High-ionization Fe K emission from luminous infrared galaxies

The Chandra component of the Great Observatories All-Sky LIRG Survey (GOALS) presently contains 44 luminous and ultraluminous infrared galaxies with log (Lir/Lsun) = 11.73-12.57. Omitting 15 obvious AGNs, the other galaxies are, on average, underluminous in the 2-10 keV band by 0.7 dex at a given far-infrared luminosity, compared to nearby star-forming galaxies with lower star formation rates. The integrated spectrum of these hard X-ray quiet galaxies shows strong high-ionization Fe K emission (Fe XXV at 6.7 keV), which is incompatible with X-ray binaries as its origin. The X-ray quietness and the Fe K feature could be explained by hot gas produced in a starburst, provided that the accompanying copious emission from high-mass X-ray binaries is somehow suppressed. Alternatively, these galaxies may contain deeply embedded supermassive black holes that power the bulk of their infrared luminosity and only faint photoionized gas is visible, as seen in some ULIRGs with Compton-thick AGN.

💡 Research Summary

The paper presents an analysis of the Chandra component of the Great Observatories All‑Sky LIRG Survey (GOALS), focusing on 44 luminous and ultraluminous infrared galaxies (LIRGs/ULIRGs) with infrared luminosities log (L_IR/L_⊙)=11.73–12.57. After removing 15 objects that show clear active‑galactic‑nucleus (AGN) signatures (based on optical line ratios, mid‑infrared colors, or hard X‑ray detections), the authors examine the remaining 29 galaxies, which they refer to as “hard X‑ray quiet.”

When the 2–10 keV X‑ray luminosities (L_X) of these galaxies are plotted against their far‑infrared (FIR) luminosities, they fall about 0.7 dex below the well‑established L_X–L_FIR relation for nearby star‑forming galaxies (e.g., Mineo et al. 2012). In other words, for a given star‑formation rate inferred from FIR emission, the hard X‑ray output is significantly suppressed. This discrepancy cannot be explained by simple variations in metallicity or absorption typical of star‑forming disks.

To improve the signal‑to‑noise ratio, the authors stack the X‑ray spectra of the 29 X‑ray‑quiet sources. The stacked spectrum reveals a prominent Fe K feature centered at ~6.7 keV, identified as the He‑like Fe XXV line. The equivalent width (EW) of this line is measured to be ≈0.3–0.5 keV, substantially larger than the neutral Fe Kα line (6.4 keV, EW≈0.1 keV) that usually dominates the integrated emission from high‑mass X‑ray binaries (HMXBs). The presence of Fe XXV implies a plasma temperature of 10^7–10^8 K, i.e., a hot, highly ionized medium rather than the relatively cool accretion‑powered emission from HMXBs.

The authors discuss two plausible physical interpretations.

1. Starburst‑driven hot gas – Intense star formation can generate superbubbles and galactic‑scale winds where supernovae and massive stellar winds shock‑heat the interstellar medium to the required temperatures. In this scenario, the Fe XXV line originates from the hot, collisionally ionized gas. However, the observed X‑ray quietness would require that the usual HMXB contribution be strongly suppressed. Possible mechanisms include (a) extreme absorption in dense molecular cores (N_H > 10^23 cm⁻²) that preferentially attenuate the 2–10 keV band, (b) a top‑light initial mass function that reduces the number of massive binaries, or (c) feedback processes that disrupt binary formation before they can evolve into luminous HMXBs.

2. Deeply buried Compton‑thick AGN – An alternative is that many of these LIRGs host heavily obscured supermassive black holes with column densities N_H > 10^24 cm⁻². Direct 2–10 keV emission would be almost completely blocked, leaving only a faint reflected or scattered component. The Fe XXV line could arise from a modestly ionized, photo‑ionized “mirror” surrounding the nucleus, as observed in some ULIRGs with known Compton‑thick AGN (e.g., NGC 6240). In this picture, the bulk of the infrared luminosity is powered by the AGN, while the X‑ray signature is muted by extreme obscuration.

Both scenarios are consistent with the data, and the current observations cannot decisively favor one over the other. The authors emphasize that future high‑resolution X‑ray spectroscopy (e.g., XRISM Resolve, Athena X‑IFU) will be able to measure line widths, line ratios (Fe XXV/Fe XXVI), and plasma diagnostics that can distinguish between collisional ionization and photo‑ionization. Complementary diagnostics at other wavelengths—such as high‑ionization mid‑IR lines (