Ionized gas and radio emission in the barred Seyfert galaxy NGC 5728

This paper presents new, highly detailed optical and radio observations of the barred Seyfert 2 galaxy NGC 5728. Broad-band CCD images reveal the presence of blue stars in a bright central ring which lies off-center with respect to the nucleus. Imaging Fabry-Perot observations of the Hα emission in the central regions show, in addition to a bright nucleus, a distinctive arc of ionized gas which is approximately coincident with the blue stellar ring. A radial velocity map derived from these data confirms the presence of large-scale deviations from simple circular motion. In addition to S-shaped twists in the isovelocity contours at radii of 5"-10", we detect a region of gas within 3" of the nucleus characterized by double-peaked emission line profiles, with a separation of ~400 km s^-1^. Long-slit spectra show that the excitation of the gas in the inner regions is generally quite high, indicating that the dominant source of ionization is nonstellar radiation from the Seyfert nucleus with some local contribution from the stars in the blue stellar ring. High-resolution radio maps reveal a morphology in the central regions which is strikingly similar to that of the Hα emission. These observations support a scenario where the active nature of NGC 5728 has arisen primarily from the inflow of gas into the nuclear region due to the perturbing influence of the nonaxisymmetric bar potential. Such a model can explain, at least qualitatively, the noncircular motions of the ionized gas. Gas streaming into the inner regions along straight shocks in the bar may account for the enhanced star formation observed in the central ring, or this feature may be related to an inner Lindblad resonance. Supernova remnants produced in a "starburst" may, in turn, explain the extended radio emission apparently associated with the ring. Nevertheless, we cannot rule out the possibility that the dynamics of the ionized gas in NGC 5728 are, at least in part, the result of the observed nuclear activity, rather than the source. This alternative hypothesis is suggested by the existence of the double emission line region near the nucleus, the jetlike extensions of radio emission, and a striking asymmetry in the ionization of the gas in the central regions.