galaxies. The other radio galaxies have 1.6 μm stellar fractions spanning the range 20–80%. The resultant stellar luminosities imply stellar masses of 1011−12M⊙, independent of redshift, indicating that radio galaxies are amoungst the most massive galaxies observed over this redshift range. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the inferred stellar contribution to their shorter-wavelength, mid-IR SEDs. The stellar fraction of the rest-frame 1.6 μm luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5 μm) luminosity. The bolometric energy output of these sources is dominated by the infrared, and the mid-IR luminosities are found to be similar to that of lower redshift (z < 1) radio galaxies. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. A weak, but significant, correlation of stellar mass with radio luminosity is found, consistent with earlier results.

Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe

We present measurements obtained with the Spitzer Space Telescope in five bands from 3.6 to 24 μm of the northern inner radio lobe of Centaurus A, the nearest powerful radio galaxy. We show that this emission is synchrotron in origin. Comparison with ultraviolet observations from GALEX shows that diffuse ultraviolet emission exists in a smaller region than the infrared but also coincides with the radio jet. We discuss the possibility that synchrotron emission is responsible for the ultraviolet emission, and we conclude that further data are required to confirm this.

A jet-cloud interaction in the 3C 196 environment

Powerful radio galaxies and radio-loud quasars at high redshifts are frequently associated with extended emission-line regions (EELRs). Here we investigate the [O II] EELR around the quasar 3C 196 at z=0.871 using integral field spectroscopy. We also detect extended [Ne II] emission at a distance of about 30 kpc from the core. The emission is aligned with the radio hot spots and shows a redshifted and a blueshifted component with a velocity difference of ~800 km s^-1. The alignment effect and large velocities support the hypothesis that the EELR is caused by a jet-cloud interaction, which is furthermore indicated by the presence of a pronounced bend in the radio emission at the location of the radio hot spots. We also report observations of two other systems which do not show as clear indications of interactions. We find a weaker alignment of an [O II] EELR from the z=0.927 quasar 3C 336, while no EELR is found around the core-dominated quasar OI 363 at z=0.63.

CO Line Width Differences in Early Universe Molecular Emission-Line Galaxies: Submillimeter Galaxies versus QSO Hosts

We investigate the CO line widths in early universe molecular emission-line galaxies. There is a clear difference in the CO line widths in QSO host galaxies compared to submillimeter galaxies, as first pointed out by Greve et al., with the QSO hosts' line widths typically a factor of 2.3 narrower in velocity. Powerful radio galaxies fall in between. At the same time, there is no major difference between the molecular gas mass distributions of the different galaxy types. We consider possible explanations for the narrower line widths in the QSO hosts and favor an explanation that the QSO hosts are systematically closer to the sky plane than the submillimeter galaxies, QSO = 18°, consistent with the lack of strong obscuration toward the active galactic nucleus. However, we cannot rule out other possible factors, such as different stages in a merger sequence or galaxy mass or size. Only high-resolution imaging of a large sample of sources can address this interesting dichotomy.

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

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 contamination and the presence of any nuclear point source, and the data analysed as a function of both redshift and the radio source properties. The galaxy colours are remarkably similar for the two populations, and the ultraviolet excess evolves with radio source size similarly in both samples, despite the fact that the alignment effect is more extensive for the more powerful 3CR radio galaxies. These results seem to suggest that the alignment effect at these redshifts does not 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 presence of relatively young (several times 10 8 -yr-old) stellar populations has considerably contaminated the K-band flux of these systems, particularly in the case of the more powerful 3CR sources, which are ∼0.5 mag more luminous than the predictions of passive evolution models at z ∼ 1. The higher luminosity of the 3CR alignment effect is balanced by emission at longer wavelengths, thereby leading to comparable colours for the two samples.

Protoclusters associated with distant radio galaxies

AbstractThe results of a large program conducted with the Very Large Telescope and the Keck telescope to search for forming clusters of galaxies near powerful radio galaxies at 2 < z < 5.2 are presented. Deep narrow‐ and broad‐band images of eight radio galaxies and their surroundings were obtained. In the images a total of 335 candidate Lyα emitting galaxies at the redshift of the radio galaxies were selected. Subsequently, follow‐up spectroscopy was performed to obtain redshifts for the candidate Lyα emitters. In at least six of eight fields (75%) an overdensity of Lyα emitters is found near the radio galaxy. Taken together with a significant clustering in redshift space of the emitters, we argue that we have discovered six forming clusters of galaxies (protoclusters). Based on the estimated masses and the volume density, we conclude that the observed structures are the likely progenitors of present‐day massive (∼1015 M⊙) clusters of galaxies. Finally, we compare the observed galaxy overdensities with numerical simulations of the development of large scale structure in the Universe. The preliminary results of this comparison provide strong additional evidence for the hypothesis that the radio galaxy protoclusters are forming massive clusters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The relation between mergers and AGN activity

AbstractThere is morphological evidence that the activity in powerful radio galaxies could be triggered by mergers and galaxy interactions. However, nothing is known about the timescales, order of events, and the type of interaction involved. It is not yet known whether there exists an evolutionary link between radio galaxies and other merger systems such as very luminous and ultraluminous infrared galaxies (VLIRGs and ULIRGs). Here, we report preliminary results obtained from the analysis of near‐ultraviolet and optical spectroscopic observations of samples of Radio Galaxies and VLIRGs‐ULIRGs to investigate, through age‐dating of their young stellar population, whether an evolutionary link exists between VLIRGs/ULIRGs–Radio Galaxies–normal elliptical galaxies. These results will help to understand the genesis events that lead to the formation of radio jet and quasar activity, and they will allow us to place radio galaxies in the context of hierarchical evolution models for the population of giant elliptical galaxies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The lifecycles of powerful radio galaxies: Triggering and feedback

AbstractI review the links between host galaxy evolution and quasar/jet activity in radio galaxies from an observational standpoint. Observations of the warm ionized gas provide evidence for powerful outflows in both the extended emission line regions (EELR: 5 < r < 100 kpc) and the narrow line regions (NLR: r < 5 kpc) that are capable of directly influencing the evolution of the stellar populations in the host galaxies. There is also evidence that the distribution of circum‐nuclear ISM undergoes significant evolution as the radio jets expand through the central bulges of the hosts. On the other hand, results for the young stellar populations in the host galaxies provide key information about the triggering merger events. They indicate that there can be a significant time lag between the major episode of merger‐induced star formation and triggering of the radio jets, and that some ultraluminous infrared galaxies may evolve into radio galaxies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolutionary models of powerful radio galaxies

AbstractIn the past, extragalactic radio jets have been modelled as interacting with a smooth, hot interstellar medium. However, it is likely that the precursors of radio sources are major or minor mergers and the resultant debris from such an event will interact with the jets that result from the fuelling of the black hole. We describe here some recent work on the interactions of jets with clumpy interstellar media consisting of a log normal distribution of dense clouds. Important features of the simulations include radio morphologies typical of powerful radio galaxies at various stages of their evolution and persistent X‐ray emission from shocked clouds which are the remnant of a circumnuclear disk. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MAMBO 1.25 mm observations of 3CR quasars at z ~ 1.5: on the debate of the unified schemes

In order to explore the nature of the 850 μm flux difference between powerful radio galaxies and steep radio-spectrum quasars at z ∼ 1.5 reported by Willott et al. (2002), we have observed 9 sources from their sample of 11 quasars at 1.25 mm. For 7 sources the 1.25 mm fluxes are much brighter than one would expect from a purely thermal dust model fitted to the submm data, providing evidence for the synchrotron nature of the observed 1.25 mm radiation. If we extrapolate a power-law synchrotron spectrum to shorter wavelengths, then for 6 of the 9 sources also the 850 μm fluxes are dominated by synchrotron radiation. We discuss how far the (sub)-millimetre data can be interpreted in accordance with the orientation-dependent unified schemes for powerful radio galaxies and quasars. In this case the results challenge the reported evidence for the receding torus model and for the evolutionary trend of a declining dust luminosity with increasing projected size of the radio lobes.

Resolving the Shocks in Radio Galaxy Nebulae:Hubble Space Telescopeand Radio Imaging of 3C 171, 3C 277.3, and PKS 2250-41

We present the results of Hubble Space Telescope (HST) WFPC2 medium-band and narrowband imaging and Very Large Array and MERLIN2 radio imaging of three powerful radio galaxies: 3C 171, 3C 277.3, and PKS 2250-41. We obtained images of the rest frame [O III] λ5007 and [O II] λ3727 line emission using the linear ramp filters on WFPC2. The correlations of the emission-line morphology and the [O III]/[O II] line ratios with the radio emission seen in ground-based observations are clarified by the HST imaging. We confirm that the radio lobes and hot spots are preferentially associated with lower ionization gas. The galaxy 3C 171 exhibits high surface brightness emission-line gas mainly along the radio source axis. The lowest ionization gas is seen at the eastern hot spot. In 3C 277.3 there is bright high-ionization gas (and continuum) offset just to the east of the radio knot K1. Our observations are consistent with previous work suggesting that this emission is produced by precursor gas ionized by the shock being driven into the cloud by the deflected radio jet. In PKS 2250-41 we resolve the emission-line arc that wraps around the outer rim of the western lobe. The lower ionization [O II] emission is nested just interior to the higher ionization [O III] emission, suggesting that we have resolved the cooling region behind the bow shock. We also detect possible continuum emission from the secondary hot spot. Thus, our observations support the hypothesis that in these sources the interaction between the expanding radio source and the ambient gas strongly influences the morphology, kinematics, and ionization of the gas.

A sensitive search for CO J = 1–0 emission in 4C 41.17: high-excitation molecular gas at z = 3.8

We report sensitive imaging observations of the CO J=1-0 line emission in the powerful high-redshift radio galaxy 4C 41.17 (z=3.8) with the NRAO Very Large Array (VLA), conducted in order to detect the large concomitant H_2 gas reservoir recently unveiled in this system by De Breuck et al (2005) via the emission of the high excitation J=4-3 line. Our observations fail to detect the J=1-0 line but yield sensitive lower limits on the R_43=(4-3)/(1-0) brightness temperature ratio of R_43 ~ 0.55 - >1.0 for the bulk of the H_2 gas mass. Such high ratios are typical of the high-excitation molecular gas phase ``fueling'' the star formation in local starbursts, but quite unlike these objects, much of the molecular gas in 4C 41.17 seems to be in such a state, and thus participating in the observed starburst episode. The widely observed and unique association of highly excited molecular gas with star forming sites allows CO line emission with large (high-J)/(low-J) intensity ratios to serve as an excellent ``marker'' of the spatial distribution of star formation in distant dust-obscured starbursts, unaffected by extinction.

Fast neutral outflows in powerful radio galaxies: a major source of feedback in massive galaxies

We report the detection of fast (similar to 1000 km s(-1)), massive outflows of neutral gas observed - using the WSRT - as 21- cm H), Hi absorption against the strong radio continuum of seven radio sources. The neutral outflows occur, in at least somes cases, at kpc distance from the nucleus, and they are most likely driven by the interactions between the expanding radio jets and the gaseous medium enshrouding the central regions. We estimate that the associated mass outflow rates are up to similar to 50 M(circle dot)yr(-1), comparable ( although at the lower end of the distribution) to the outflow, rates found for starburst- driven superwinds in Ultra Luminous IR Galaxies ( ULIRG). This suggests that massive, jet- driven outflows of neutral gas in radio- loud AGN can have as large an impact on the evolution of the host galaxies as the outflows associated with starbursts. A radio- loud phase of the AGN is likely a relatively common, albeit short, phase in the life of many ( or even all) massive ellipticals. Jet- driven neutral outflows may represent one of the main feedback mechanisms in these galaxies.

When Less Is More: Are Radio Galaxies below the Fanaroff-Riley Break More Polarized on Parsec Scales?

We present images showing the first detections of polarization on parsec scales in the nuclei of four Fanaroff-Riley type I (low-luminosity) radio galaxies. Observations with Very Long Baseline Interferometry at \lambda 3.6cm reveal the presence of ordered magnetic fields within ~1650 Schwarzchild radii of the putative central supermassive black hole. The relatively high fractional polarization in the pc-scale jets of these galaxies is consistent with the standard scheme unifying low-luminosity radio galaxies with BL Lac objects. This result also suggests that these radio galaxies lack the obscuring tori that apparently depolarize the nuclear emission in the more powerful FRII radio galaxies, and that their supermassive blackholes are poorly fed and/or inefficient radiators.

A jet-induced outflow of warm gas in 3C 293

Using long-slit emission-line spectra we detect a fast outflow of ionized gas, with velocities up to 1000 km s −1 ,i nthe nearby powerful radio galaxy 3C 293 (z = 0.045). 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 1 × 10 5 M� , the total H I mass involved in the neutral outflow is about 100 times higher (10 7 M� ). 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. Ke yw ords: line: profiles ‐ ISM: jets and outflows ‐ ISM: kinematics and dynamics ‐ galaxies: active ‐ galaxies: individual: 3C 293 ‐ galaxies: ISM.

Time variability of active galactic nuclei and heating of cooling flows

There is increasing evidence that active galactic nucleus (AGN) feedback is important in the energetics of cooling flows in galaxies and galaxy clusters. It is possible that, in most cooling-flow clusters, radiative losses from the thermal plasma are balanced, over the cluster lifetime, by mechanical heating from the central radio source. We investigate the implications of the variability of AGN mechanical luminosity L m on observations of cooling flows and radio galaxies in general. It is natural to assume that l = In (L m /L X ) is a Gaussian process. Then L m will be lognormally distributed at fixed cooling luminosity L X , and the variance in a measure of L m will increase with the time resolution of the measure. We test the consistency of these predictions with existing data for cooling flows and radio galaxies. These tests hinge on the power spectrum P(ω) of the Gaussian process l(t). General considerations suggest that P is a power law P ∼ ω -β . Long-term monitoring of Seyfert galaxies combined with estimates of the duty cycle of quasars imply that β ≃ 1, which corresponds to flicker noise. The power spectra ofmicroquasars have similar values of β. We combine a sample of sources in cooling flows that have cavities with the assumption that the average mechanical luminosity of the AGN equals the cooling flow's X-ray luminosity. Given that the mechanical luminosities are characterized by flicker noise, we find that their spectral amplitudes ωP(ω) lie between the estimated amplitudes of quasars and the measured values for the radio luminosities of microquasars. The model, together with the observation that powerful radio galaxies lie within a narrow range in optical luminosity, predicts the luminosity function of radio galaxies. Both the shape and the normalization of the predicted function are in agreement with observations. Forthcoming radio surveys will test the prediction that the luminosity function turns over at about the smallest luminosities so far probed.

HSTand UKIRT imaging observations ofz∼ 1 6C radio galaxies - II. Galaxy morphologies and the alignment effect

(abridged) Powerful radio galaxies often display enhanced optical/UV emission regions, elongated and aligned with the radio jet axis. The aim of this series of papers is to separately investigate the effects of radio power and redshift on the alignment effect, together with other radio galaxy properties. In this second paper, we present a deeper analysis of the morphological properties of these systems, including both the host galaxies and their surrounding aligned emission. The host galaxies of our 6C subsample are well described as de Vaucouleurs ellipticals, with typical scale sizes of ~10kpc. This is comparable to the host galaxies of low-z radio sources of similar powers, and also the more powerful 3CR sources at the same redshift. The contribution of nuclear point source emission is also comparable, regardless of radio power. The 6C alignment effect is remarkably similar to that seen around more powerful 3CR sources at the same redshift in terms of extent and degree of alignment with the radio source axis, although it is generally less luminous. The bright, knotty features observed in the case of the z~1 3CR sources are far less frequent in our 6C subsample; neither do we observe such strong evidence for evolution in the strength of the alignment effect with radio source size/age. However, we do find a very strong link between the most extreme alignment effects and emission line region properties indicative of shocks, regardless of source size/age or power. In general, the 6C alignment effect is still considerably stronger than that seen around lower redshift galaxies of similar radio powers. (abridged)

Direct evidence of the receding “torus” around central nuclei of powerful radio sources

The presence of obscuring material (or a dusty `torus') in active galactic nuclei (AGN) is central to the unification model for AGN. Two models, the multi-population model for radio sources and the receding torus model, are capable of describing observational properties of powerful radio galaxies and radio quasars. Here, I study the changes of the opening angle of the obscuring torus with radio luminosity at 151 MHz, [O III] emission-line luminosity and cosmic epoch aiming to discriminate between two working models. An analytical expression relating the half opening angle of the torus to the mean projected linear sizes of double radio galaxies and quasars is derived. The sizes of powerful double radio sources taken from the combined sample of 3CRR, 6CE and 7CR complete samples are used to estimate the torus opening angle. I found a statistically significant correlation between the half opening angle of the torus and [O III] emission-line luminosity. The opening angle increases from 20 to 60 degrees with increasing [O III] emission-line luminosity. This correlation is interpreted as direct evidence of the receding torus around central engines of powerful double radio sources.