On the nature of X-ray “unobscured” Seyfert 2 galaxies

Abstract

We present a systematic study to reveal the effect of host galaxy contamination to the identification of X-ray “unobscured” Seyfert 2 galaxies. We compiled a sample of 14 X-ray “unobscured” Seyfert 2 galaxies and 29 X-ray obscured Seyfert 2s, with infrared [Ne II] and [O IV] emission line flux measurements. In this work we classify Seyfert 2s as “unobscured” or obscured in X-ray simply based on X-ray spectral fitting. We find that our X-ay “unobscured” Seyfert 2s have statistically higher [Ne II]/[O IV] line ratios, compared with obscured ones. Since the [Ne II]/[O IV] line ratio reflects the relative strength of star formation activity comparing with the SMBH accretion, the observed differences in [Ne II]/[O IV] line ratios clearly indicate relatively stronger X-ray contamination from star formation in the host galaxies in X-ray “unobscured” Seyfert 2s. Therefore we could attribute the X-ray “unobscured” appearance to host galaxy contamination for most (if not all) of our X-ray “unobscured” Seyfert 2s. Further analysis indicates that “unobscured” Seyfert 2s have intrinsically lower AGN luminosity but similar star formation rates, compared with obscured ones. This is also consistent with previous studies showing most X-ray “unobscured” Seyfert 2s are at lower luminosities, and relatively stronger contaminations from the host galaxies are thus expected. Finally we develop a screening criterion to distinguish potential pure Seyfert 2s from X-ray “unobscured” appearances which could be caused by strong host galaxy contamination.