Fundamental Relationships in Galactic Disks

Stuart D. Ryder, PASA, 14 (2), in press.

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Title/Abstract Page: Fundamental Relationships in Galactic
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Contents Page: Volume 14, Number 2


We have demonstrated that the following three (azimuthally-averaged) observational parameters are correlated at each radius in the disks of nearby spiral galaxies:

  1. tex2html_wrap_inline404 - the surface density of old stars;
  2. tex2html_wrap_inline406 - the recent formation rate of massive stars per unit area; and
  3. tex2html_wrap_inline408 - the mean gas-phase abundance of oxygen in HII regions, the current sites of star formation,
such that:
where tex2html_wrap_inline410 is the HI mass per unit area of the star-forming disk. As a consequence of these new observational relationships, and our attempts to interpret them using galactic evolution models, two important results have emerged:
  1. The return of oxygen to the ISM by dying low-mass stars late in the galaxy's lifetime can make a significant contribution to the gas-phase oxygen abundance at late epochs.
  2. The ``classical'' Schmidt Law for star formation cannot simultaneously satisfy both the (tex2html_wrap_inline412) and the (tex2html_wrap_inline414) relationships; some dependence on the total mass surface density (or some other quantity that scales with it) is also required.
It should be emphasised that these results have only been shown to hold in the disks of spiral galaxies; they break down in the bulge region (perhaps due to the different dynamical environment there), and thus may not necessarily lead to a global relationship. While we may never come up with a fundamental relationship for spiral galaxies as conceptually simple as the Hertzsprung-Russell diagram for stars, the relationships presented here could well form the basis of a ``Fundamental Plane'' for galactic disks.

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