2 research outputs found
Artillery Shells over Circinus
The recently identified Circinus Galaxy is the nearest (about 4 Mpc) Seyfert
2 galaxy known and we now demonstrate to be one of the best laboratories for
studying the effects of nuclear activity on the surrounding environment. Here
we present new imaging Fabry-Perot observations of Circinus which confirm the
existence of an ionization cone in this object but also show for the first time
a complex of ionized filaments extending radially from the nucleus out to
distances of 1 kpc. Arcs suggestive of bow shocks are observed at the terminus
of some of these filamentary structures. Most spectacular of all, one of the
structures appears to be a scaled-up version of a Herbig-Haro jet. The velocity
field of the filaments confirms that they represent material expelled from the
nucleus (possibly in the form of `bullets') or entrained in a wide-angle wind
roughly aligned with the polar axis of the galaxy. The motions observed across
the ionization cone are highly supersonic, so high-velocity shocks are likely
to contribute to the ionization of the line emitting gas. However, it is not
clear at present whether shock ionization dominates over photoionization by the
Seyfert 2 nucleus. Extrapolation of the filaments to smaller radii comes to
within 1 arcsec (about 20 pc) of the infrared nucleus, therefore suggesting a
AGN or nuclear starburst origin to these features. The complex of radial
filaments detected in the Circinus galaxy is unique among active galaxies. The
frequency of such events is unknown since only a handful of galaxies have been
observed at the sensitivity level of our present observations. The event in the
Circinus galaxy may represent a relatively common evolutionary phase in the
lives of gas-rich active galaxies during which the dusty cocoon surrounding the
nucleus is expelled by the action of jet or wind phenomena.Comment: 22 pages including 5 figures, Latex, requires aasms4.sty. To appear
in ApJ Letters 479 (1997