X-ray Bright Optically Normal Galaxies with the Chandra Source Catalog

We have analyzed the X-ray bright optically normal galaxies (XBONG) using the Chandra Source Catalog (CSC) version 2. Crossmatching the SDSS sample of spectroscopically classified galaxies, we identified 817 XBONG candidates with L_X > 10⁴² erg s^-1 and F_XO (X-ray to optical flux ratio) > 0.1.To explore the nature of XBONGs, we examine the X-ray hardness ratios (HR). Interestingly, we find that the fraction of XBONGs with high HR values is significantly (at the 7σ level) higher than that of normal galaxies and QSOs, indicating that the XBONG sample does consist of an absorbed population. The fraction of the obscured (with N_H> 10²² cm^-2) XBONG sample is 43%, a factor of two higher than that of the QSO sample.

About 50% of XBONGs show no clear evidence of obscuration. Given that their X-ray spectra are still harder than that of the normal galaxies and that they have no AGN-like optical spectral evidence, they may be unidentified groups and clusters, including fossil systems. We have eye-examined the individual X-ray images of the ~100 XBONGs with an extended flag set and present a few clear cases of extended hot gas. Most are not easily discernable because they are faint or at large off-axis angles with a large PSF. The approximate upper limit of extended XBONGs is 20%. We also estimate the approximate fraction of XBONGs whose AGN signatures are diluted by stellar light of host galaxies to be 30%, based on their redshift and deviation from the extrapolation of the QSO L­_X-L_R relation.

We also explore XBONGs with WISE colors and NIR, optical, and UV luminosities. The XBONG sample lies between the normal galaxy and QSO samples in all multi-wavelength studies. In particular, the WISE color (W1-W2) distribution suggests that about 50% of XBONGs are possibly obscured, like type 2 AGNs and Compton-thick AGNs. In the L_X - L_UV and L_X – L_K relations, XBONGs follow the same relation of QSOs at the low luminosity end.