New Lessons from the HI Size-Mass Relation of Galaxies
  We revisit the HI size-mass (D$_{\rm HI}$-M$_{\rm HI}$) relation of galaxies
with a sample of more than 500 nearby galaxies covering over five orders of
magnitude in HI mass and more than ten $B$-band magnitudes. The relation is
remarkably tight with a scatter $\sigma \sim$0.06 dex, or 14%. The scatter does
not change as a function of galaxy luminosity, HI richness or morphological
type. The relation is linked to the fact that dwarf and spiral galaxies have a
homogenous radial profile of HI surface density in the outer regions when the
radius is normalised by D$_{\rm HI}$. The early-type disk galaxies typically
have shallower HI radial profiles, indicating a different gas accretion
history. We argue that the process of atomic-to-molecular gas conversion or
star formation cannot explain the tightness of the D$_{\rm HI}$-M$_{\rm HI}$
relation. This simple relation puts strong constraints on simulation models for
galaxy formation.
