Near-infrared colours of active galactic nuclei

Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan e-mail: kouzuma@phys.kyushu-u.ac.jp

Received: 10 July 2009
Accepted: 9 October 2009

Abstract

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 redshift. We first 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 the 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 an other four types of objects (namely, microquasars, cataclysmic variables, low mass X-ray binaries, and massive young stellar objects), 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 we can differentiate between the four types of objects on the basis of their 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.