Near-infrared colors of hard X-ray-selected active galactic nuclei

Chisato Watanabe, Kouji Ohta, Masayuki Akiyama, Yoshihiro Ueda

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18 Citations (Scopus)


We present results of near-infrared photometry (J, H, Ks) 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) × 10-13 ergs s -1 cm-2 (2-10 keV). The sample covers the AGNs at 0.1 ≲ z ≲ 1 with L2-10 keV ∼ 1042-10 46 ergs s-1with 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-KS > 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. (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 L2-10 keV ≥ 1044.5ergs s-1) in the systematically surveyed sample in hard X-rays. For all the QSOs with an X-ray absorption of NH ≥ 1022 cm -2 in our sample, the values of AV / NH are 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.

Original languageEnglish
Pages (from-to)128-139
Number of pages12
JournalAstrophysical Journal
Issue number1 I
Publication statusPublished - 2004 Jul 20


  • Galaxies: active
  • Infrared: galaxies
  • Quasars: general
  • Surveys
  • X-rays: galaxies


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