Starts With A Bang

Why Do The Tiniest Galaxies Have The Most Dark Matter? (Synopsis)

Image credit: The Millenium Simulation, V. Springel et al., of the cosmic web of dark matter and the large-scale structure it forms.

“Men occasionally stumble over the truth, but most of them pick themselves up and hurry off as if nothing ever happened.” -Winston Churchill

When we look out at the Universe on the largest scales, from large-scale structure to the fluctuations in the CMB to lensed clusters of galaxies and to giant spirals and ellipticals, we find the same thing everywhere we look: dark matter outmasses normal matter by a 5-to-1 ratio. It’s a finding that’s independent of direction, scale or distance.

Images credit: X-ray: NASA/ CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right). These four separate groups and clusters all show the separation between dark matter (blue) and normal matter (pink).

But when we go smaller, to tiny dwarf galaxies, we find that dark matter plays an even greater role, outmassing normal matter by factors of dozens, hundreds or even thousands-to-one. The tiniest galaxies somehow have the most dark matter, with the smallest galaxies of all containing only a few hundred low-mass stars yet nearly a million solar masses of dark matter. Yet this is no mystery; the physics of how normal matter gets ejected by star formation and other electromagnetic violence while dark matter is unaffected explains it all!

Image credit: ESA/Hubble & NASA, of dwarf galaxy NGC 5477.

Go get the whole story over on Forbes.