Anisotropic ionizing radiation in Seyfert galaxies. I - The extended narrow-line region in Markarian 573

Abstract

Narrow-band images and a grid of long-slit spectra are used to study the morphology, kinematics and ionization state of the nuclear extended narrow-line region (ENLR) of the Seyfert 2 galaxy Mrk 573. The entire ENLR is mapped spectroscopically, and velocity structure is studied. The velocity field map shows a typical galactic rotation picture with some important deviations. A simple geometric model, in accordance with the "unified schemes," is used to study effects of various parameters on the observed picture. The best match is achieved when a biconical radiation field illuminates the ISM of the host galaxy that takes part in a normal galaxy rotation but also has radial motions close to the nucleus. The emission-line images reveal a ENLR elongated along the radio axis in the northwest-southeast direction, but a map of the flux ratio R = [O III] λ5007/(Hα + [N II]) shows a different structure, with the highest excitation peak offset by 4" along the radio axis to the southeast. Several line diagnostic diagrams are produced from the grid of spectra and are used to study the ionization mechanism and angular dependence of the ionizing field. Observed line ratios are compared with photoionization models. It is shown that an accretion disk or hot blackbody (T_BB_ > 150,000 K) incident spectrum produces a better fit to the line ratios than a single power-law spectrum. The general trend of the line ratios requires that the ionization parameter U initially decreases with radial distance up to approximately 2" (~1 kpc) and then increases further outward, the latter effect probably implying that the density is decreasing faster than r^-2^. The [S II) electron density is used in conjunction with U derived from the photoionization models to estimate the angular dependence of the ionizing flux. It is shown that the number of ionizing photons is higher along the axis than in a perpendicular direction, again suggesting the presence of an anisotropic, biconical, shaped ionizing field as in the unified model of AGNs.