Galaxies appear simpler than expected

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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 holes1. The hierarchical theory of galaxy formation holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter2,3,4. 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 from optical selection effects5, 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 data set. Such a degree of organization appears to be at odds with hierarchical galaxy formation, a central tenet of the cold dark matter model in cosmology6.

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Figure 1: Scatter plots showing correlations between five measured variables, not including colour.
Figure 2: Scatter plots showing correlations between the five measured variables and the principal components.
Figure 3: Scatter plots showing correlations between the previous five measured variables (Fig. 1), but now including colour.
Figure 4: Scatter plots showing correlations between all the six measured variables (including colour) and the principal components.


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We would like to thank the HIPASS team, and especially R. Ekers, A. Wright and L. Staveley-Smith of the Australian National Telescope at CSIRO Radiophysics in Sydney for their foresight and enterprise in getting the Multibeam project started. M.J.D. would like to thank M. Disney of the Geography Department at University College London for first pointing out the one-dimensional nature of this data.

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Correspondence to M. J. Disney.

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Disney, M., Romano, J., Garcia–Appadoo, D. et al. Galaxies appear simpler than expected. Nature 455, 1082–1084 (2008) doi:10.1038/nature07366

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