Universality of galactic surface densities
You will find an intriguing paper on today’s issue of Nature (subscription required):
It was recently discovered that the mean dark matter surface density within one dark halo scale-length (the radius within which the volume density profile of dark matter remains approximately flat) is constant across a wide range of galaxies. This scaling relation holds for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. Here we report that the luminous matter surface density is also constant within one scale-length of the dark halo. This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale-length it is constant. Our finding can be interpreted as a close correlation between the enclosed surface densities of luminous and dark matter in galaxies.
See also the Editor’s Summary.
As noted by the authors:
A large central luminous density thus implies a large core radius, and in turn a small central dark matter density. This precise balance must be the result of some unknown, fine-tuned process in galaxy formation, because it is a priori difficult to envisage how such relations between dark and baryonic galaxy parameters can be achieved across galaxies that have experienced significantly different evolutionary histories, including numbers of mergers, baryon cooling or feedback from supernova-driven winds.
Update: I thought it would be interesting to point to a previous work of mine and collaborators (back from 2003) which shows that the central dark matter halo densities for a large data sample ranging from dwarf ellipticals to clusters of galaxies, based on the application of the two-component virial theorem (2VT) to these systems, do not show universality. Only the abstract is available:
Title: The case against scale-invariant central halo densities: implications for the self-interacting dark matter scenarios in the context of the two-component virial theorem
Authors: Ribeiro, A. L. B.; Dantas, C. C.; Capelato, H. V.; Carvalho, R. R.
Publication: Boletim da Sociedade Astronômica Brasileira (ISSN 0101-3440), vol.23, no.1, p. 163-163
I will attempt to find the poster PDF and make it opportunely available here.
More on the 2VT can be found here:
Title: The Two-Component Virial Theorem and the Physical Properties of Stellar Systems
Authors: Dantas, Christine C.; Ribeiro, André L. B.; Capelato, Hugo V.; de Carvalho, Reinaldo R.
Publication: The Astrophysical Journal, Volume 528, Issue 1, pp. L5-L8.
Update 2: Interesting discussions here.