Gas follows stars in resonance-ring galaxies
NGC 1433 and NGC 6300 are two of the best examples of the ringed barred
galaxies. Previously inaccessible to aperture synthesis telescopes, they
have now been studied in HI with the Australia Telescope Compact Array.
The results have allowed us to determine, for both, the pattern speed of
the bar: a parameter fundamental for simulations of galaxy dynamics but
still poorly constrained.
About two thirds of disk galaxies are barred (SB) or weakly barred (SAB)
systems. Many of these galaxies have one or more ring-like features in their
stellar distributions. The rings fall into three main categories: nuclear
rings, inner rings encircling the bar, and outer or pseudo-rings. All three
types are thought to arise from the galaxy's internal dynamics (unlike the
rarer polar rings and collisional rings). Each one is produced by a resonance
between the speed at which the bar potential rotates and the period of small
epicyclic (radial) oscillations of stars and gas clouds at that ring's radius.
Thus, the location and dynamics of these resonance rings can be used to
determine the pattern speed of the bar.
The southern spiral NGC 1433 is the prototypical ringed barred galaxy:
it has all three of the main ring types. It has been well studied, yet its
dynamics have not been completely analysed. NGC 6300 is almost identical
in angular size and distance, but differs markedly in its features: it has
no nuclear or outer pseudo-ring and its inner ring is exceptionally broad
and dusty.
The optical and HI images of NGC 1433 and NGC 6300 are shown below.
The stellar and gas rings are strikingly similar: the gas clearly feels
the same resonant effects as the stars, accumulating near the rings and
being swept out of other regions. Although predicted, this effect could
not be confirmed before the advent of the AT Compact Array.
The bar pattern speed for both galaxies, derived by linear perturbation
analysis, is 20-30 km s-1 kpc-1.
S. Ryder (UNSW);
R. Buta (Uni. Alabama); L.
Staveley-Smith, W. Walsh
(ATNF)
Comparison of blue-light images (top, obtained with the CTIO 1.5-m
telescope) with the surface density of neutral hydrogen (bottom, from ATCA
observations) for NGC 1433 (left) and NGC 6300 (right). A
colour composite of this data for NGC 1433 is available.
