The kinematics of the Seyfert 2 galaxy NGC 5728 circumnuclear region

Abstract

We secured spectral images of the circumnuclear, 12 x 10 arcsec 2 region of the Seyfert 2 galaxy NGC 5728 with the Optically Adaptive System for Imaging Spectroscopy at the Canada-France-Hawaii Telescope 3.6-m telescope. The radial velocity distribution of the two bright zones, main central (C) core and north-west (NW) core, and strategically important points along the position angles, PA 33°, 90°, 123° and 143°, are analysed. The large-scale reconstructed monochromatic images of the Ha, Hβ, [OIII] and [NII] lines show a bipolar morphology along PA ∼ 123°. The prominent elliptical ring is seen in the Ha and Hβ monochromatic images. Our analysis implies that this ring is rotating in the same clockwise direction as that of the galactic disc; this elliptical ring is in fact a projected result of a circular ring of radius ∼5.4 arcsec, with i ∼ 50° (nearly aligned with the host galactic plane). We used the [O III]5007 line profiles for a detailed analysis of kinematics in the circumnuclear region of the NGC 5728. The [O III]5007 line profiles reveal distinct kinematical structures. (1) The strongest component of Doppler peaks: two core components, consisting of the receding main C-core and the approaching NW-core with the radial velocities, V r ≃ +250 and -240 (or -250) km s -1 , respectively. These two cores consist of a roughly spherically symmetric distribution of the gas with radii of r ∼ 1.0 arcsec (at V r ≃ +240 km s -1 ) and 0.9 arcsec (at V r ≃ -240 km s -1 ), respectively. Hints of the presence of inflows around these cores are observed. (2) The next strong line profile component: this component corresponds to the bipolar cone aligned PA ∼ 123°. (3) The reverse S-shaped kinematical boundary which runs through the double-peaked zone of 6-7 arcsec long in the galactic major axis direction is likely to be caused by a combination of the disc rotation of the host galaxy and the revolution of the two central cores.