Natures of hard X-ray selected active galactic nuclei (AGNs) are discussed, based on the sample from optical identification of X-ray sources detected in the ASCA Large Sky Survey.
Optical spectroscopic observations were done for 34 X-ray sources which were detected with the SIS in the 2-7 keV band above 3.5 sigma. The flux limit corresponds to 1x10-13/ erg s-1 cm-2 in the 2-10 keV band. The sources are identified with 30 AGNs, 2 clusters of galaxies, and 1 galactic star. Only 1 source is still unidentified.
All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the Cosmic X-ray Background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of log NH (cm-2) = 22 - 23 at the object's redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3+-0.3. The hardness may be explained by the reflection component of a type 1 Seyfert. The hard X-ray spectra may also be explained by absorption with log NH (cm-2) = 22 - 23 at the object's redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet.
Based on the logN-logS relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH (cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3x10-11 erg s-1 cm-2 to 2x10-13 erg s-1 cm-2. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%.
In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs are limited below z=0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z=0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of log NH (cm-2) = 22 to 23 in the redshift range between 0.5 and 2, or in the X-ray luminosity range larger than 10^44 erg s-1, or both. Considering the luminosity and redshift distribution of absorbed and less-absorbed AGNs of the HEAO1 A2 sample, the dependence on the luminosity is suggested. However, if the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.
We made optical photometric and imaging observations for 6 absorbed and/or narrow-line AGNs in the LSS sample. 5 of them which are at low and intermediate redshift universe reside in early-type (E or S0) galaxies. Luminosities of the 5 objects distribute from 1/2 $L* to 3 L* and peak at 2 L*, if we adopt the absolute magnitude of the knee of the galaxy luminosity function as MR = -22.2 mag. Their radial surface brightnesses are well described by the de Vaucouleurs r1/4 law and the surface brightnesses and the effective radii follow the Kormendy relation of E and S0 galaxies in the nearby universe. However, the colors of host galaxies are not fully consistent with those of old elliptical galaxies at their redshift. They show bluer colors than the old elliptical galaxy model. The remaining highest redshift luminous absorbed AGN (absorbed QSO, it should be noted that the object is not "absorbed" AGNs because the best fit column density is 6x1021 cm-2) shows two components, a point-like nucleus which is thought to originate from scattered nuclear light and a peculiar extended component whose colors suggest post-starburst nature of the host galaxy.