arXiv subjects

M. S. Longair

Publications and source records attributed to M. S. Longair.

At least 19 recordsLinked to original sources

arXiv2005

Deviations from passive evolution - star formation and the UV 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 UV excess evolves with radio source size similarly in both samples, depsite 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 10^8 years 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 ~0.5mag 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.

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arXiv2005

HST and UKIRT imaging observations of z~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)

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arXiv2004

On the jet speeds of classical double radio sources

A simple integral relation is obtained for the distribution functions of jet speeds and jet-counterjet flux asymmetries of classical double radio sources. Analytic expressions relating the mean jet speed and the mean jet flux asymmetry, and their variances are derived. The results are considered in the light of orientation-based unified schemes, and an analytical equation for the critical angle separating quasars and radio galaxies is derived. The mean jet speeds of classical double radio sources are estimated using the asymmetry of jet-counterjet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is ~0.54c (with a standard error SE=0.03c), while for FRII low-redshift radio galaxies and intermediate redshift quasars the values found are ~0.4c (SE=0.06c) and >0.6c respectively.

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arXiv2003

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 luminosity of the aligned emission appears 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; line emission and related nebular continuum emission, however, often provide a significant contribution to the aligned emission.

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arXiv2002

The evolution of radio galaxy emission line regions: Breaking the P-z degeneracy

The ionization and kinematic properties of the emission line regions of three subsamples of 6C and 3CR radio galaxies have been compared. The degeneracy between redshift and radio power is broken, and the relative importance of radio power (P), radio size (D) and redshift (z) for the emission line region properties is determined.

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arXiv2002

Deep spectroscopy of z~1 6C radio galaxies - I. The effects of radio power and size on the properties of the emission line gas

(abridged) The results of deep long-slit optical spectroscopy for a sample of 8 6C radio galaxies at z~1 are presented. Emission line ratios are derived for emission lines with rest-frame wavelengths of 1500 - 4500Ang, and the kinematic propertiess of the emission line gas are derived from an analysis of the 2-d structure of thee [OII]3727 Ang emission line at ~5Ang spectral resolution. In general, the 6C spectra display many characteristics similar to those of more powerful 3CR sources at the same redshifts. The emission line region gas kinematics are more extreme for the smaller radio sources in the sample, which often display distorted velocity profiles. The ionization state of the emission line region also varies with radio size: the spectra of large radio sources (>120kpc) are consistent with photoionization by an obscured AGN, whilst smaller (<120kpc) sources typically exist in a lower ionization state and have spectra which are better explained by additional ionization due to shocks associated with the expanding radio source. The kinematic and ionization properties of the 6C radio galaxies are clearly linked. As for the 3CR sources, smaller radio sources also typically possess more extensive emission line regions, with enhanced emission line luminosities. It is clear that the best interpretation of the spectra of radio sources requires a combination of ionization mechanisms.

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arXiv2002

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 galaxies, chosen to be matched in radio luminosity to the 6C sources studied at z~1, thus enabling the P-z degeneracy to be broken. Partial rank correlations and principal component analysis have been used to determine which of z and P are the critical parameters underlying the observed variation of the ionization state andd kinematics of the emission line gas. [OII]/H-beta is 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 z is observed, implying that the ionization state of the emission line gas is solely a function of the AGN properties rather than the hostt galaxy and/or environment. Statistical analysis of the kinematic properties of the emission line gas shows that these are strongly correlated independently withh both P and z. The correlation with redshift is the stronger of the two, suggesting that host galaxy 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 thee galaxies, the independent correlations observed with radio size are strongest. Radio source age is a determining factor for the extended emission line regions.

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arXiv2002

Formation of internal shock waves in bent jets

We discuss the circumstances under which the bending of a jet can generate an internal shock wave. The analysis is carried out for relativistic and non-relativistic astrophysical jets. The calculations are done by the method of characteristics for the case of steady simple waves. This generalises the non-relativistic treatment first used by Icke (1991). We show that it is possible to obtain an upper limit to the bending angle of a jet in order not to create a shock wave at the end of the curvature. This limiting angle has a value of 75 degrees for non-relativistic jets with a polytropic index k = 4/3, 135 degrees for non-relativistic jets with k = 5/3) and 50 degrees for relativistic jets with k = 5/3. We also discuss under which circumstances jets will form internal shock waves for smaller deflection angles.

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arXiv2001

Infrared Magnitude-Redshift Relations for Luminous Radio Galaxies

Infrared magnitude-redshift relations for the 3CR and 6C samples of radio galaxies are presented for a wide range of plausible cosmological models, including those with non-zero cosmological constant OmegaLambda. Variations in the galaxy formation redshift, metallicity and star formation history are also considered. The results of the modelling are displayed in terms of magnitude differences between the models and no-evolution tracks, illustrating the amount of K-band evolution necessary to account for the observational data. Given a number of plausible assumptions, the results of these analyses suggest that: (i) cosmologies which predict T_0xH_0>1 (where T_0 denotes the current age of the universe) can be excluded; (ii) the star formation redshift should lie in the redshift interval 5<z<20, values towards the lower end of the range being preferred in cosmologies with larger values of T_0xH_0; (iii) the Einstein-de Sitter model provides a reasonable fit to the data; (iv) models with finite values of OmegaLambda can provide good agreement with the observations only if appropriate adjustments of other parameters such as the galaxy metallicities and star-formation histories are made. Without such modifications, even after accounting for stellar evolution, the high redshift radio galaxies are more luminous (ie. more massive) than those nearby in models with finite OmegaLambda, including the favoured model with Omega=0.3, OmegaLambda=0.7. For cosmological models with larger values of T_0xH_0, the conclusions are the same regardless of whether any adjustments are made or not. The implications of these results for cosmology and models of galaxy formation are discussed.

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arXiv2001

Deflection of Jets Induced by Jet-Cloud Interactions

A non--relativistic and relativistic model in which astrophysical jets are deflected on passing through an isothermal high density region is analysed. The criteria for the stability of jets due to the formation of internal shocks are discussed.

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arXiv2001

Deflection of jets induced by jet-cloud & jet-galaxy interactions

The model first introduced by Raga & Canto (1996) in which astrophysical jets are deflected on passing through an isothermal high density region is generalised by taking into account gravitational effects on the motion of the jet as it crosses the high density cloud. The problem is also generalised for relativistic jets in which gravitational effects induced by the cloud are neglected. Two further cases, classical and relativistic, are discussed for the cases in which the jet is deflected on passing through the interstellar gas of a galaxy in which a dark matter halo dominates the gravitational potential. The criteria for the stability of jets due to the formation of internal shocks are discussed.

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arXiv2000

Starburst galaxies and structure in the submillimetre background towards the Hubble Deep Field

We use an 850-micron SCUBA map of the HDF to study the dust properties of optically-selected starburst galaxies at high redshift. Although the 850-micron map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S_{850}=0.2 mJy for an apparent UV star-formation rate of 1 h^{-2} msun yr^{-1}. This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-micron background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-micron background originates over an extremely wide range of redshifts -- perhaps 1 < z < 6.

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arXiv2000

An asymmetric relativistic model for classical double radio sources

There is substantial observational evidence against the symmetric relativistic model of FRII radio sources. An asymmetric relativistic model is proposed which takes account of both relativistic effects and intrinsic/environmental asymmetries to explain the structural asymmetries of their radio lobes. A key parameter of the model is the jet-side of the double sources, which is estimated for 80% of the FRII sources in the 3CRR complete sample. Statistical analyses of the properties of these sources show that the asymmetric model is in agreement with a wide range of observational data, and that the relativistic and intrinsic asymmetry effects are of comparable importance. Intrinsic/environmental asymmetry effects are more important at high radio luminosities and small physical scales. The mean translational speed of the lobes is found to be 0.11c, consistent with the speeds found from spectral ageing arguments. According to a Gaussian model, the standard deviation of lobe speeds is 0.04c. The results are in agreement with an orientation-based unification scheme in which the critical angle separating the radio galaxies from quasars is about 45 degrees.

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arXiv1999

Deep spectroscopy of distant 3CR radio galaxies: the data

Deep long-slit spectroscopic data are presented for a sample of 14 3CR radio galaxies at redshift z ~ 1, previously studied in detail using the Hubble Space Telescope, the Very Large Array, and UKIRT. Analysis of the [OII] 3727 emission line structures at 5 Ang spectral resolution is carried out to derive the kinematic properties of the emission line gas. In line with previous, lower resolution, studies, a wide variety of kinematics are seen, from gas consistent with a mean rotational motion through to complex structures with velocity dispersions exceeding 1000 kms. The data confirm the presence of a high velocity gas component in 3C265 and detached emission line systems in 3C356 and 3C441, and show for the first time that the emission line gas in the central regions of 3C324 is composed of two kinematically distinct components. Emission line fluxes and the colour of the continuum emission are determined down to unprecedently low observed wavelengths, lambda < 3500 Ang, sufficiently short that any contribution of an evolved stellar population is negligible. An accompanying paper investigates the variation in the emission line ratios and velocity structures within the sample, and draws conclusions as to the origin of the ionisation and kinematics of these galaxies.

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arXiv1999

Ionisation, shocks and evolution of the emission line gas of distant 3CR radio galaxies

An analysis of the kinematics and ionisation state of the emission line gas of a sample of 14 3CR radio galaxies with redshifts z~1 is carried out. The data used for these studies, deep long--slit spectroscopic exposures from the WHT, are presented in an accompanying paper. It is found that radio sources with small linear sizes (<150 kpc) have lower ionisation states, higher emission line fluxes and broader line widths than larger radio sources. The emission line ratios of small radio sources are in agreement with theoretical shock ionisation predictions and their velocity profiles are distorted. Together with the other emission line properties this indicates that shocks associated with the radio source dominate the kinematics and ionisation of the emission line gas during the period that the radio source is expanding through the ISM. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of the clouds and the presence of the ionising photons associated with the shocks combine to lower the ionisation state of the emission line gas. By contrast, in larger sources the shock fronts have passed well beyond the emission line regions; their emission line gas has much more settled kinematical properties, indicative of rotation, and emission line ratios consistent with the dominant source of ionising photons being the active galactic nucleus. (Abridged)

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arXiv1998

The emission line properties of the 3CR radio galaxies at redshift one: shocks, evolution, and the alignment effect

The results of a deep spectroscopic campaign on powerful radio galaxies with redshifts z ~ 1, to investigate in detail their emission line gas properties, are presented. Both the 2-dimensional velocity structure of the [OII] 3727 emission line and the ionisation state of the gas are found to be strongly dependent upon the linear size (age) of the radio source in a manner indicative of the emission line properties of small (young) radio sources being dominated by the passage of the radio source shocks. The consequences of this evolution throughout the few times 10-to-the-7 year lifetime of the radio source are discussed, particularly with relation to the alignment of the UV--optical continuum emission of these objects along their radio axis, the nature of which shows similar evolution.

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arXiv1998

Studies of a sample of 6C radio galaxies at redshift one, I -- Deep multi-frequency radio observations

Deep radio observations at 5 and 8 GHz are presented of a complete sample of 11 radio galaxies with redshifts 0.85 < z < 1.5, selected from the 6C sample of Eales. The radio data, taken using the VLA in A, B and C array configurations, provide a best angular resolution of 0.25'' and reach an rms noise level of order 20 microJy. Radio core candidates are detected in 8 of the 11 sources. 9 of the 11 sources display deviations from `standard double radio source' morphologies, with multiple hotspots or a hotspot withdrawn from the leading edge of the radio emission. The sources are typically polarised at the 5 to 15% level. The mean rotation measures of the individual lobes are less than 50 rad/m^2 but strong asymmetries between the two lobes indicate that the Faraday rotation does not have a Milky Way origin; rather, the distant 6C radio sources lie in a relatively dense clumpy environment. The sources are compared with the more radio powerful distant 3CR radio galaxies and with low redshift sources. The ratio of core to extended radio flux is almost independent of the radio source size and only weakly inversely correlated with the total radio power. This indicates that the high radio luminosity of the most powerful radio sources must originate in the AGN, in contrast to the suggestion that they are so luminous only due to confinement by a dense surrounding environment. Environmental effects play a secondary role. (Abridged)

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arXiv1998

Deep radio observations of 3C324 and 3C368: evidence for jet-cloud interactions

High resolution, deep radio images are presented for two distant radio galaxies, 3C324 (z=1.206) and 3C368 (z=1.132), which are both prime examples of the radio-optical alignment effect seen in powerful radio galaxies with redshifts z > 0.6. Radio observations were made using the Very Large Array in A-array configuration at 5 and 8 GHz, and using the MERLIN array at 1.4 and 1.65 GHz. Radio spectral index, radio polarisation, and rotation measure maps are presented for both sources. Radio core candidates are detected in each source, and by aligning these with the centroid of the infrared emission the radio and the optical/infrared images can be related astrometrically with 0.1 arcsec accuracy. In each source the radio core is located at a minimum of the optical emission, probably associated with a central dust lane. Both sources also exhibit radio jets which lie along the directions of the bright strings of optical knots seen in high resolution Hubble Space Telescope images. The northern arm of 3C368 shows a close correlation between the radio and optical emission, whilst along the jet direction of 3C324 the bright radio and optical knots are co-linear but not co-spatial. These indicate that interactions between the radio jet and its environment play a key role in producing the excess ultraviolet emission of these sources, but that the detailed mechanisms vary from source to source. 3C368 is strongly depolarised and has an average rest-frame rotation measure of a few hundred rad/m^2, reaching about 1000 rad/m^2 close to the most depolarised regions. 3C324 has weaker depolarisation, and an average rest-frame rotation measure of between 100 and 200 rad/m^2. Both sources show large gradients in their rotation measure structures, with variations of up to 1000 rad/m^2 over distances of about 10 kpc.

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