Figure 8: Size and surface density. The characteristic surface density of baryons as defined in
Figure 5 is plotted against their dynamical scale length in the left panel. The dark-blue
points are star-dominated galaxies and the light-blue ones gas-dominated. High characteristic
surface densities at low in the left panel are typical of bulge-dominated galaxies. The
stellar disk component of most spiral galaxies is well approximated by the exponential disk with
. This disk-only central surface density and the exponential scale length of the
stellar disk are plotted in the right panel. Galaxies exist over a wide range in both size and
surface density. There is a maximum surface density threshold (sometimes referred to as Freeman’s
limit) above which disks become very rare [264]. This is presumably a stability effect, as purely
Newtonian disks are unstable [343, 415]. Stable disks only appear below a critical surface density
[299, 77].
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