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

Abstract

Abstract We present Chandra ACIS-S imaging spectroscopy results of the extended (1.″5–8″, 300–1600 pc) hard X-ray emission of NGC 5728, the host galaxy of a Compton-thick active galactic nucleus. We find spectrally and spatially resolved features in the Fe Kα complex (5.0–7.5 keV) redward and blueward of the neutral Fe line at 6.4 keV in the extended narrow-line region bicone. A simple phenomenological fit of a power law plus Gaussians gives a significance of 5.4σ and 3.7σ for the red and blue wings, respectively. Fits to a suite of physically consistent models confirm a significance of ≥3σ for the red wing. The significance of the blue wing may be diminished by the presence of rest-frame highly ionized Fe xxv and Fe xxvi lines (1.4σ–3.7σ range). A detailed investigation of the Chandra ACIS-S point-spread function and comparison with the observed morphology demonstrates that these red and blue wings are radially extended (∼5″, ∼1 kpc) along the optical bicone axis. If the wing emission is due solely to redshifted and blueshifted high-velocity neutral Fe Kα, then the implied line-of-sight velocities are +/− ∼0.1c, and their fluxes are consistent with being equal. A symmetric high-velocity outflow is then a viable explanation. This outflow has deprojected velocities ∼100 times larger than the outflows detected in optical spectroscopic studies, potentially dominating the kinetic feedback power.