Papers in Submission
P. Gandhi and A.C. Fabian
We present a synthesis model of the X-ray background based on the cross-correlation between mid-infrared and X-ray surveys, where the distribution of type 2 sources is assumed to follow that of luminous infrared galaxies while type 1 sources are traced by the observed ROSAT distribution. The best fits to both the X-ray number counts and background spectrum require at least some density evolution. We explore a limited range of parameter space for the evolutionary variables of the type 2 luminosity function. Matching to the observed deep fields' redshift distribution, we find weak residuals as a signature of Fe emission from sources in a relatively peaked range of redshift. This extends the recent work of Franceschini et al., and emphasizes the possible correlation between obscured AGN and star-forming activity.
Refereed Publications
P. Gandhi, C.S. Crawford, A.C. Fabian
We present near-infrared long-slit spectra of eight optically-dim X-ray sources obtained with ISAAC on the Very Large Telescope. Six of the sources have hard X-ray emission with a significant fraction of the counts emerging above 2 keV. All were discovered serendipitously in the fields of three nearby galaxy clusters observed with Chandra, and identified through near-infrared imaging. The X-ray fluxes lie close to the break in the source counts. Two of the sources show narrow emission lines, and a third has a broad line. One of the narrow line-emitting sources has a clear redshift identification at z=2.18, while the other has a tentative determination based on the highest redshift detection of He I 10830 at z=1.26. The remainder have featureless spectra to deep limiting equivalent widths of 20--60 angstroms and line flux approx= 5 x 10^{-17} erg/s/cm^2 in the K-band. High-quality J, H and Ks--band images of the sources were combined with archival optical detections or limits to estimate a photometric redshift for six. Two sources show complex double morphology. The hard sources have spectral count ratios consistent with heavily obscured AGN, while the host galaxy emits much of the optical and near-infrared flux. The most likely explanation for the featureless continua is that the line photons are being scattered or destroyed by optically-thick gas and associated dust with large covering fractions.
C.S. Crawford, P. Gandhi, A.C. Fabian, R.J. Wilman, R.M. Johnstone, A.J. Barger, L.L. Cowie
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.
Wilman RJ, Fabian AC, Gandhi P
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 1000 K), with an inner radius within a parsec of the nucleus. Some warm dust models with inner temperatures of 200 K~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.
Crawford CS, Fabian AC, Gandhi P, Wilman RJ, Johnstone RM
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.
Conference Proceedings
P. Gandhi, A.C. Fabian and C.S. Crawford
Models for the origin of the hard X-ray background have suggested that sources with the most accretion activity lie hidden in highly obscured AGN. We report on our study of hard, serendipitous sources in the fields of Chandra clusters with fluxes close to the turn-over in the source-counts. These include two Type II quasars with measured X-ray luminosities >10^{45} ergs per second and column-densities >10^{23} cm^{-2}, one possibly being Compton-thick. Both show indications of redshifted Fe K\alpha line emission. Radiative transfer modelling of the broad-band spectrum of a highly-magnified source with deep ISOCAM detections implies the presence of warm-to-hot dust obscuring a quasar with L_{UV}>10^{45} ergs per second. Multi-wavelength spectroscopic and photometric follow-up of the optically-faint sources suggests that these objects are found in the centres of massive evolved galaxies at a range of redshifts, with red optical / near-infrared colours dominated by the host galaxy. Detailed source identification is difficult due to the paucity of strong emission features, especially in the infrared. We present the main results from a sample of near-infrared spectra of optically-faint sources obtained with 4 m and 8 m telescopes. Through the study of the harder and brighter X-ray background source population, we are likely to be viewing the most intense phase of the growth of supermassive black holes.
Proc. Symposium "New Visions of the X-ray Universe in the XMM-Newton and Chandra Era" 26-30 November 2001, ESTEC, The Netherlands astro-ph/0201543
P. Gandhi, C.S. Crawford, A.C. Fabian, R.J. Wilman, R.M. Johnstone, A.J. Barger, L.L. Cowie
We describe progress in a programme to study a sample of sources typical of those which contribute a large fraction of the hard X-ray background. The sources are selected from the fields of approximately 10 ks Chandra cluster observations with follow-up in the near-infrared and optical. The X-ray data indicate that many of these are powerful, obscured sources. Two hard objects which are lensed by the cluster are good candidates for X-ray type-2 quasars, with luminosities of the order 10^{44}-10^{45} erg s^-1 and obscuring column densities of the order 10^23 cm^-2. We find that the sources are bright in the infra-red (K typically 17-18). From Keck optical spectra and photometric redshifts, we find that the host galaxies are consistent with being early-type, massive hosts at redshifts ranging from 0.2 to 3, with median redshift approximately 1.5. The two obscured quasars also have mid-infrared detections and 850-micron upper-limits implying that the surrounding dust is at warm-to-hot temperatures (T of the order 1000 K).
Proceedings of the XXI Moriond Astrophysics Meeting: "Galaxy Clusters and the High Redshift Universe Observed in X-rays", edited by D. Neumann, F.Durret & J. Tran Thanh Van, astro-ph/0106139
Posters Presented
Last Modified on
2002-10-21
X-ray background synthesis: the infrared connection (gzipped version) (pdf)