Stuart D. Ryder, PASA, 14 (2), in press.
| Next Section: A Relationship Between Stellar Title/Abstract Page: Fundamental Relationships in Galactic Previous Section: Fundamental Relationships in Galactic | Contents Page: Volume 14, Number 2 |
Introduction
Fundamental relationships between observable parameters in astrophysical objects can serve as powerful tools for helping us interpret the universe around us. One of the most familiar examples of such a relationship is the Hertzsprung-Russell diagram, which relates a star's luminosity with its temperature (as determined from its colour/spectral type). Once it was realised that the H-R diagram charted a star's evolutionary path, astronomers were able to utilise it for deriving other fundamental quantities, such as the ages and distances of star clusters.
The search for a simple galactic analog of the H-R diagram has not been quite so fruitful. For elliptical galaxies, the central velocity dispersion, the effective radius (or alternatively, the total luminosity), and the mean surface brightness are all related, and define what is commonly referred to as the ``Fundamental Plane'' (Djorgovski & Davis 1987; Jørgensen, Franx, & Kjærgaard 1996). For dwarf galaxies, correlations have been claimed between surface brightness and luminosity, colour and luminosity, metallicity and luminosity, and thus, metallicity and surface brightness, but it is not yet clear which of these (if any) is the truly fundamental correlation, and which may be just a selection effect (Edmunds & Phillipps 1989). In the case of spiral galaxies, initial hopes that the Hubble type of a galaxy would prove to be tightly correlated with other parameters have not panned out. There is of course the Tully-Fisher relation between a spiral galaxy's luminosity and its maximum rotational velocity. However, in view of the unknown role played by dark matter in maintaining this rotational velocity at large radii, the T-F relation is really only of limited use in analysing the evolution of spiral galaxies.
Assuming such a fundamental relationship exists and can be found for spiral galaxies, it would:
- Argue for a common origin, and provide clues to the formation process.
- Serve as an empirical endpoint for galaxy evolution models, and thereby help to constrain the plethora of star formation prescriptions, Initial Mass Functions, gas exchange processes, etc.
- Provide a useful (relative) distance indicator.
- Define truly abnormal galaxies as those which depart significantly from the mean trend.
| Next Section: A Relationship Between Stellar Title/Abstract Page: Fundamental Relationships in Galactic Previous Section: Fundamental Relationships in Galactic | Contents Page: Volume 14, Number 2 |
© Copyright Astronomical Society of Australia 1997