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Chandra Observations of Excess Fe Kα Line Emission in Galaxies with High Star Formation Rates: X-Ray Reflection on Galaxy Scales?

Yan, Wei and Hickox, Ryan C. and Chen, Chien-Ting J. and Ricci, Claudio and Masini, Alberto and Bauer, Franz E. and Alexander, David M. (2021) 'Chandra Observations of Excess Fe Kα Line Emission in Galaxies with High Star Formation Rates: X-Ray Reflection on Galaxy Scales?', The Astrophysical Journal, 914 (2). p. 83.


In active galactic nuclei (AGNs), fluorescent Fe Kα (iron) line emission is generally interpreted as originating from obscuring material around a supermassive black hole on the scale of a few parsecs. However, recent Chandra studies indicate the existence of iron line emission extending to kiloparsec 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 parsec-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 10 M M > * ( )  at 0.5 < z < 2, obtaining 25 sources with high-infrared (IR) luminosity (star formation rate, SFRFIR … 17 M☉ yr−1 ) and 32 sources below this threshold. We find that the equivalent width (EW) of the iron line EW(Fe) is a factor of three higher with 3σ significance for high-IR 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 of the EW(Fe) on M* or X-ray luminosity, suggesting that the reflection of AGN X-ray emission over large scales in their host galaxies may be widespread.

Item Type:Article
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Publisher statement:© 2021. The American Astronomical Society. All rights reserved.
Date accepted:20 April 2021
Date deposited:23 August 2021
Date of first online publication:17 June 2021
Date first made open access:23 August 2021

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