Showing papers by "Stuart D. Ryder published in 1996"

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.

Magnetic fields and hot gas in the spiral galaxy ngc 1566 as derived from atca radio polarization and rosat x-ray observations

Abstract: NGC 1566, a southern galaxy with an exceptionally regular spiral pattern, was observed in radio continuum at λ3.5, 6.2, 12.7 and 21.7 cm wavelengths with the Australia Telescope Compact Array (ATCA), and in the optical Hα line with the MSSSO 1.0-m telescope. The total radio emission has a smooth nonthermal component and a thermal component associated with Hα emission. Even at λ3.5 cm the thermal emission dominates only in the western spiral arm. The nonthermal intensities require an equipartition magnetic field of 13 ±2 μG average strength, and ≃ 15-20 μG in spiral arms. For the first time linearly polarized radio emission has been detected in NGC 1566. At λ6.2 cm the highest polarized intensities with a degree of polarization p≃ 30% emerge from interarm regions, possibly a result of magnetic field compression between two spiral arms. No such phenomenon has been observed for any other spiral galaxy. As Faraday rotation is low, the B-vectors at λ6.2 cm indicate the orientations of the magnetic field lines which almost perfectly follow the optical spiral structure. No systematic variations of the field pitch angles from spiral arm to interarm regions were found, in contrast to density-wave models. The polarized emission at λ12.7 cm is 2.5-10 times lower than at λ6.2 cm. If this is due to internal Faraday dispersion in the halo of NGC 1566, we would need electron densities of ne ≳ 0.03 cm. We observed NGC 1566 in X-rays with the Position Sensitive Proportional Counter (PSPC) aboard the Rontgen Satellit (ROSAT). The bright nuclear region with a luminosity of L(0.1 -2.4 keV)= 1.0 · 10 erg s can best be fitted with a power-law spectral model of index 2.3 thai is in good agreement with the mean index for Seyfert type I galaxies. Besides the nuclear source we detected point sources in the spiral arms of NGC 1566 and extended emission (L = 1.4 · 10 erg s) surrounding the nucleus. The extent of the radio continuum emission is similar to that of the extended X-ray emission, suggesting a link between hot gas and magnetic field. Assuming that the extended soft X-ray emission is due to a hot gaseous component, we derive an electron density ≦ 1.0 · 10/ √η cm with an unknown volume filling factory η. This is too small to explain the observed depolarization at λ12.7 cm, indicating that the hot gas is far from radiative equilibrium, or that another component of ionized gas with lower temperature exists in the halo.

Neutral Gas in Resonance-Ring Galaxies

Abstract: We have mapped the distribution and kinematics of H I in NGC 1433 and NGC 6300 with the Australia Telescope Compact Array, in order to test the association between resonances and ring phenomena. We find a bar pattern speed in NGC 1433 of ftp = 26 ± 5 km s kpc 1 , and postulate that NGC 6300 is still evolving to a mature resonance-ring galaxy.

NGC 7421: Surfing the Intracluster Medium?

Abstract: NGC 7421 is a barred spiral displaced from the centre of its optical envelope, with a 'bow-shock'-like western boundary, both suggestive of interaction with an intracluster medium. We have used five configurations of the ATCA to map the distribution and kinematics of HI in NGC 7421, and find supportive evidence for this scenario in the form of an HI 'wake'. When compared with ROSAT PSPC observations of the diffuse hot gas in the surrounding group of galaxies, these new ATCA results will allow us to place new constraints on the density and dynamics of the intracluster medium.