Narrow-line X-ray-selected galaxies in the Chandra-COSMOS field. Ii. Optically elusive X-ray AGNs

In the Chandra-COSMOS (C-COSMOS) survey, we have looked for X-ray-selected active galactic nuclei (AGNs), which are not detected as such in the optical, the so-called elusive AGNs. A previous study based on XMM-Newton and Sloan Digital Sky Survey observations has found a sample of 31 X-ray AGNs optically misclassified as star-forming (SF) galaxies at $z<0.4$, including 17 elusive Sy2s. Using Chandra observations provides a sample of fainter X-ray sources and so, for a given X-ray luminosity, extends to higher redshifts. To study the elusive Sy2s in the C-COSMOS field, we have removed the NLS1s that contaminate the narrow-line sample. Surprisingly, the contribution of NLS1s is much lower in the C-COSMOS sample (less than 10% of the optically misclassified X-ray AGNs) than in Pons and Watson. The optical misclassification of the X-ray AGNs ($L_X>{10}^{42}erg{s}^{-1}$) can be explained by the intrinsic weakness of these AGNs, in addition to, in some cases, optical dilution by the host galaxies. Interestingly, we found the fraction of elusive Sy2s (narrow emission-line objects) optically misclassified as SF galaxies up to $z\sim 1.4$ to be 10% ± 3% to 17% ± 4%, compared to the 6% ± 1.5% of the Pons and Watson work (up to $z\sim 0.4$). This result seems to indicate an evolution with redshift of the number of elusive Sy2s.