Starts With A Bang

Dark Matter Riches? Why Some Galaxies Have More Than Others (Synopsis)

An image of galaxy Dragonfly 44, recently discovered to have the largest offset between normal matter and dark matter of any known, large galaxy. Image credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA.

“Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there.” -Pieter van Dokkum

Everything in the Universe was born with the same ratios of dark matter to normal matter: approximately 5-to-1. It shows up in everything from the cosmic microwave background to galaxy clustering to internal motions of spiral and elliptical galaxies, including the Milky Way.

From simulations and inferred maps, dark matter (blue) may form some clumps, but overall exists in a massive, diffuse halo around the luminous, disk-like part of galaxies we’re familiar with. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).

But some galaxies, particularly dwarf galaxies and rapidly moving galaxies within clusters, show a significant enhancement of dark matter over what’s expected. Ratios of even hundreds-to-1 aren’t unheard of. This is a difficulty for theorists to explain, but it doesn’t mean that dark matter is wrong.

Dwarf galaxies, like the one imaged here, have a much greater than 5-to-1 dark matter to normal matter ratio, as bursts of star formation have expelled much of the normal matter. Image credit: ESO / Digitized Sky Survey 2.

On the contrary, it might mean that astrophysics, and how these galaxies evolve, explains exactly what the Universe gives us!