We present an HST 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 [ OIII ] \lambda 5007 , H \alpha and H _ { 2 } v=1–0 S ( 1 ) emission lines , and in the green ( 5470 Å ) , red ( 8140 Å ) and near infrared ( 2.04 and 2.15 \mu m ) continua .
An image in the [ Fe II ] \lambda 1.644 \mu m line has been taken with a ground-based telescope .
The [ OIII ] and H \alpha 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 , outflowing 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 outflow must occur within 2 pc of the apex of the cone , presumed to be the location of the nucleus .

The H \alpha image shows a complex structure of HII 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 HII 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 A _ { V } = 28 \pm 7 mag .