From Ethan Siegel: “Why do the tiniest galaxies have the most dark matter?”

Starts with a bang
Starts with a Bang

Ethan Siegel

Dark matter halo  Image credit: Virgo consortium / A. Amblard / ESA
Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
Dark matter cosmic web and the large-scale structure it forms. The Millenium Simulation, V. Springel et al

Everything else has a 5:1 dark matter-to-normal matter ratio. But get a smaller and smaller galaxy, and dark matter skyrockets!

“For the moment we might very well can them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere).” –Bill Bryson

When we look out at the Universe, in any direction and with a variety of methods, we find the same ratios of dark matter to normal matter all over the place: 5-to-1. Whether we’re looking at the fluctuations in the cosmic microwave background, the lensing-to-X-ray ratios of colliding clusters, the way large-scale structure clumps together or the rotation properties of the largest spiral and elliptical galaxies, that same ratio — of dark matter outmassing normal matter by a 5-to-1 ratio — exists everywhere.

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).

NASA/Chandra Telescope
NASA/Chandra Telescope

CFHT Telescope, Mauna Kea, Hawaii, USA
CFHT Interior
CFHT Telescope, Mauna Kea, Hawaii, USA

NASA/ESA Hubble Telescope
NASA/ESA Hubble Telescope

ESA/XMM Newton
ESA/XMM Newton

Everywhere, that is, until you start looking at the tiniest galaxies in the Universe. All the way down to Milky Way-sized galaxies, which represents the vast majority of galaxies we’ve discovered in the Universe, that 5-to-1 ratio remains constant. But when you go to smaller galaxies, down to dwarf galaxies in clusters or ultra-low-mass galaxies visible only in our local group (due to their tiny light output), you find that the less mass there is overall, the greater the dark matter fraction is.

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

In other words, the lower in mass your galaxy is, the smaller the percentage of stars and normal matter you’ll find inside, and the more dominated by dark matter it will turn out to be! This might seem paradoxical, since gravity affects both normal and dark matter equally. When you start from an overdense region, whether it’s a tiny one that grows into a miniature galaxy or a giant one that grows into a supermassive cluster, it should attract normal and dark matter equally.

But if we think about it a little bit deeper — and consider the following two pictures — it might start to make sense why dark matter comes to dominate the tiniest galaxies. It isn’t because these little ones start out with more dark matter; initially, they have that same 5-to-1 ratio that everything does. But because their gravitational pull is so weak, they have a very difficult time holding on to their matter. Unfortunately for normal matter, it interacts with both light and with other normal matter, making it incredibly easy to strip away.

Messier 82 Cigar starburst galaxy
Image credit: NASA, ESA, The Hubble Heritage Team, (STScI / AURA); acknowledgement: M. Mountain (STScI), P. Puxley (NSF), J. Gallagher (U. Wisconsin), of the starburst galaxy Messier 82, with matter being expelled as shown by the red jets.

When you get a large burst of star formation, you create intense, ultraviolet radiation. When the most massive stars die, they create bursts of supernovae, which ionize matter and accelerate it to near-relativistic speeds. And when you funnel matter into a black hole, it can cause jets, which eject matter into the intergalactic medium. All of these factors are at play in all galaxies, and yet these matter-ejecting effects only touch the normal matter. Because dark matter is transparent to all electromagnetic phenomena, only the normal matter gets ejected whenever you have a star-formation, stellar-death or black-hole-infalling event. On the other hand, these effects simply pass through the dark matter, and so it remains in these low-mass galaxies.

Image credit: NASA, ESA Acknowledgements: Ming Sun (UAH), and Serge Meunier, of spiral galaxy ESO 137–001 having its normal matter stripped away as it speeds through the intracluster medium.

This discrepancy is compounded when you have a galaxy inside of a large cluster. The intergalactic medium there is dense and full of matter, and when these galaxies pass through, they do so at high speeds. Just as a strong wind can easily blow the loosely-held seeds off of a dandelion, the intra-cluster medium easily blows the normal matter off of the smaller galaxies in the Universe, leaving only the dark matter behind.

Take all of these effects into account, and you’ll find that the smaller and lower-mass your galaxy is, the more tenuously the normal matter is held onto in the first place, making the dark matter-to-normal matter ratio that much larger. For the smallest mini-galaxies in the Universe, ratios in the thousands-to-one are common, while if you come up to Milky Way-sized galaxies, you’re back to the 5-to-1 ratio that everything else in the Universe holds to. Everything might be born with the same ratio of dark matter to normal matter, but it’s only the big winners that hang onto their normal matter for long!

See the full article here .

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan