  Galaxies are complex systems the evolution of which apparently results from
the interplay of dynamics, star formation, chemical enrichment, and feedback
from supernova explosions and supermassive black holes. The hierarchical theory
of galaxy formation holds that galaxies are assembled from smaller pieces,
through numerous mergers of cold dark matter. The properties of an individual
galaxy should be controlled by six independent parameters including mass,
angular-momentum, baryon-fraction, age and size, as well as by the accidents of
its recent haphazard merger history. Here we report that a sample of galaxies
that were first detected through their neutral hydrogen radio-frequency
emission, and are thus free of optical selection effects, shows five
independent correlations among six independent observables, despite having a
wide range of properties. This implies that the structure of these galaxies
must be controlled by a single parameter, although we cannot identify this
parameter from our dataset. Such a degree of organisation appears to be at odds
with hierarchical galaxy formation, a central tenet of the cold dark matter
paradigm in cosmology.
