Monthly Notices of the Royal Astronomical Society · 2022
MNRAS 511, 1420-1430 (2022)
Scientific summary
Cloudy photoionization models predict spatial profiles for high-ionization optical and infrared lines formed in gas over the same ionization range as important soft X-ray lines. For NGC 4151, archival Hubble/STIS spectroscopy and WFPC2 imaging are combined with physical parameters from a published Chandra/HETG analysis to test whether these footprint lines can trace the extended X-ray-emitting gas.
Key results
The models identify high-ionization optical and infrared footprint lines, including [Fe X] λ6375 and [Si X] 1.43 μm, that form over the same ionization range as important soft X-ray emission lines.
For NGC 4151, the modeled [Fe X] radial-flux profile agrees with measurements from the Hubble/STIS G750L spectrum. Predicted spatially integrated fluxes for [Al IX] 2.04 μm and [Si X] 1.43 μm also agree with published ground-based measurements, while discrepancies for other lines indicate that the agreement does not extend uniformly to every predicted footprint line.
The footprint method gives a spatially integrated X-ray-emitting gas mass of (7.8 ± 2.1) × 10⁵ M⊙, consistent with the independent Chandra/HETG value of (5.4 ± 1.1) × 10⁵ M⊙. This agreement supports the use of the [Fe X] profile to recover the mass distribution of the extended X-ray-emitting gas.
The available STIS G750L spectrum does not provide sufficient spectral resolution and signal-to-noise to measure reliable [Fe X] kinematics. Consequently, this study derives the gas-mass profile but does not independently derive the kinematics or mass-outflow-rate profile of NGC 4151.
