Evidence for ultra-fast outflows in radio-quiet AGNs: III - location and energetics

Using the results of a previous X-ray photo-ionization modelling of blue-shifted Fe K absorption lines on a sample of 42 local radio-quiet AGNs observed with XMM-Newton, in this letter we estimate the location and energetics of the associated ultra-fast outflows (UFOs). Due to significant uncertainties, we are essentially able to place only lower/upper limits. On average, their location is in the interval ~0.0003-0.03pc (~10^2-10^4 r_s) from the central black hole, consistent with what is expected for accretion disk winds/outflows. The mass outflow rates are constrained between 0.01-1 M_{\odot} yr^{-1}, corresponding to >5-10% of the accretion rates. The average lower-upper limits on the mechanical power are log\dot{E}_K42.6-44.6 erg s^{-1}. However, the minimum possible value of the ratio between the mechanical power and bolometric luminosity is constrained to be comparable or higher than the minimum required by simulations of feedback induced by winds/outflows. Therefore, this work demonstrates that UFOs are indeed capable to provide a significant contribution to the AGN cosmological feedback, in agreement with theoretical expectations and the recent observation of interactions between AGN outflows and the interstellar medium in several Seyferts galaxies.

💡 Research Summary

This paper builds on a previous X‑ray photo‑ionization analysis of blue‑shifted Fe K absorption lines detected in a sample of 42 nearby radio‑quiet active galactic nuclei (AGNs) observed with XMM‑Newton. The authors use the derived ionization parameter (ξ), column density (N_H), and outflow velocity (v) to estimate the location, mass‑outflow rate, and mechanical power of the associated ultra‑fast outflows (UFOs). Because the gas density (n) and covering factor are not directly measurable, the authors adopt a conservative approach: they calculate lower limits on the distance (r) by pairing the highest plausible ξ with the lowest N_H, and upper limits by pairing the lowest ξ with the highest N_H. This yields a distance range of roughly 0.0003–0.03 pc, equivalent to about 10²–10⁴ Schwarzschild radii (r_s) from the central black hole. Such distances are precisely where accretion‑disk winds are expected to be launched, supporting the interpretation of UFOs as highly ionized, high‑velocity extensions of disk outflows.

The mass‑outflow rate is estimated using the expression Ṁ_out ≈ Ω μ m_p N_H v r⁻¹, where Ω is the solid angle subtended by the wind (assumed ≈0.5 sr), μ the mean atomic weight, and m_p the proton mass. The resulting Ṁ_out spans 0.01–1 M_⊙ yr⁻¹, corresponding to at least 5–10 % of the accretion rate inferred from the AGN bolometric luminosity. The mechanical (kinetic) power follows from Ė_K = ½ Ṁ_out v² and lies between log Ė_K ≈ 42.6 and 44.6 erg s⁻¹. When compared to the bolometric luminosity (L_bol), even the most conservative estimate gives Ė_K/L_bol ≥ 0.5 %, a threshold commonly cited in cosmological simulations as the minimum required for effective AGN feedback on galactic scales.

These findings have several important implications. First, the inferred distances and velocities are fully consistent with theoretical models of magnetically or radiation‑driven disk winds, suggesting that UFOs are the innermost, most energetic component of a continuous outflow structure that also includes the slower, lower‑ionization warm absorbers observed in many AGNs. Second, the mass‑loading and kinetic power indicate that UFOs can carry a substantial fraction of the accretion energy back into the host galaxy, potentially heating or expelling interstellar gas, suppressing star formation, and contributing to the observed scaling relations between black hole mass and galaxy properties. Third, the analysis highlights the dominant sources of uncertainty: the unknown gas density and covering factor, the assumption of spherical symmetry, and the measurement errors on ξ and N_H. By providing both lower and upper bounds, the authors demonstrate that even under pessimistic assumptions UFOs remain energetically significant.

The paper concludes that ultra‑fast outflows are capable of delivering a mechanical power comparable to, or exceeding, the minimum required by feedback models, thereby playing a non‑negligible role in the cosmological impact of AGNs. The authors advocate for future high‑resolution X‑ray spectroscopy (e.g., XRISM, Athena) and multi‑wavelength campaigns to resolve the geometry, variability, and interaction of UFOs with the surrounding interstellar medium. Such observations will be essential to refine distance estimates, constrain covering factors, and ultimately quantify the contribution of UFOs to galaxy evolution.