arXiv subjects

R. J. Wilman

Publications and source records attributed to R. J. Wilman.

46 records · Page 3Linked to original sources

arXiv2001

Multi-wavelength observations of serendipitous Chandra X-ray sources in the field of A2390

We present optical spectra and near-infrared imaging of a sample of serendipitous X-ray sources detected in the field of Chandra observations of the A2390 cluster of galaxies. The sources have 0.5-7 keV fluxes of 0.6-8 x 10^{-14} erg cm^{-2} s^{-1} and lie around the break in the 2-10 keV source counts. They therefore are typical of sources dominating the X-ray Background in that band. Twelve of the fifteen targets for which we have optical spectra show emission lines, most of which have soft X-ray spectra. Including photometric redshifts and published spectra we have redshifts for seventeen of the sources, ranging from z~0.2 up to z~3 with a peak between z=1-2. Ten of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic 2-10 keV luminosities of 0.2-3 x 10^{45} erg/s and absorbing columns of N_H>10^{23} cm^{-2}. Both were detected by ISO, showing that the absorbed flux emerges in the far-infrared.

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arXiv2000

Hot dust in two hard Chandra X-ray sources

The two brightest hard X-ray sources discovered serendipitously by Chandra in the field of the lensing cluster A2390 are found to have ISOCAM counterparts at 6.7 and 15 micron. We use this fact, together with their non-detection by SCUBA at 850 micron, as the basis for DUSTY radiative transfer modelling of their infrared spectral energy distributions. For the best-fitting models, we find that the dust which reprocesses the optical--UV light in these Compton-thin AGN is heated to near its sublimation temperature (above 1000K), with an inner radius within a parsec of the nucleus. Some warm dust models with inner temperatures of 200K are also formally acceptable. These findings strongly support the obscured AGN hypothesis for the new hard X-ray Chandra sources, which lack both strong emission lines and 850 micron SCUBA detections.

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arXiv2000

Extended line emission around seven radio-loud quasars at redshift z~2

We present near-infrared spectra of seven radio-loud quasars with a median redshift of 2.1, five of which were previously known to have Ly alpha nebulae. Extended [OIII]5007 and H alpha emission are evident around six objects, at the level of a few times 1E-16 erg/cm^2/arcsec^2/s within 2 arcsec of the nucleus (16 kpc in the adopted cosmology). Nuclear [OII]3727 is detected in three of the 5 quasars studied at this wavelength, and clearly extended in one of them. The extended [OIII] tends to be brighter on the side of the nucleus with the stronger, jet-like radio emission, indicating at least that the extranuclear gas is distributed anisotropically. It is also redshifted by several hundred km/s from the nuclear [OIII], perhaps because the latter is blueshifted from the systemic velocity of the host galaxy. Alternatively, the velocity shifts could be due to infall (which is suggested by line widths ~1000 km/s FWHM) in combination with a suitable dust geometry. Lyalpha/Halpha ratios well below the case B value suggest that some dust is present. Photoionization modelling of the [OIII]/[OII] ratios in the extended gas suggests that its pressure is around or less than a few times 1E7 cm^-3 K; any confining intracluster medium may thus host a strong cooling flow. A comparison with lower-redshift work suggests that there has been little evolution in the nuclear emission line properties of radio-loud quasars between redshifts 1 and 2.

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arXiv2000

A physical model for the hard X-ray background

We combine a semi-analytic galaxy formation model (Nulsen and Fabian 1997) with a prescription for the obscured growth of massive black holes (Nulsen and Fabian 2000; Fabian 1999), to reproduce the hard X-ray background (XRB), the local 2-10keV AGN luminosity function and the source counts, including recent Chandra results. The model also complies with constraints on the AGN contribution to the far-infrared and sub-mm backgrounds. The co-moving density of luminous AGN [L(2-10keV)>1E44 erg/s, in the unabsorbed rest- frame] in the model declines sharply since z=2, mimicking the observed evolution of the quasar population. The abundance of lower luminosity AGN simultaneously increases, and we identify such objects with the Seyfert galaxies. These features reflect an epoch-dependent rate of Bondi accretion from the hot, cooling atmosphere: at z<2.5 most AGN are fed at much less than 10 per cent of the Eddington rate, whilst at z>2.5 most accrete at 10-50 per cent Eddington. Unlike other models for quasar and galaxy formation, we do not consider major mergers between normal galaxies. The model produces an excess soft XRB below 4keV which can be removed by modifying the wind expulsion model of Fabian (1999) to incorporate anisotropic ejection.

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arXiv2000

Molecular hydrogen emission in Cygnus A

We present J, H and K-band spectroscopy of Cygnus A, spanning 1.0-2.4 micron in the rest-frame and hence several rovibrational H2, H recombination and [FeII] emission lines. The lines are spatially extended by up to 6kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H2 and [FeII]) are not wholly produced in the same gas. The broadest line, [FeII]1.644, exhibits a non-gaussian profile with a broad base (FWHM=1040 km/s), perhaps due to interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines. Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H2 emission. The results of Maloney, Hollenbach and Tielens (1996) are thus used to infer the total mass of gas in H2 v=1-0 S(1)-emitting clouds as a function of radius, for gas densities of 10^3 and 10^5 cm^-3, and stopping column densities N_H=10^22-10^24 cm^-2. Assuming azimuthal symmetry, at least 2.3E8 Msun of such material is present within 5kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. (1994) arises in a circumnuclear torus, the implied gas mass rises to 10^10 Msun. The latter plausibly accounts for 10^9 yr of mass deposition from the cluster cooling flow, for which Mdot \simeq 10 Msun/yr within this radius.

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arXiv2000

Infrared observations of serendipitous hard Chandra X-ray sources

We present observations of a sample of optically-faint, hard X-ray sources of the kind likely to be responsible for much of the hard X-ray background. We confirm that such sources are easily detected in the near-infrared, and find that they have a featureless continuum suggesting that the active nucleus is heavily obscured. The infrared colours of the majority of the targets observed are consistent with absorbed elliptical host galaxies at z=1-2. It is likely that we are observing some of the brighter members of the important new class of X-ray Type II quasars.

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arXiv2000

Testing the connection between the X-ray and submillimetre source populations using Chandra

The powerful combination of the Chandra X-ray telescope, the SCUBA submillimetre-wave camera and the gravitational lensing effect of the massive galaxy clusters A2390 and A1835 has been used to place stringent X-ray flux limits on six faint submillimetre SCUBA sources and deep submillimetre limits on three Chandra sources which lie in fields common to both instruments. One further source is marginally detected in both the X-ray and submillimetre bands. For all the SCUBA sources our results are consistent with starburst-dominated emission. For two objects, including SMMJ14011+0252 at z=2.55, the constraints are strong enough that they can only host powerful active galactic nuclei if they are both Compton-thick and any scattered X-ray flux is weak or itself absorbed. The lensing amplification for the sources is in the range 1.5-7, assuming that they lie at z>1. The brightest detected X-ray source has a faint extended optical counterpart (I~22) with colours consistent with a galaxy at z~1. The X-ray spectrum of this galaxy is hard, implying strong intrinsic absorption with a column density of about 1e23 cm-2 and an intrinsic (unabsorbed) 2-10 keV luminosity of 3e44 erg/s. This source is therefore a Type-II quasar. The weakest detected X-ray sources are not detected in HST images down to I~26.

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arXiv1999

Mapping the gas kinematics and ionization structure of four ultraluminous IRAS galaxies

(Abridged) We analyse the morphology, kinematics and ionization structure of the extended emission-line regions in four ULIRGs at 0.118 1000 km/s within 1" of the nucleus; an isolated blob ~8kpc from the nucleus with a much lower FWHM may lie in a photoionization cone. Line ratios suggest AGN-photoionization for the systemic gas, and shock+precursor ionization for a second, spatially-unresolved, nuclear component blueshifted by 990 km/s. The gas in IRAS F23060+0505 is in dipolar rotation but with an asymmetric rotation curve and an increased FWHM on one side; an asymmetry to the narrow-line profiles is attributed to a spatially-resolved outflow, and shock+precursor ionization is dominant. At lower luminosity, IRAS F01217+0122 has a featureless velocity field with a high FWHM, an AGN-like emission-line spectrum and a ~1 Gyr old stellar population. IRAS F01003-2238 has a dipolar velocity field and an HII-region emission-line spectrum with a strong blue continuum which can be reproduced with ~10^7 O5 stars, sufficient to account for the bolometric luminosity of the entire galaxy. This diversity is accommodated within the merger-driven evolutionary scenario for ULIRGs: the merger status is assessed from the kinematics in a way which is consistent with the colour and morphology of the galaxy, or with the inferred age of the stellar population and the dominance of the AGN.

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arXiv1999

Fitting the spectrum of the X-ray background: the effects of high metallicity absorption

Recent work by Risaliti et al.(1999) suggests that more than half of all Seyfert 2 galaxies in the local universe are Compton-thick (N_H > 10^24 cm^-2). This has implications for AGN synthesis models for the X-ray background (XRB), the flexibility of which for the inclusion of large numbers of high-z type 2 sources we examine here. We highlight the importance of Compton down-scattering in determining the individual source spectra and the fit to the XRB spectrum, and demonstrate how parameter space opens up considerably if a super-solar iron abundance is assumed for the absorbing material. This is illustrated with a model which satisfies the present constraints, but which predicts substantial numbers of type 2 source at the faint flux levels soon to be probed for the first time by the Chandra and XMM missions. We demonstrate also how a strong negative K-correction facilitates the detection of sources with 10^~24 < N_H < 10^25 cm^-2 out to the highest redshifts at which they could plausibly exist.

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arXiv1998

Limits on the X-ray emission from several hyperluminous IRAS galaxies

We report long, pointed ROSAT HRI observations of the hyperluminous galaxies IRAS F00235+1024, F12514+1027, F14481+4454 and F14537+1950. Two of them are optically classified as Seyfert-like. No X-ray sources are detected at the positions of any of the objects, with a mean upper limit L(X)/L(Bol) of 2.3 times 10^-4. This indicates that any active nuclei are either atypically weak at X-ray wavelengths or obscured by column densities N(H)>10^23 cm^-2. They differ markedly from `ordinary' Seyfert 2 galaxies, bearing a closer resemblance in the soft X-ray band to composite Seyfert 2 galaxies or to some types of starburst.

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