We present a highly complete and reliable mid-infrared (MIR) colour selection of luminous active galactic nucleus (AGN) candidates using the 3.4, 4.6 and 12 µm bands of the Widefield Infrared Survey Explorer (WISE) survey. The MIR colour wedge was defined using the wide-angle Bright Ultrahard XMM-Newton survey (BUXS), one of the largest complete flux-limited samples of bright (f 4.5-10 keV > 6 × 10 −14 erg s −1 cm −2 ) 'ultrahard' (4.5-10 keV) X-ray-selected AGN to date. The BUXS includes 258 objects detected over a total sky area of 44.43 deg 2 of which 251 are spectroscopically identified and classified, with 145 being type 1 AGN and 106 type 2 AGN. Our technique is designed to select objects with red MIR power-law spectral energy distributions (SEDs) in the three shortest bands of WISE and properly accounts for the errors in the photometry and deviations of the MIR SEDs from a pure power-law. The completeness of the MIR selection is a strong function of luminosity. At L 2-10 keV > 10 44 erg s −1 , where the AGN is expected to dominate the MIR emission, 97.1 +2.2 −4.8 and 76.5 +13.3 −18.4 per cent of the BUXS type 1 and type 2 AGN, respectively, meet the selection. Our technique shows one of the highest reliability and efficiency of detection of the X-rayselected luminous AGN population with WISE amongst those in the literature. In the area covered by BUXS our selection identifies 2755 AGN candidates detected with signal-to-noise ratio ≥5 in the three shorter wavelength bands of WISE with 38.5 per cent having a detection at 2-10 keV X-ray energies. We also analysed the possibility of including the 22 µm WISE band to select AGN candidates, but neither the completeness nor the reliability of the selection improves. This is likely due to both the significantly shallower depth at 22 µm compared with the first three bands of WISE and star formation contributing to the 22 µm emission at the WISE 22 µm sensitivity.

Aims. Pointed observations with XMM-Newton provide the basis for creating catalogues of X-ray sources detected serendipitously in each field. This paper describes the creation and characteristics of the 2XMM catalogue. Methods. The 2XMM catalogue has been compiled from a new processing of the XMM-Newton EPIC camera data. The main features of the processing pipeline are described in detail. Results. The catalogue, the largest ever made at X-ray wavelengths, contains 246 897 detections drawn from 3491 public XMM-Newton observations over a 7-year interval, which relate to 191 870 unique sources. The catalogue fields cover a sky area of more than 500 deg 2 . The non-overlapping sky area is ∼360 deg 2 (∼1% of the sky) as many regions of the sky are observed more than once by XMM-Newton. The catalogue probes a large sky area at the flux limit where the bulk of the objects that contribute to the X-ray background lie and provides a major resource for generating large, well-defined X-ray selected source samples, studying the X-ray source population and identifying rare object types. The main characteristics of the catalogue are presented, including its photometric and astrometric properties

Abstract. We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the EB−V /NH ratio is nearly always lower than Galactic by a factor ranging from ∼3 up to ∼100. Other observational results indicate that the AV /NH ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7 µm (silicates) and at 2175Å (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the EB−V /NH and AV /NH ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

Abstract. We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the range 8 × 10 −15 −4 × 10 −13 erg cm −2 s −1 and the total area surveyed is 0.9 square degrees. One of the most interesting results (which is found also in deeper sourveys) is that about 20% of the hard X-ray selected sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more than one order of magnitude lower than the HELLAS2XMM survey sources, many of the extreme X/O sources in our sample have R < ∼ 25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with X/O > ∼ 10 (to be compared with 9 sources known from the deeper, pencil-beam surveys). Eight of them are narrow line QSO (seemingly the extension to very high luminosity of the type 2 Seyfert galaxies), four are broad line QSO. The results from our survey are also used to make reliable predictions about the luminosity of the sources not yet spectroscopically identified, both in our sample and in deeper Chandra and XMM-Newton samples. We then use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, Chandra Deep Field North 1Msec, Chandra SSA13 and XMM-Newton Lockman Hole flux limited samples), 221 with measured redshifts, to evaluate the cosmological evolution of the hard X-ray source's number and luminosity densities. Looking backward in time, the low luminosity sources (log L 2−10 keV = 43−44 erg s −1 ) increase in number at a much slower rate than the very high luminosity sources (log L 2−10 keV > 44.5 erg s −1 ), reaching a maximum around z = 1 and then levelling off beyond z = 2. This translates into an accretion driven luminosity density which is dominated by sources with log L 2−10 keV < 44.5 erg s −1 up to at least z = 1, while the contribution of the same sources and of those with log L 2−10 keV > 44.5 erg s −1 appear, with yet rather large uncertainties, to be comparable between z = 2 and 4.

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XMM-Newton observations of 10 ultraluminous infrared galaxies (ULIRGs) from a 200-ks mini-survey programme are reported. The aim is to investigate in hard X-rays a complete ULIRG sample selected from the bright IRAS 60-µm catalogue. All sources are detected in Xrays, five of which for the first time. These observations confirm that ULIRGs are intrinsically faint X-ray sources, their observed X-ray luminosities being typically L 2-10 keV 10 42 -10 43 erg s −1 , whereas their bolometric (mostly infrared) luminosities are L bol > 10 45 erg s −1 . In all sources we find evidence for thermal emission from hot plasma with a rather constant temperature kT 0.7 keV, dominating the X-ray spectra below 1 keV, and probably associated with a nuclear or circumnuclear starburst. This thermal emission appears uncorrelated with the far-infrared luminosity, suggesting that, in addition to the ongoing rate of star formation, other parameters may also affect it. The soft X-ray emission appears to be extended on a scale of ∼30 kpc for Mrk 231 and IRAS 19254−7245, possible evidence of galactic superwinds. In these two sources, IRAS 20551−4250 and 23128−5919, we find evidence for the presence of hidden active galactic nuclei (AGNs), while a minor AGN contribution may be suspected also in IRAS 20100−4156. In particular, we have detected a strong (EW ∼ 2 keV) Fe K line at 6.4 keV in the spectrum of IRAS 19254−7245 and a weaker one in Mrk 231, suggestive of deeply buried AGNs. For the other sources, the X-ray luminosities and spectral shapes are consistent with hot thermal plasma and X-ray binary emissions of mainly starburst origin. We find that the 2-10 keV luminosities in these sources, most probably due to high-mass X-ray binaries, are correlated with L FIR : both luminosities are good indicators of the current global star formation rate in the Galaxy. The composite nature of ULIRGs is then confirmed, with hints for a predominance of the starburst over the AGN phenomenon in these objects even when observed in hard X-rays.

We present the morphological analysis based on HST-NIC2 (0.075 arcsec pixel −1 ) images in the F160W filter of a sample of nine massive field (>10 11 M ) galaxies spectroscopically classified as early-types at 1.2 < z < 1.7. Our analysis shows that all of them are bulgedominated systems. In particular, six of them are well fitted by a de Vaucouleurs profile (n = 4) suggesting that they can be considered pure elliptical galaxies. The remaining three galaxies are better fitted by a Sérsic profile with index 1.9 < n fit < 2.3 suggesting that a disc-like component could contribute up to 30 per cent to the total light of these galaxies. We derived the effective radius R e and the mean surface brightness (SB) μ e within R e of our galaxies and we compared them with those of early-types at lower redshifts. We find that the SB μ e of our galaxies should get fainter by 2.5 mag from z ∼ 1.5 to ∼0 to match the SB of the local ellipticals with comparable R e , that is, the local Kormendy relation. Luminosity evolution without morphological changes can only explain half of this effect, as the maximum dimming expected for an elliptical galaxy is ∼1.6 mag in this redshift range. Thus, other parameters, possibly structural, may undergo evolution and play an important role in reconciling models and observations. Hypothesizing an evolution of the effective radius of galaxies we find that R e should increase by a factor of 1.5 from z ∼ 1.5 to ∼0.

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10 −14 erg cm −2 s −1 . Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as "FIORE P3". The analysis of the broadband energy distribution suggests the presence of a heavily obscured active nucleus.