Astronomy and Astrophysics, volume 487, 119-130 (2008/8-3)
The cosmological properties of AGN in the XMM-Newton Hard Bright Survey.
DELLA CECA R., CACCIANIGA A., SEVERGNINI P., MACCACARO T., BRUNNER H., CARRERA F.J., COCCHIA F., MATEOS S., PAGE M.J. and TEDDS J.A.
Abstract (from CDS):
We investigate here the X-ray luminosity function (XLF) of absorbed (NH between 4x1021 and 1024/cm2) and unabsorbed (NH<4x1021/cm2) AGN, the fraction of absorbed AGN as a function of LX (and z), the intrinsic NH distribution of the AGN population, and the XLF of Compton thick (NH>1024/cm2) AGN. To carry out this investigation, we have used the XMM-Newton Hard Bright Serendipitous Sample (HBSS), a complete sample of bright X-ray sources (fx>7x10–14erg/cm2/s) at high galactic latitude (|b|>20°) selected in the 4.5-7.5 keV energy band. The HBSS sample is now almost completely identified (97% spectroscopic identifications) and it can be safely used for a statistical investigation. The HBSS contains 62 AGN out of which 40 are unabsorbed (or marginally absorbed; NH<4x1021/cm2) and 22 are absorbed (NH between 4x1021 and ∼1024/cm2). Absorbed and unabsorbed AGN are characterised by two different XLF with the absorbed AGN population being described by a steeper XLF, if compared with the unabsorbed ones, at all luminosities. The intrinsic fraction F of absorbed AGN (i.e., the fraction of sources with NH between 4x1021 and 1024/cm2 divided the sources with NH below 1024/cm2, corrected for the bias due to the photoelectric absorption) with L2–10keV>3x1042erg/s is 0.57±0.11; we find that F decreases with the intrinsic luminosity, and probably, increases with the redshift. Our data are consistent with a flat Log NH distribution for NH between 1020 and 1024/cm2. Finally, by comparing the results obtained here with those obtained using an optically-selected sample of AGN we derive, in an indirect way, the XLF of Compton thick AGN; the latter is well described by a XLF similar, in shape, to that of absorbed AGN, but having a normalization of about a factor of 2 above. The density ratio between Compton thick AGN (NH≥1024/cm2) and Compton thin AGN (NH≤1024/cm2) decreases from 1.08±0.44 at ∼1043erg/s to 0.57±0.22 at ∼1044erg/s to 0.23±0.15 at ∼1045erg/s. The results presented here on the anti-correlation between F and -Lx are fully consistent with the hypothesis of a reduction of the covering factor of the gas as a function of the luminosity and are clearly inconsistent with the simplest unified scheme of AGN. These results strongly support the recently proposed radiation-limited clumpy dust torus model although alternative physical models are also consistent with the observations.
surveys - galaxies: active - galaxies: evolution - X-rays: diffuse background - X-ray: galaxies - quasars: general
VizieR on-line data:
<Available at CDS (J/A+A/487/119): table1.dat>
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