Showing papers by "Lister Staveley-Smith published in 1996"

The Parkes 21 cm Multibeam Receiver

Abstract: Several extragalactic HI surveys using a λ21 cm 13-beam focal plane array will begin in early 1997 using the Parkes 64 m telescope. These surveys are designed to detect efficiently nearby galaxies that have failed to be identified optically because of low optical surface brightness or high optical extinction. We discuss scientific and technical aspects of the multibeam receiver, including astronomical objectives, feed, receiver and correlator design and data acquisition. A comparison with other telescopes shows that the Parkes multibeam receiver has significant speed advantages for any large-area λ21 cm galaxy survey in the velocity range range 0–14000 km s−1.

An approach to interferometric mosaicing

Abstract: In radio astronomy, mosaicing is the practice of combining multiple pointings of a telescope to produce an image significantly larger than the telescope's primary beam. We consider an approach to imaging and deconvolving interferometric mosaic observations. This approach directly deconvolves a linear mosaic of dirty images. We consider the advantages and limitations of this technique. Projection issues for mosaicing are also discussed. We present an example from a 320-pointing, spectral-line, mosaiced observation of the Small Magellanic Cloud.

Neutral Hydrogen in the Ringed Barred Galaxies NGC 1433 and NGC 6300

Abstract: We have made observations of the \ion{H}{1} in the southern ringed barred spiral galaxies NGC~1433 and NGC~6300 with the Australia Telescope Compact Array (ATCA), the main goal being to test the resonance theory for the origin of these rings. NGC~1433 is the prototypical ringed barred spiral, and displays distinct \ion{H}{1}~counterparts to its nuclear ring, inner ring, outer pseudoring, and plume-like features. The $L_{4}$ and $L_{5}$ regions at corotation, as well as the bar itself, are relatively devoid of neutral atomic hydrogen. By associating the inner ring of NGC~1433 with the inner second harmonic resonance, and its outer pseudoring with the outer Lindblad resonance, we are able to infer a bar pattern speed for NGC~1433 of $26\pm5$~km~s$^{-1}$~kpc$^{-1}$. By way of contrast, NGC~6300 possesses a much more extended \ion{H}{1}~disk than NGC~1433. There is a gas ring underlying the inner pseudoring, but it is both broader and slightly larger in diameter than the optical feature. By again linking this inner ring feature to the inner second harmonic resonance, we derive a bar pattern speed for NGC~6300 of $27\pm8$~km~s$^{-1}$~kpc$^{-1}$, but in this case, neither the outer pseudoring nor the nuclear ring predicted by the resonance-ring theory can be identified in NGC~6300. Although it may be the case that the ring in NGC~6300 is not related to a resonance with the bar at all, we postulate instead that NGC~6300 is merely a less well-developed example of a resonance-ring galaxy than is NGC~1433.

Radio Emission from SN 1987A

Abstract: We review the first six years of radio observations of Supernova 1987A. The evolution can be divided into two phases: the initial radio outburst which lasted a few weeks, and the period from mid-1990 to the present, during which the radio emission has steadily increased. Both phases can be explained by a small fraction (0.1-0.5%) of the post-shock thermal energy being converted to energy in relativistic particles and magnetic fields, which give rise to synchrotron radiation. The optical depths, densities and density profiles for the pre-shocked circumstellar material are somewhat different for the two phases, but consistent with models of the density structure of the material within the circumstellar ring. New high-resolution radio observations show that the SN shock front is already at about three-quarters of the radius of the circumstellar ring, and that there exists a bright equatorial component of emission aligned with this ring which is probably due to a polar density gradient in the ‘hourglass’ structure.

The asymmetric radio remnant of SN 1987A

Abstract: We present seven years of radio observations of SN 1987A made with the Australia Telescope Compact Array. At 1.4, 2.4, 4.8 and 8.6 GHz, the flux density of the radio remnant has increased monotonically since emission was redetected 1200 days after the explosion. On day 3200, the remnant was expanding at 2800 +/- 400 km/s, which we interpret as indicating significant deceleration of the fastest moving ejecta. Since day 1787 the spectral index has remained constant at alpha = -0.95 +/- 0.04. These observations are all consistent with the shock having encountered a denser, shocked, component of the progenitor's stellar wind. At the current rate of expansion, the shock is expected to encounter the inner optical ring in the year 2006 +/- 3. Using super-resolution, we have also obtained 9 GHz images of the remnant (resolution approx 0".5) at four epochs. The emission is distributed around the rim of a near-circular shell, but has become increasingly asymmetric with time. There are two "hotspots" to the east and west, aligned along the major axis of the optical ring. This morphology is most likely indicative of an axisymmetric circumstellar medium into which the shock is expanding, consistent with present understanding of the progenitor star and its environment.