Figure 8

Figure 8: Size and surface density. The characteristic surface density of baryons as defined in Figure 5 is plotted against their dynamical scale length Rp 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 R p 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 Σ (R) = Σ0e −R∕Rd. 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 Σ † ≈ a0∕G [299, 77].