Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.269..683f&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 269, 683 (1994)
Astronomy and Astrophysics
Astronomy
25
Active Galactic Nuclei: Luminosity Functions, Evolution, X-Ray: Agns, Quasars, Optical: Agns
Scientific paper
We compare statistical and evolutionary properties of Active Galactic Nuclei and quasars selected at optical and X-ray wavelengths. We show that the often suggested interpretation for the lower observed evolution of X-ray AGNs in terms of a non-linear dependence of the X-ray emission on optical luminosity brings to severe inconsistencies between the luminosity functions at various redshifts. A deeper insight into the recently published dataset on X-ray selected AGNs suggests that both questions probably require major revisions. From evolution studies based on ROSAT deep surveys and from a re-analysis of the Einstein EMSS survey we find indications that evolution rates of X-ray AGNs could have been underestimated. We also find that optical and X-ray emissions of X-ray selected AGNs scale linearly. Adoption of our new best solutions obtainde independently for the optical and X-ray evolution models and for the $L_O/L_X$ dependence leads to an impressive agreement of the LFs over wide luminosity and redshift intervals: optical and soft X-ray observations appear to detect essentially the same population of AGNs. On this basis, we argue that X-ray observations provide crucial advantages with respect to the usual optical selection techniques. a) Because of the small fraction of spurious contaminants (some \% of stars, galaxies and galaxy clusters in faint X-ray samples), it allows unbiased sampling at low redshifts and low luminosities, where the completeness of optical samples may be questionable. b) Due to the simple power-law behaviour of X-ray spectra, X-ray detection of high redshift objects is not affected by selection biases or hardly quantifiable effects characterizing optical methods. Our knowledge of the formation epoch and early evolution of AGNs, and of the global fraction of galaxies hosting an active nucleus, will benefit a lot by current and future X-ray missions.
Cristiani Stefano
Franceschini Alberto
La Franca Fabio
Martin-Mirones J. M.
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