HST Observations of the Seyfert Nucleus of the Barred Spiral Galaxy NGC 5728

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Session 4 -- AGNs
Display presentation, Monday, 9:20-6:30, Pauley Room

[4.07] HST Observations of the Seyfert Nucleus of the Barred Spiral Galaxy NGC 5728

J.A. Braatz (UMd), A.S. Wilson (UMd/STScI)

Both continuum and line emission were observed in the nucleus of the barred Seyfert~2 galaxy NGC 5728 using the WFPC of the Hubble Space Telescope. Integrations of 1200 seconds were taken through the F664N and F492M filters to observe the [NII]+H$\alpha$ and [OIII] emission, respectively, and 600 second exposures were taken through the F718M and F547M filters to get continuum reference images. The continuum images show interesting structure, including an elliptical ring (8$^{\prime \prime}$ $\times$ 10$^{\prime \prime}$) which surrounds the nucleus and a bar-shaped feature which extends from the ring in towards the nucleus. The line images both reveal a bi-conical emission line region indicative of an anisotropic source of ionizing radiation. The apex of these cones is the most probable location of the central engine, and hence the ``true'' nucleus. This location of the nucleus is supported by the observation that the ring seen in the continuum images is centered on the apex of the cones, and not on the brightest peak of the optical emission. Only very weak-line and continuum emission is found at the apex, implying the nucleus is blocked from view. Recent observations of other Seyfert 2 nuclei suggest that the ionization cones may be produced by shadowing of the nuclear radiation source by an optically thick molecular torus. The torus blocks the Seyfert~2 nucleus from our view, but nuclear radiation which escapes through the holes of the torus can still be scattered into our line of sight by dust or free electrons. In fact, bright peaks which may result from this scattering appear in the continuum images. The orientation of the cones can be inferred by noting that the ionization cone to the SE of the nucleus has a greater extent and brightness than the NW cone. This difference is probably due to extinction by dust in the outskirts of the torus or in the disk of the galaxy, and in this case the brighter side is the near side. A similar difference in brightness is evident in the two peaks associated with the scattering zones on the continuum images.

Monday program listing