2004ApJ...610..128W

We present results of near-infrared photometry (J, H, K_s) for a hard X-ray-selected sample of active galactic nuclei (AGNs) obtained from optical identification of the sources detected in ASCA surveys (total ∼75 deg²) with a flux limit of (1-3)x10^–13 ergs/s/cm2 (2-10 keV). The sample covers the AGNs at 0.1≲z≲1 with L_2–10keV∼10⁴²-10⁴⁶ ergs/s with very high completeness. The near-infrared photometric data of the sample are obtained from the Two Micron All Sky Survey (2MASS) and observations with Kitt Peak National Observatory 2.1 m telescope and the University of Hawaii 2.2 m telescope. The fraction of red (J-K_S>2 mag) AGNs in our sample is ∼2%±1%, which is comparable with that for optically or UV-selected quasi-stellar objects (QSOs, i.e., luminous AGNs). The number of red AGNs found in our sample is consistent with that expected from the surface density of red AGNs found in 2MASS by Cutri et al. ((ref???)2001). We find that the anomalously small dust-to-gas ratios in circumnuclear gas, which is seen in some AGNs with Seyfert-class luminosity, also occur in the QSOs (AGNs with luminosity of L_2–10keV≥10^44.5 ergs/s) in the systematically surveyed sample in hard X-rays. For all the QSOs with an X-ray absorption of N_H≥10²²/cm2 in our sample, the values of A_V/N_Hare smaller than the Galactic value by a factor of ∼5 to ∼100. Since the fraction of this population among the QSOs in our sample is about 30%, the fraction of optical/UV-selected type 1 QSOs known to date may show a type 2 nature in X-ray.