  Galaxies evolve continuously under the influence of self-gravity, rotation,
accretion, mergers and feedback. The currently favored cold dark matter
cosmological framework, suggests a hierarchical process of galaxy formation,
wherein the present properties of galaxies are decided by their individual
histories of being assembled from smaller pieces. However, recent studies have
uncovered surprising correlations among the properties of galaxies, to the
extent of forming a one-parameter set lying on a single fundamental line. It
has been argued in the literature that such simplicity is hard to explain
within the paradigm of hierarchical galaxy mergers. One of the puzzling
results, is the simple linear correlation between the neutral hydrogen mass and
the surface area, implying that widely different galaxies share very similar
neutral hydrogen surface densities. In this work we show that self-regulated
star formation, driven by the competition between gravitational instabilities
and mechanical feedback from supernovae, can explain the nearly constant
neutral hydrogen surface density across galaxies. We therefore recover the
simple scaling relation observed between the neutral hydrogen mass and surface
area. This result furthers our understanding of the surprising simplicity in
the observed properties of diverse galaxies.
