Showing papers by "Joss Bland-Hawthorn published in 1997"

Artillery Shells over Circinus

Abstract: The recently identified Circinus galaxy is the nearest (~4 Mpc) Seyfert 2 galaxy known, and we now demonstrate it 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 that 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'' (~20 pc) of the infrared nucleus, therefore suggesting an active galactic nucleus or nuclear starburst origin for 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.

Velocity-resolved Fabry-Pérot imaging of molecular hydrogen emission in OMC-1

Abstract: We present a velocity-resolved data cube of the distribution of the V= 1-0 S(1) line of molecular hydrogen from the source OMC-1. With", 1.5-arcsec spatial and 14 km S-1 velocity resolution, obtained with a scanning Fabry-Perot interferometer, it provides the most extensive data base yet obtained on both the distribution and the dynamics of the shocked molecular gas in this source. The line profiles are broad and generally smooth allover the source, with total extent (FWZI) up to '" 150 km S-1. We have identified, however, two distinct components to the line profiles which have different spatial distributions. Over the entire source, strong emission from a central velocity component is present, close to the ambient cloud velocity, while emission from high-velocity components (both red and blueshifted) is confined to discrete condensations. The central velocity component peaks, on average, at VLSR '" + 12 km s-\ with an average FWHM of '" 37 km s-\ but shows subtle variation across the source. In particular, the strongest emission region, Peak 1 to the north-west, is both slightly blueshifted and broader than the secondary Peak 2 to the south-east of OMC-1. We interpret this emission as the result of an isotropic steady wind from the IRc2 complex being loosely collimated by a disc, so that it flows close to the plane of the sky but with Peak 1 pointing slightly towards us and interacting with ambient molecular gas to excite the molecular hydrogen emission. In addition, high-velocity emission components (centred at '" - 35 km S-1 and + 40 km S-1 with FWHM of '" 30 km S-1) are found in discrete locations, primarily along an emission ridge running north along the Peak 1 region and towards the circumstellar disc about the IRc2 complex. We develop an analytical shock model to demonstrate that these discrete emission knots closely resemble the emission expected from partially resolved bow shocks. We ascribe these features to additional 'bullets' to those identified by Allen & Burton in [Fe II] at greater distances from the source, but which have been revealed by their interaction with dense, ambient molecular gas. They likely originate from a temporally limited event ~ 1000 yr ago. We provide new estimates on the energetics and the momenta of all the bullets, and speculate on their origin.

TTF survey of galaxy populations

Abstract: The TAURUS Tunable Filter (TTF) affords a new approach to observational cosmology, allowing a wide field (10 arcmin) to be imaged monochromatically in contiguous wavelength intervals (6 - 60 Angstrom bandpass) over the R and I bands. In a 200 s exposure at the AAT, the TTF can detect H-alpha emission powered by star formation rates as low as 0.1 solar mass per year at z = 0.08 and 1 solar mass per year at z = 0.24 in 2 arcsec seeing (cf. 0.26 solar mass per year for the LMC). In this paper we describe an emission-line survey currently underway using the TTF on the AAT to detect redshifted H-alpha over the ranges z = 0.06 - 0.1 and z = 0.22 - 0.26. Such detections will be of timely interest to the Southern HI Sky Survey which is motivated along similar lines.

Artillery Shells over Circinus

Abstract: 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.