This paper reports the first clear detection of very highly ionized iron in the active galaxy NGC 5548. Using XRISM’s Resolve instrument, the team found absorption lines from Fe XXV and Fe XXVI—iron atoms that have lost many electrons—appearing in the X‑ray band around 6–9 keV. Those lines reveal a hot, structured outflow of gas rising from the galaxy’s center.

The researchers combined a deep XRISM observation (301 kiloseconds of exposure in July 2025) with higher-resolution soft X‑ray spectra from XMM‑Newton’s Reflection Grating Spectrometer and ultraviolet spectra from the Hubble Space Telescope’s Cosmic Origins Spectrograph. XRISM’s Resolve microcalorimeter has very good energy resolution (about 4.5 electronvolts full width at half maximum), which makes it possible to pick out faint, narrow iron absorption features that previous instruments could not measure well in this energy band.

From the joint spectral fits they identify four distinct ionization components. The ionization parameter (ξ), a simple measure of how strongly the gas is ionized by the central source, spans log ξ ≈ 0.9 to 3.4 (logarithmic units). Three components split into two separate velocity subcomponents, showing the gas is multiphase and clumpy. Measured outflow speeds range from about 240 to 2,730 kilometers per second. The data show a general trend: gas with higher ionization tends to have higher column density (more absorbing material) and to move faster.

XRISM also delivers a much more detailed absorption measure distribution (AMD), which is a way to show how much absorbing gas sits at each ionization level. The AMD exhibits two different slopes below and above log ξ ≈ 2.6, which suggests that the hot and warm parts of the wind are not simply a single smooth flow but have different physical behavior. A comparison of the Fe XXV X‑ray profile with ultraviolet absorption lines (C IV and Lyman α) from HST shows partial overlap but also clear differences, reinforcing the picture of a multiphase outflow.