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Session 84 - Dwarf Galaxies.
Oral session, Wednesday, January 15
I discuss the properties of a sample of 19 late-type Low Surface Brightness (LSB) disk galaxies. These have exponential disks with scalelengths of a few kpc and central surface brightnesses \mu_0 \simeq 23.5 B-mag arcsec^-2.
LSB galaxies are extended, low density, dark-matter-dominated, slowly evolving galaxies, that are still in an earlier stage of galaxy evolution than the ``normal'' High Surface Brightness (HSB) galaxies. Colours are blue, metallicities are low, they have high gas fractions and only small amounts of star formation. 21-cm synthesis observations show that HI surface densities are \sim 3 times lower than in late-type HSB galaxies and usually below the critical density for star formation (Kennicutt (1989)). Despite this the gas disks are so extended that LSB galaxies still rank among the most gas-rich galaxies. The gas is dynamically at least as important as the stellar component.
Despite the different evolutionary scenarios LSB galaxies lie on the same Tully-Fisher relation as HSB galaxies. This implies a systematic relation between surface brightness and dynamical mass-to-light ratio, with LSB galaxies having the highest ratios. The Tully-Fisher relation and HI rotation curves of LSB galaxies both show that LSB galaxies are very dark-matter dominated with respect to HSB galaxies. Mass models derived from the rotation curves of LSB and HSB galaxies show that LSB galaxies inhabit less dense and more extended halos. They have dynamical masses comparable to those of HSB galaxies.
The fact that LSB galaxies do have luminosities and masses comparable to those of HSB galaxies, evolve much slower, and are found in low density regions of the general galaxy distribution, shows that galaxy evolution does not just depend on mass (observed as luminosity), but just as much on density (observed as surface brightness) which might also be related to the environment where a galaxy is formed.
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