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2nd of May 2016
Coupling between baryons and dark matter in galaxies
by Paolo Serra (CASS)
The mass budget of the Universe is dominated by a dark matter of unknown nature. An important clue about the properties of this dark matter is that, within galaxies, its spatial distribution is coupled to that of baryons. So far this coupling has been observed in spiral and dwarf irregular galaxies. In a new study now accepted for publication P. Serra and collaborators show for the first time that the same holds also in ellipticals and lenticulars.

The key observational result is a tight, linear correlation between the velocity dispersion of the stars and the circular velocity of the neutral hydrogen gas in these galaxies (see figure). The stellar velocity dispersion is a probe of the distribution of mass in the central regions of galaxies, where baryons dominate the potential. The neutral hydrogen circular velocity is a probe of the distribution of mass at very large radius, where dark matter dominates. The fact that the two quantities are so tightly correlated (12% observed scatter!) implies a strong coupling between the two types of matter. Unlike other galaxy scaling relations, the one shown here is relatively free of systematics as it is independent of distance and stellar mass-to-light ratio. For this reason, it can be used as a strong test of galaxy formation models.

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