PHYSICAL CONDITIONS IN THE INNER NARROW-LINE REGION OF THE SEYFERT 2 GALAXY MARKARIAN 573

Abstract

We have examined the physical conditions within a bright emission-line knot\nin the inner narrow-line region (NLR) of the Seyfert 2 galaxy Mrk 573 using\noptical spectra and photoionization models. The spectra were obtained with the\nHubble Space Telescope/Space Telescope Imaging Spectrograph with the G430L and\nG750M gratings. Comparing the spatial emission-line profiles, we found [Fe X]\n6734 barely resolved, [O III] 5007 centrally peaked, but broader than [Fe X],\nand [O II] 3727 the most extended. The spectra reveal that [Fe X] is broader in\nvelocity width and blue-shifted compared with lines from less ionized species.\nOur estimate of the bolometric luminosity indicates that the active galactic\nnucleus (AGN) is radiating at or above its Eddington Luminosity, which is\nconsistent with its identification as a hidden Narrow-Line Seyfert 1. We were\nable to successfully match the observed emission line ratios with a\nthree-component photoionization model. Two components, one to account for the\n[O III] emission and another in which the [Fe X] arises, are directly ionized\nby the AGN, while [O II] forms in a third component, which is ionized by a\nheavily absorbed continuum. Based on our assumed ionizing continuum and the\nmodel parameters, we determined that the two directly-ionized components are ~\n55 pc from the AGN. We have found similar radial distances for the central\nknots in the Seyfert 2 galaxies Mrk 3 and NGC 1068, but much smaller radial\ndistances for the inner NLR in the Seyfert 1 galaxies NGC 4151 and NGC 5548.\nAlthough in general agreement with the unified model, these results suggest\nthat the obscuring material in Seyfert galaxies extends out to at least tens of\nparsecs from the AGN.\n