Hubble Space Telescope imaging of the circinus galaxy

Abstract

We present a Hubble Space Telescope imaging study of the nearby (4 Mpc distant) Circinus galaxy, which contains the nearest type 2 Seyfert nucleus and prominent circumnuclear star formation. Images have been obtained in the [O III] λ5007, Hα, and H2v=1−0 S(1) emission lines, and in the green (5470 Ŏ ), red (8140 Ŏ ), and near-infrared (2.04 and 2.15 μm) continua. An image in the [Fe II] λ1.644 μm line has been taken with a ground-based telescope. The [O III] and Hα images reveal the detailed structure of the complex of streamers and knots of high-excitation gas, which extends out of the galaxy disk. The morphology some 250 pc from the nucleus strongly suggests that the high-excitation gas is concentrated on the surface of a hollow cone with apex close to the nucleus. Such a structure may result through entrainment of dense gas from a circumnuclear torus in the galaxy disk by a low-density, outÑowing wind or jet. Within 40 pc of the nucleus, the high-excitation gas takes the form of a striking, filled V-shaped structure extending in the same direction as the larger scale high-excitation emission. This V can be described as an ionization cone, though a matter-bounded structure is also possible. The implied collimation of the ionizing photons or gaseous outÑow must occur within 2 pc of the apex of the cone, presumed to be the location of the nucleus. The Hα image shows a complex structure of H II regions, including the well-known starburst ring of radius 150−270 pc. In addition, there is a more compact (40 pc radius), elliptical ring of H II regions around the ionization cone. We argue that this latter ring, which we call the nuclear ring, is intrinsically circular and located in the plane of the galaxy disk. Much of the [Fe II] emission is associated with this nuclear, star-forming ring and is presumably powered by supernova remnants. Hot molecular hydrogen extends to within 10 pc of the nucleus, and possibly closer. The intrinsic infrared−optical continuum colors in the inner regions of the Circinus galaxy are, in many locations, bluer than is typical of bulges, indicating a relatively young stellar population is present. We confirm the presence of a compact (<2 pc), very red nuclear source in the K band. Its properties are consistent with a type 1 Seyfert nucleus viewed through an obscuration of AV=28±7 mag.