Chandra Observations of Excess Fe Kα Line Emission in Galaxies with High Star Formation Rates: X-Ray Reflection on Galaxy Scales?
2021
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.
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