Insights Into AGN and Host Galaxy Co-evolution From Hard X-ray Emission

TL;DR

This study investigates the relationship between hard X-ray emission from AGNs and the stellar populations of their host galaxies, revealing that older stellar populations are associated with harder X-ray spectra, indicating a link between galaxy evolution and AGN activity.

Contribution

The paper provides new evidence of a correlation between host galaxy stellar age and AGN X-ray spectral hardness, using combined optical and X-ray spectral analysis on a sample of obscured AGNs.

Findings

Older stellar populations correlate with harder X-ray emission.

The correlation suggests decreased Compton cooling with stellar age.

Line ratios [O I]/Hα and [S II]/Hα relate to X-ray spectral slope.

Abstract

We study the AGN-host co-evolution issue here by focusing on the correlation between the hard X-ray emission from central AGNs and the stellar populations of the host galaxies. By focusing on the galaxies with strong H$\alpha$ line emission (EW(H$\alpha)>5$\AA), both X-ray and optical spectral analysis are performed on 67 (partially) obscured AGNs that are selected from the \it XMM-Newton\rm\ 2XMMi/SDSS-DR7 catalog originally cross-matched by Pineau et al. The sample allows us to study central AGN activity and host galaxy directly and simultaneously in individual objects. Combining the spectral analysis in both bands reveals that the older the stellar population of the host galaxy, the harder the X-ray emission will be, which was missed in our previous study where the \it ROSAT\rm\ hardness ratios are used. By excluding the contamination from the host galaxies and from the jet beaming emission, the correlation indicates that the Compton cooling in the accretion disk corona decreases with the mean age of the stellar population. We argue that the correlation is related to the correlation of $L/L_{\mathrm{Edd}}$ with the host stellar population. In addition, the [\ion{O}{1}]/H$\alpha$ and [\ion{S}{2}]/H$\alpha$ narrow-line ratios are identified to correlate with the spectral slope in hard X-ray, which can be inferred from the currently proposed evolution of the X-ray emission because of the confirmed tight correlations between the two line ratios and stellar population age.