The AGN Ionization Cones of NGC 5728. I. Excitation and Nuclear Structure

Abstract

Abstract We explore the gas morphology and excitation mechanisms of the ionization cones of the Type II Seyfert galaxy NGC 5728. Near-IR and optical data from the SINFONI and MUSE integral field units on the Very Large Telescope are combined with Hubble Space Telescope optical images, Chandra X-ray data, and Very Large Array radio observations. The complex nuclear structure has a star-forming (SF) ring with a diameter of 2 kpc. A radio jet impacts on the interstellar medium at about 200 pc from the nucleus, with the supernova remnants in the SF ring also present. Emission-line ratios of [Fe ii] and H ii show heavy extinction toward the nucleus, moderate extinction in the SF ring, and reduced extinction in the ionization cones. The active galactic nucleus (AGN) is hidden by a dust bar with up to 19 mag of visual extinction; the dust temperature at the nuclear position is ∼870 K. An X-ray jet is aligned with the ionization cones and associated with high-excitation emission lines of [Si vi] in a coronal line region extending 300 pc from the nucleus. Molecular hydrogen is spatially independent of the cones, concentrated in a disk equatorial to the SF ring, but also showing entrainment along the sides of the bicone. Gas masses for warm and cold H2, H i, and H ii are estimated, and the excitation mechanisms for ionized and molecular gas are elucidated, from both optical (which shows a clean SF–AGN mixing sequence) and infrared diagnostics (which show more complicated, multicomponent excitation regimes).