arXiv subjects

P. N. Best

Publications and source records attributed to P. N. Best.

At least 181 records · Page 10Linked to original sources

arXiv2005

A sample of radio-loud AGN in the Sloan Digital Sky Survey

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. The comparison is carried out in a multi-stage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multi-component radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95% completeness and 98.9% reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud AGN and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000Ang break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star forming galaxies brighter than 5mJy at 1.4GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-IR correlation, the radio luminosity function of star forming galaxies is also compared to the luminosity function derived in the far-infrared. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to far-IR correlation breaks down below about 10^22 W/Hz at 1.4GHz.

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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

The Search for AGN in Distant Galaxy Clusters

We are undertaking the first systematic study of the prevalence of AGN activity in a large sample of high redshift galaxy clusters. Local clusters contain mainly red elliptical galaxies, and have little or no luminous AGN activity. However, recent studies of some moderate to high redshift clusters have revealed significant numbers of luminous AGN within the cluster. This effect may parallel the Butcher-Oemler effect - the increase in the fraction of blue galaxies in distant clusters compared to local clusters. Our aim is to verify and quantify recent evidence that AGN activity in dense environments increases with redshift, and to evaluate the significance of this effect. As cluster AGN are far less prevalent than field sources, a large sample of over 120 cluster fields at z > 0.1 has been selected from the Chandra archives and is being analysed for excess point sources. The size of the excess, the radial distribution and flux of the sources and the dependence of these on cluster redshift and luminosity will reveal important information about the triggering and fueling of AGN.

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arXiv2004

CENSORS: The VLT/VLA mJy source survey for high redshift AGN evolution

The Combined EIS-NVSS Survey Of Radio Sources (CENSORS) has been produced with the primary goal of investigating the cosmological evolution of the radio luminosity function. This 1.4GHz sample, complete to the 7.2 mJy level, contains 150 radio sources. Host galaxies are almost entirely identified in optical and near-IR bands and the sample is now approaching 70% spectroscopic completeness. We show preliminary results demonstrating how CENSORS will improve upon previous work and how it is applicable to other projects.

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arXiv2004

VLT spectropolarimetry of two powerful radio galaxies at z~1.4: UV continuum, emission-line properties and the nature of high-redshift dust

(Abridged) Deep VLT spectropolarimetric observations are presented for two powerful radio galaxies, 0850-206 and 1303+091. These observations cover the rest-frame wavelength range ~ 1450 - 3750 A. New radio observations and continuum images of the same sources are also presented. These galaxies are the first two observed from a complete sample of nine radio sources with redshifts in the range 1.3 < z < 1.5 (selected from the equatorial sample of powerful radio sources of Best, Rottgering & Lehnert), as part of a project aimed to investigate the multi-component nature of the UV continuum in radio galaxies and any variations of the continuum properties with the radio source age. The larger radio source of the two, 0850-206, presents a high continuum fractional polarization, averaging 17% across the observed wavelength range and reaching 24% at rest-frame wavelengths of <2000 A. The smaller radio source, 1303+091, shows a lower continuum polarization, averaging 8% and rising to 11% for rest-frame wavelengths >3000 A. For both galaxies, the position angle of the electric vector is generally constant with wavelength and within ~15 degrees of perpendicular to the radio axis. Both their total flux spectra and polarized flux spectra reveal the 2200 A dust feature, and comparison with dust scattering models suggests that the composition of the dust in these galaxies is similar to that of Galactic dust. In 0850-206, scattered quasar radiation dominates the UV continuum emission, with the nebular continuum accounting for no more than ~22% and no requirement for any additional emission component such as emission from young stars. By contrast, in 1303+091, unpolarized radiation could be a major constituent of the UV continuum emission, with starlight accounting for up to ~50% and the nebular continuum accounting for ~11%.

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arXiv2004

The environmental dependence of radio-loud AGN activity and star formation in the 2dFGRS

By combining the 2-degree Field Galaxy Redshift Survey with the NRAO VLA Sky Survey at 1.4 GHz, the environments of radio loud AGN in the nearby Universe are investigated using both local projected galaxy densities and a friends-of-friends group finding algorithm. Radio-loud AGN are preferentially located in galaxy groups and poor-to-moderate richness galaxy clusters. The AGN fraction appears to depend more strongly on the large-scale environment (group, cluster, etc) in which a galaxy is located than on its more local environment, except at the lowest galaxy surface densities where practically no radio-loud AGN are found. The ratio of absorption-line to emission-line AGN changes dramatically with environment, with essentially all radio-loud AGN in rich environments showing no emission lines. This result could be connected with the lack of cool gas in cluster galaxies, and may have important consequences for analyses of optically-selected AGN, which are invariably selected on emission line properties. The local galaxy surface density of the absorption-line AGN is strongly correlated with radio luminosity, implying that the radio luminosities may be significantly boosted in dense environments due to confinement by the hot intracluster gas. The environments of a radio-selected sample of star forming galaxies are also investigated to provide an independent test of optical studies. In line with those studies, the fraction of star forming galaxies is found to decrease strongly with increasing local galaxy surface density; this correlation extends across the whole range of galaxy surface densities, with no evidence for the density threshold found in some optical studies.

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arXiv2003

The depolarisation properties of powerful radio sources: breaking the radio-power vs. redshift degeneracy

We define 3 samples of extragalactic radio sources of type FRII, containing 26 objects in total. The control sample consists of 6C and 7C sources with radio powers of around 10e27 W\Hz at 151 MHz and redshifts of around z=1. The other samples contain 3CRR sources with either comparable redshifts but radio powers about a decade larger or with comparable radio powers but redshifts around z = 0.4. We use these samples to investigate the possible evolution of their depolarisation and rotation measure properties with redshift and radio power independently. We used VLA data for all sources at 4800 MHz and two frequencies within the 1400 MHz band, either from our own observations or from the archive. We present maps of the total intensity flux, polarised flux, depolarisation, spectral index, rotation measure and magnetic field direction where not previously published. Radio cores were detected in twelve of the twenty-six radio sources. Fourteen of the sources show a strong Laing-Garrington effect but almost all of the sources show some depolarisation asymmetry. All sources show evidence for an external Faraday screen being responsible for the observed depolarisation. We find that sources at higher redshift are more strongly depolarised. Rotation measure shows no trend with either redshift or radio power, however variations in the rotation measure across individual sources increase with the redshift of the sources but do not depend on their radio power.

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arXiv2003

The final two redshifts for radio sources from the equatorial BRL sample

Best, Rottgering and Lehnert (1999, 2000a) defined a new sample of powerful radio sources from the Molonglo Reference Catalogue, for which redshifts were compiled or measured for 177 of the 178 objects. For the final object, MRC1059-010 (3C249), the host galaxy is here identified using near-infrared imaging, and the redshift is determined from VLT spectroscopy. For one other object in the sample, MRC0320+053 (4C05.14), the literature redshift has been questioned: new spectroscopic observations of this object are presented, deriving a corrected redshift. With these two results, the spectroscopic completeness of this sample is now 100%. New redshifts are also presented for PKS0742+10 from the Wall & Peacock 2.7 GHz catalogue, and PKS1336+003 from the Parkes Selected Regions. PKS0742+10 shows a strong neutral hydrogen absorption feature in its Lyman-alpha emission profile.

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arXiv2003

CENSORS: A Combined EIS-NVSS Survey Of Radio Sources. I. Sample definition, radio data and optical identifications

A new sample of radio sources, with the designated name CENSORS (A Combined EIS-NVSS Survey Of Radio Sources), has been defined by combining the NRAO VLA Sky Survey (NVSS) at 1.4 GHz with the ESO Imaging Survey (EIS) Patch D, a 3 by 2 degree region of sky centred at 09 51 36.0, -21 00 00 (J2000). New radio observations of 199 NVSS radio sources with NVSS flux densities S(1.4GHz) > 7.8mJy are presented, and compared with the EIS I-band imaging observations which reach a depth of I~23; optical identifications are obtained for over two-thirds of the ~150 confirmed radio sources within the EIS field. The radio sources have a median linear size of 6 arcseconds, consistent with the trend for lower flux density radio sources to be less extended. Other radio source properties, such as the lobe flux density ratios, are consistent with those of brighter radio source samples. From the optical information, 30-40% of the sources are expected to lie at redshifts z >~ 1.5. One of the key goals of this survey is to accurately determine the high redshift evolution of the radio luminosity function. These radio sources are at the ideal flux density level to achieve this goal; at redshifts z~2 they have luminosities which are around the break of the luminosity function and so provide a much more accurate census of the radio source population at those redshifts than the existing studies of extreme, high radio power sources. Other survey goals include investigating the dual--population unification schemes for radio sources, studying the radio luminosity dependence of the evolution of radio source environments, and understanding the radio power dependence of the K-z relation for radio galaxies.

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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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arXiv2003

The AGN content of the z=0.83 cluster MS1054-0321

We present a survey of X-ray point sources in a 91 ksec Chandra ACIS-S observation of the z=0.83 cluster MS1054-0321. We detect 47 X-ray sources within the 8.3 arcmin by 8.3 arcmin field, of which two are immediately confirmed from pre-existing spectroscopy to be at the redshift of the cluster. At fluxes brighter than S_(0.5 - 8 keV) = 5 x 10^{-15} erg s^{-1} cm^{-2} we find a ~ 2 sigma excess compared to predictions from field surveys, consistent with an excess of approximately 6 AGN. If these sources are associated with the cluster, they too are AGN with luminosities of order L_(0.5 - 8 keV) ~ 10^{43} erg s^{-1}. Combined with the identification of 7 cluster AGN from deep radio observations (Best et al. 2002), these observations suggest significantly enhanced AGN activity in MS1054-03 compared to local galaxy clusters. Interestingly, the excess of X-ray detected AGN is found at radial distances of between 1 and 2 Mpc, suggesting they may be associated with infalling galaxies. The radio AGN are seen within the inner Mpc of the cluster and are largely undetected in the X-ray, suggesting they are either intrinsically less luminous and/or heavily obscured.

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arXiv2003

Red galaxy overdensities and the varied cluster environments of powerful radio sources with z~1.6

The environments of a complete subsample of 6 of the most powerful radio-loud AGN at redshifts z~1.6 are investigated, using deep RJK imaging to depths of R~26, J~22.4 and K~20.6. An excess of galaxy counts in the K-band is seen across these fields; these are predominantly associated with red galaxies (R-K>4) of magnitudes 17.5 4) galaxies, suggesting that the morphology-density relation is imprinted into the centres of clusters at a high redshift. (ii) weaker large-scale excesses extending out to between 1 and 1.5 Mpc radius. The presence or absence of galaxy excesses on these two scales, however, differs greatly between the six different fields. The implications of these results for both cluster formation and the nature of high redshift AGN are discussed (abridged).

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arXiv2002

Clustering around radio galaxies at z~1.5

The importance of studying old elliptical galaxies at redshift z ~ 1.5 is reviewed, considering both what can be learned by extending studies of the evolution of cluster galaxy scaling relations to earlier cosmic epochs, and the age-dating of old elliptical galaxies at high redshifts. Following this, the first results are provided of an on-going project to find such distant elliptical galaxies, through an investigation of the cluster environments of powerful radio sources with redshifts 1.44 < z < 1.7. These studies show a considerable excess of red galaxies in the radio sources fields, with the magnitudes (K >~ 17.5) and colours (R-K > 4) expected of old passively evolving galaxies at the radio source redshift. The red galaxy overdensities are found on two different scales around the radio sources; a pronounced small-scale peak at radial distances of <~ 150 kpc, and a weaker large-scale excess extending out to 1 - 1.5 Mpc. The presence and richness of these red galaxy excesses varies considerably from source to source. An interpretation of these results is provided.

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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

An excess of sub-millimetre sources towards z~1 clusters

Deep sub-millimetre observations using SCUBA are presented of the central regions of four high redshift clusters which have been extensively studied optically: CL0023+0423 (z=0.84), J0848+4453 (z=1.27), CL1604+4304 (z=0.90) and CL1604+4321 (z=0.92). 10 sub-millimetre sources are securely detected towards these four clusters at 850 microns, with two further tenuous detections; the raw 850 micron source counts exceed those determined from blank-field surveys by a factor of 3-4. In particular, towards CL1604+4304, 6 sources are detected with S(850) > 4 mJy making this the richest sub-mm field discovered to date. Corrections for gravitational lensing by these high redshift clusters reduce these excess sources counts, but are unlikely to account for more than about half of the excess, with the remainder presumably directly associated with cluster galaxies. The 450 to 850 micron flux density ratios of the detected sources are systematically higher (at a significance level > 98%) than those determined for blank-field selected sources, consistent with them being at the cluster redshifts. If subsequent identifications confirm cluster membership, these results will demonstrate that the optical Butcher-Oemler effect is also observed at sub-mm wavelengths.

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arXiv2001

The evolution of the cluster environments of radio sources at z<1.8

An analysis of the cluster environments around distant radio galaxies is presented, in particular the results from new NTT deep optical-IR imaging of the fields of radio sources at z ~ 1.6. A net overdensity of K-band galaxies is found, together with a sharp peak in the angular cross-correlation amplitude, centred on the radio galaxies. This excess clustering is associated predominantly with red galaxies, with colours consistent with being old ellipticals at the radio source redshift. A large excess of such red galaxies is seen, particularly within 100 kpc of the radio source. These comprise amongst the most distant red sequences of cluster ellipticals yet discovered, that is, the highest redshift `normal' clusters.

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