Chandra Observations of Excess Fe K$\alpha$ Line Emission in Galaxies with High Star Formation Rates: X-ray Reflection on Galaxy Scales?

TL;DR

This study finds that galaxies with high star formation rates exhibit significantly stronger Fe Kα line emission on large scales, suggesting widespread X-ray reflection from star-forming material in galaxy hosts.

Contribution

It provides evidence linking large-scale Fe Kα emission to star-forming regions in galaxies, expanding understanding beyond the traditional AGN torus model.

Findings

Fe Kα equivalent width is three times higher in high SFR galaxies

No significant dependence of Fe line strength on stellar mass or X-ray luminosity

Large-scale X-ray reflection may be common in star-forming galaxy hosts

Abstract

In active galactic nuclei (AGN), fluorescent Fe K$\alpha$ (iron) line emission is generally interpreted as originating from obscuring material around a supermassive black hole (SMBH) on the scale of a few parsecs (pc). However, recent Chandra studies indicate the existence of iron line emission extending to kpc scales in the host galaxy. The connection between iron line emission and large-scale material can be spatially resolved directly only in nearby galaxies, but could be inferred in more distant AGNs by a connection between line emission and star-forming gas and dust that is more extended than the pc-scale torus. Here we present the results from a stacking analysis and X-ray spectral fitting performed on sources in the Chandra Deep Field South (CDFS) 7 Ms observations. From the deep stacked spectra, we select sources with stellar mass $\log(M_*/M_\odot)>10$ at $0.5<z<2$, obtaining 25 sources with high infrared luminosity ($ {\rm SFR}_{\rm FIR} \geq 17\;M_{\odot}\;{\rm yr}^{-1}$) and 32 sources below this threshold. We find that the equivalent width of the iron line EW(Fe) is a factor of three higher with 3$\sigma$ significance for high infrared luminosity measured from Herschel observations, indicating a connection between iron line emission and star-forming material on galaxy scales. We show that there is no significant dependence in EW(Fe) on $M_*$ or X-ray luminosity, suggesting the reflection of AGN X-ray emission over large scales in their host galaxies may be widespread.