Discovery of Kiloparsec-scale Semi-relativistic Fe Kα Complex Emission in NGC 5728

A phenomenological continuum-plus-line model gives significances of 5.4σ and 3.7σ for the red and blue Fe Kα excesses, respectively. Across the suite of physically motivated models, the red component remains significant at ≥3σ, while the significance of the blue component ranges from 1.4σ to 3.7σ because rest-frame Fe XXV and Fe XXVI emission may contribute to that energy range.

Comparison with the Chandra point-spread function and radial profiles shows that both components extend approximately 5 arcsec, or 1 kpc, along the optical bicone and beyond the nuclear point source.

If both components arise entirely from Doppler-shifted neutral Fe Kα emission, their energies imply line-of-sight velocities approaching ±0.1c. Their comparable fluxes make a symmetric high-velocity outflow a viable interpretation, but the possible contribution of highly ionized iron to the blue component means that this interpretation is not unique.

Under the high-velocity-outflow interpretation, the deprojected velocities are approximately 100 times larger than those measured for the optical emission-line outflow. Because kinetic power scales as the cube of velocity, an equal-mass comparison would imply up to approximately 10⁶ times the kinetic power of the optical phase. The paper does not independently measure the mass or kinetic luminosity of this proposed component, so its feedback significance remains an inference rather than a direct energetic measurement.