arXiv subjects

C. N. Tadhunter

Publications and source records attributed to C. N. Tadhunter.

At least 55 records · Page 3Linked to original sources

arXiv2007

IC5063: AGN driven outflow of warm and cold gas

We present new ATCA 17- and 24-GHz radio images and ESO-NTT optical spectra of the radio-loud Seyfert galaxy IC5063, the first galaxy in which a fast (~ 600 km/s) outflow of neutral hydrogen was discovered. The new radio data confirm the triple radio structure with a central core and two resolved radio lobes. This implies that the previously detected fast outflow of neutral gas is occurring off-nucleus, near a radio lobe about 0.5 kpc from the core. The ionised gas shows complex kinematics in the region co-spatial with the radio emission. Broad and blueshifted (~ 500 km/s) emission is observed in the region of the radio lobe, at the same location as the blueshifted HI absorption. The velocity of the ionised outflow is similar to the one found in HI. The first order correspondence between the radio and optical properties suggests that the outflow is driven by the interaction between the radio jet and the ISM. Despite the high outflow velocities, no evidence is found for the ionisation of the gas being due to fast shocks in the region of the outflow, indicating that photoionisation from the AGN is likely to be the dominant ionisation mechanism. The outflow rate of the warm (ionised) gas is small compared to that of the cold gas. The mass outflow rate associated with the HI is in the same range as for ``mild'' starburst-driven superwinds in ULIRGs. However, in IC5063, the AGN-driven outflow appears to be limited to the inner kpc region of the galaxy. The kinetic power associated with the HI outflow is a small fraction (a few x 10^-4) of the Eddington luminosity of the galaxy but is a significant fraction (~ 0.1) of the nuclear bolometric luminosity. In IC5063, the outflows may have sufficient kinetic power to have a significant impact on the evolution of the ISM in the host galaxy.

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arXiv2007

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. VIMOS IFU spectroscopy is used to study the morphology, kinematics and ionisation state of the EELR surrounding this source, and also a companion galaxy at a similar redshift. Near- and far-IR imaging observations obtained using the NTT and SPITZER are used to analyse the underlying galaxy morphologies and the nature of the AGN. The host galaxy is identified as an ~M* elliptical. Combining Spitzer 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 BLRG. However, given its relatively large [OIII] luminosity it is likely that the AGN was substantially more luminous in the recent past (<10^4 years 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 ionised by a mixture of AGN photoionisation and emission from young stars. We confirm the presence of a series of star-forming knots extending N-S from the host galaxy, with more prodigious star formation occuring in the merging companion galaxy to the northeast, which has sufficient luminosity at mid- to far-IR wavelengths to be classified as a 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.

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arXiv2007

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

Abridged. We present high-quality optical spectra for 12 powerful radio sources at low and intermediate redshifts (z < 0.7) that show evidence for a substantial UV excess. These data were taken using the WHT and VLT to determine 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 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 auxilary polarimetric and imaging data, are required to avoid degeneracies in the model solutions. In 3/12 sources in our sample we find 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. In the remaining 9 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% 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%) 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 measureable extent of the galaxy (typically 5-30 kpc), suggesting galaxy-wide starbursts.

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arXiv2007

Large-scale HI in nearby radio galaxies: segregation in neutral gas content with radio source size

We present results of a study of neutral hydrogen (HI) in a complete sample of nearby non-cluster radio galaxies. We find that radio galaxies with large amounts of extended HI (M_HI >= 10^9 M_solar) all have a compact radio source. The host galaxies of the more extended radio sources, all of Fanaroff & Riley type-I, do not contain these amounts of HI. We discuss several possible explanations for this segregation. The large-scale HI is mainly distributed in disk- and ring-like structures with sizes up to 190 kpc and masses up to 2 x 10^10 M_solar. The formation of these structures could be related to past merger events, although in some cases it may also be consistent with a cold-accretion scenario.

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arXiv2006

A census of young stellar populations in the warm ULIRG PKS1345+12

We present a detailed investigation of the young stellar populations(YSP) in the radio-loud ultra luminous infrared galaxy (ULIRG) PKS1345+12, based on high resolution HST imaging and long slit spectra taken with the WHT. While the images clearly show bright knots suggestive of super star clusters(SSC), the spectra reveal the presence of YSP in the diffuse light across the full extent of the halo of the merging-double nucleus system. Spectral synthesis modelling has been used to estimate the ages of the YSP for both the SSC and the diffuse light sampled by the spectra. For the SSC we find ages t{SSC} < 6 Myr with reddenings 0.2 < E(B-V) < 0.5 and masses 10e6 < M{SSC} < 10e7 M{solar}. However, in some regions of the galaxy we find that the spectra of the diffuse light component can only be modelled with a relatively old post-starburst YSP (0.04 - 1.0 Gyr) or with a disk galaxy template spectrum. The results demonstrate the importance of accounting for reddening in photometric studies of SSC, and highlight the dangers of focussing on the highest surface brightness regions when trying to obtain a general impression of the star formation activity in the host galaxies of ULIRGs. The case of PKS1345+12 provides clear evidence that the star formation histories of the YSP in ULIRGs are complex. Intriguingly, our long-slit spectra show line splitting at the locations of the SSC, indicating that they are moving at up to 450km s-1 with respect to the local ambient gas. Given their kinematics, it is plausible that the SSC have been formed either in fast moving gas streams/tidal tails that are falling back into the nuclear regions as part of the merger process, or as a consequence of jet-induced star formation linked to the extended, diffuse radio emission detected in the halo of the galaxy

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arXiv2006

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

In this paper 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 Msun, diameter = 190 kpc). We interpret this as the result of a major merger that occurred > 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.

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arXiv2006

The co-evolution of the obscured quasar PKS1549-79 and its host galaxy: evidence for a high accretion rate and warm outflow

ABRIDGED: We use deep optical, IR and radio observations to explore the symbiosis between the nuclear activity and galaxy evolution in the southern compact radio source PKS1549-79 (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. This is further supported by the detection of a young stellar population which was formed 50-250Myr ago and makes up 1-30% of the total stellar mass. Despite the core-jet structure of the radio source we detect HI absorption associated with both the core and the jet. The luminous, quasar-like AGN (M_V < -23.5) is highly extinguished (A_v > 4.9) at optical wavelengths and shows many properties in common with narrow line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines (FWHM ~ 1940 km/s), highly blueshifted [OIII] lines (~680 km/s), and evidence that the putative supermassive black hole is accreting at a high Eddington ratio. This suggests that accretion at high Eddington ratio does not prevent the formation of powerful jets. Our 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; the major growth phase largely hidden by the natal gas and dust. Although the models predict that AGN-driven outflows will eventually remove the gas from the bulge of the host galaxy, the visible warm outflow in PKS1549-79 is not currently capable of doing so. ABRIDGED

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arXiv2005

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

We report the detection of fast (~ 1000 km/s), massive outflows of neutral gas observed -- using the WSRT -- as 21-cm 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 ~50 M_sun/yr, 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.

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arXiv2005

Outflows and shocks in compact radio sources

We report some key results from the optical emission line study of a complete sample of compact radio sources. We find strong evidence for jet-driven outflows in the circum-nuclear emission line gas namely: 1) highly broadened and blueshifted emission line components (up to 2000 km/s), 2) shock ionised gas (broader, shifted components), 3) consistency in the scales of the emission line gas and the radio source and 4) trends between the maximum outflow velocity and radio source size (and orientation). Full details can be found in Holt (2005).

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arXiv2005

Merger-origin of radio galaxies investigated with H I observations

We present results of an HI study of a complete sample of nearby radio galaxies. Our goal is to investigate whether merger or interaction events could be at the origin of the radio-AGN activity. Around five of our radio galaxies, hosted mainly by early-type galaxies, we detect extended HI in emission. In most cases this HI is distributed in large (up to 190 kpc) and massive (up to M(HI) ~ 10^10 M(sun)) disk- or ring-like structures, that show fairly regular rotation around the host galaxy. This suggests that in these systems a major merger did occur, but at least several Gyr ago. For the HI-rich radio galaxy B2 0648+27 we confirm such a merger origin through the detection of a post-starburst stellar population that dominates the visible light throughout this system. The timescale of the current episode of radio-AGN activity in our HI-rich radio galaxies is many orders of magnitude smaller than the merger timescales. Therefore the radio-AGN activity either started late in the lifetime of the merger event, or is not directly related to the merger event at all. Another intriguing result is that the HI-rich (> 10^9 M(sun)) radio galaxies in our sample all have compact radio sources, while none of the extended radio sources contain these amounts of extended HI. This strongly suggests that there is a relation between the size of the radio jet and the presence of large amounts of neutral gas associated with the host galaxy.

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arXiv2005

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, in the 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 x 10e5 M_sun, the total HI mass involved in the neutral outflow is about 100 times higher (10e7 M_sun). This shows that, despite the high energies that must be involved in driving the outflow, most of the gas remains, or becomes again, 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 reveal a rotating ionized gas disk 30 kpc in extent.

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arXiv2005

Fast outflow of neutral and ionized gas from the radio galaxy 3C 293

We detect a fast outflow of neutral and ionized gas with velocities up to about 1000 km/s from the central region of radio galaxy 3C 293. With optical spectroscopy we locate the bulk of the ionized gas outflow at the position of a bright radio hot-spot in the inner radio jet, about 1 kpc east of the nucleus. Given the presence of large amounts of cold gas and the distorted morphology of the radio jet in this region, we argue that the ISM is pushed out by a severe interaction with the radio plasma. The similarity of the outflow of HI with the ionized gas outflow that we see at the position of the radio hot-spot suggests that despite the high energies involved in the jet-ISM interaction, part of the gas stays, or becomes again, neutral. In this paper we also present the detection of HI emission in three nearby companions of 3C 293.

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arXiv2005

The location of the broad HI absorption in 3C305: clear evidence for a jet-accelerated neutral outflow

We present high-spatial resolution 21-cm HI VLA observations of the radio galaxy 3C305 (z=0.041). These new high-resolution data show that the ~1000 km/s broad HI absorption, earlier detected in low-resolution WSRT observations, is occurring against the bright, eastern radio lobe, about 1.6 kpc from the nucleus. We use new optical spectra taken with the WHT to make a detailed comparison of the kinematics of the neutral hydrogen with that of the ionised gas. The striking similarity between the complex kinematics of the two gas phases suggests that both the ionised gas and the neutral gas are part of the same outflow. Earlier studies of the ionised gas had already found evidence for a strong interaction between the radio jet and the interstellar medium at the location of the eastern radio lobe. Our results show that the fast outflow produced by this interaction also contains a component of neutral atomic hydrogen. The most likely interpretation is that the radio jet ionises the ISM and accelerates it to the high outflow velocities observed. Our observations demonstrate that, following this strong jet-cloud interaction, not all gas clouds are destroyed and that part of the gas can cool and become neutral. The mass outflow rate measured in 3C~305 is comparable, although at the lower end of the distribution, to that found in Ultra-Luminous IR galaxies. This suggests that AGN-driven outflows, and in particular jet-driven outflows, can have a similar impact on the evolution of a galaxy as starburst-driven superwinds.

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arXiv2005

From young massive star cluster to old globular: the L_V-sigma_0 relationship as a diagnostic tool

We present a new analysis of the properties of the young massive star clusters, which demonstrates that these objects are plausible progenitors of old globular clusters (GCs). The method is based on the tight relationship for old GCs between their V-band luminosities and (central) velocity dispersions, sigma_0. We improve the significance of the relationship and find that its functional form, L_V/Lsun \propto sigma_0^(1.57 +- 0.10) (km/s), is fully consistent with previous determinations for smaller Galactic and M31 GC samples. The tightness of the relationship for a GC sample drawn from environments as diverse as those found in the Local Group implies that its origin must be sought in intrinsic properties of the GC formation process itself. We evolve the luminosities of the young massive star clusters (YMCs) in the local Universe to an age of 12 Gyr and find that most YMCs will evolve to loci close to, or to slightly fainter luminosities than the improved GC relationship. In the absence of significant external disturbances, this implies that these objects may potentially survive to become old GC-type objects over a Hubble time. The main advantage of our new method is its simplicity; the only observables required here are the system's velocity dispersion and luminosity. The most important factor affecting the robustness of our conclusions is the adopted form of the initial mass function. We use N-body simulations to confirm that dynamical evolution of the clusters does not significantly alter our conclusions. [ABRIDGED]

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arXiv2005

HST/STIS low dispersion spectroscopy of three Compact Steep Spectrum sources Evidence for jet-cloud interaction

We present Hubble Space Telescope Imaging Spectrograph long-slit spectroscopy of the emission line nebulae in the compact steep spectrum radio sources 3C 67, 3C 277.1, and 3C 303.1. We derive BPT (Baldwin- Philips-Terlevich; Baldwin et al. 1981) diagnostic emission line ratios for the nebulae which are consistent with a mix of shock excitation and photoionization in the extended gas. In addition, line ratios indicative of lower ionization gas are found to be associated with higher gas velocities. The results are consistent with a picture in which these galaxy scale radio sources interact with dense clouds in the interstellar medium of the host galaxies, shocking the clouds thereby ionizing and accelerating them.

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arXiv2004

The unfriendly ISM in the radio galaxy 4C12.50 (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$ 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.

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arXiv2004

A giant molecular cloud falling through the heart of Cygnus A: clues to the triggering of the activity

We present intermediate resolution near-IR long-slit spectroscopic data for the nearby radio galaxy Cygnus A (3C 405) (obtained with the NIRSPEC spectrograph on the Keck II telescope). The data reveal considerable complexity in the near-IR emission line kinematics, including line splittings of 200-350 km/s and a mixture of narrow (FWHM ~200 km/s) and broad (FWHM ~700 km/s) components to the emission lines. It is notable that the Pa alpha and H2 emission lines show markedly different kinematics, both on- and off-nucleus. Overall, the data provide evidence for the presence of a giant molecular cloud falling through the heart of the Cygnus A host galaxy, the motion of which is not driven by the AGN itself. We suggest that this cloud may be connected to the triggering of the activity in this highly powerful AGN. We also detect split H2 components on the nucleus that are likely to originate in the circum-nuclear torus.

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arXiv2004

Fast Outflows of Neutral Hydrogen in Radio Galaxies

AGN activity is known to drive fast outflows of gas. We report the discovery of fast outflows of neutral gas with velocities over 1000 km/s in a number of radio galaxies. In the best studied object, 3C~293, the kinematical properties of the neutral and ionised outflows are similar, indicating a common origin. Moreover, the outflow appears to be located near the radio lobes and not near the nucleus. This suggests that the interaction between the radio jet and the ISM is driving the outflow.

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