Near-Infrared Colours of Active Galactic Nuclei
  We propose near-infrared colour selection criteria to extract Active Galactic
Nuclei (AGNs) using the near-infrared Colour-Colour Diagram (CCD) and predict
near-infrared colour evolution with respect to redshift. First, we
cross-identify two AGN catalogues with the 2MASS Point Source Catalogue, and
confirm both the loci of quasars/AGNs in the near-infrared CCD and
redshift-colour relations. In the CCD, the loci of over 70 - 80% of AGNs can be
distinguished from the stellar locus. To examine the colours of quasars, we
simulate near-infrared colours using Hyperz code. Assuming a realistic quasar
SED, we derive simulated near-infrared colours of quasars with redshift (up to
z ~ 11). The simulated colours can reproduce not only the redshift-colour
relations but also the loci of quasars/AGNs in the near-infrared CCD. We
finally discuss the possibility of contamination by other types of objects. We
compare the locus of AGNs with the other four types of objects (namely,
microquasars, CVs, LMXBs, and MYSOs) which have a radiation mechanism similar
to that of AGNs. In the near-infrared CCD, each type of object is located at a
position similar to the stellar locus. Accordingly, it is highly probable that
the four types of objects can be distinguished on the basis of the locus in a
near-infrared CCD. We additionally consider contamination by distant normal
galaxies. The near-infrared colours of several types of galaxies are also
simulated using the Hyperz code. Although galaxies with z ~ 1 have
near-infrared colours similar to those of AGNs, these galaxies are unlikely to
be detected because they are very faint. In other words, few galaxies should
contaminate the locus of AGNs in the near-infrared CCD. Consequently, we can
extract reliable AGN candidates on the basis of the near-infrared CCD.
