Galaxies appear simpler than expected

• Published in: Nature Vol. 455; no. 7216; pp. 1082 - 1084
• Main Authors: Disney, M. J., Romano, J. D., Garcia–Appadoo, D. A., West, A. A., Dalcanton, J. J., Cortese, L.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23.10.2008; Nature Publishing; Nature Publishing Group
• Subjects: Acquisitions; Astronomy; Classification and classification systems; Cold dark matter; Complex systems; Dynamical systems; Earth, ocean, space; Enrichment; Exact sciences and technology; Explosions; Galaxies; Galaxy formation; Humanities and Social Sciences; letter; Mathematical models; multidisciplinary; Normal galaxies. Extragalactic objects and systems (by type); Science; Science (multidisciplinary); Star & galaxy formation; Stars & galaxies; Stellar systems. Galactic and extragalactic objects and systems. The universe; Variables; Supernovae explosion; Galaxies; Complex system; Baryons; Chemical enrichment; Neutral hydrogen; Radio emission; Correlation analysis; Feedback; Galaxy formation; Cold dark matter; Correlations; Angular momentum; Models; Observables; Age; Stellar dynamics
• ISSN: 0028-0836; 1476-4687; 1476-4687; 1476-4679
• DOI: 10.1038/nature07366

Galactic structure: a simple answer At first glance a galaxy looks pretty complicated: each contains billions of stars subject to many influences including gravity, rotation and radiation. The hierarchical theory of galaxy formation — currently fashionable — holds that they are assembled from smaller pieces, through many mergers of cold dark matter, and that the properties of an individual galaxy are determined by six independent parameters. Now a survey of 200 individual galaxies suggests that things are simpler than that, and that galactic structure is controlled by one single parameter. Though frustratingly, it is not yet clear which parameter that is. This level of organization is at odds with hierarchical galaxy formation and the predictions of cold dark matter cosmology. This paper reports that a sample of galaxies (first detected from neutral hydrogen emission) shows five independent correlations amongst six independent observables. This implies that the structure of such galaxies must be controlled by a single parameter, which cannot yet be identified. Such a degree of organization is at odds with hierarchical galaxy formation. 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 1 . The hierarchical theory of galaxy formation holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter 2 , 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 effects 5 , 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 cosmology 6 .