New lessons from the H I size-mass relation of galaxies

Abstract

We revisit the H I size-mass (D_{H I}-M<SUB>H I</SUB>) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in H I mass and more than 10 B-band magnitudes. The relation is remarkably tight with a scatter σ ∼ 0.06 dex, or 14 per cent. The scatter does not change as a function of galaxy luminosity, H I richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogeneous radial profile of H I surface density in the outer regions when the radius is normalized by D<SUB>H I</SUB>. The early-type disc galaxies typically have shallower H I 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<SUB>H I</SUB>-M<SUB>H I</SUB> relation. This simple relation puts strong constraints on simulation models for galaxy formation.