Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
How we know some galaxies have more than others.
“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
One of the biggest surprises about galaxies in our Universe is that stars only make up a tiny fraction of their mass.
Traceable stars, neutral gas, and (even farther out) globular clusters all point to the existence of dark matter, which has mass but exists in a large, diffuse halo well beyond the normal matter’s location. Image credit: Wikimedia Commons user Stefania.deluca.
Looking to gas, dust, plasma, black holes and other non-luminous forms fails to account for what’s missing.
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).
For that, you need dark matter, which has mass but is completely invisible to all non-gravitational interactions.
Dark matter, in a 5:1 ratio to normal matter, accounts for everything from the formation of the largest cosmic structures to galaxies’ internal motions to the fluctuation patterns in the cosmic microwave background.
The fluctuations across the entire sky in the cosmic microwave background, the Big Bang’s leftover glow. Image credit: ESA and the Planck collaboration.
Present in a large, diffuse halo surrounding galaxies and clusters, its gravity is observable even when collisions separate out the normal matter.
Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. 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).
The smallest galaxies are richer in dark matter, as episodes of star formation expel the normal matter.
Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. In low-mass galaxies, this material easily escapes the galaxy’s gravitational pull. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA), of the Cigar Galaxy, Messier 82.
What’s left behind is mostly dark.
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.
The lowest-mass galaxy known, Segue 3, has 600 times more dark matter than normal matter.
Only approximately 1000 stars are present in the entirety of dwarf galaxy Segue 3, which has a gravitational mass of 600,000 Suns. Image credit: Marla Geha and Keck Observatories, of the stars making up the dwarf satellite Segue 1.
But large galaxies can lose their normal matter too, by speeding through the intergalactic medium.
A Hubble (visible light) and Chandra (X-ray) composite of galaxy ESO 137–001 as it speeds through the intergalactic medium, becoming stripped of stars and gas, while its dark matter remains intact. Image credit: NASA, ESA, CXC.
“Dark galaxy” Dragonfly 44, with the same number of stars as the Large Magellanic Cloud but the mass of the Milky Way, as imaged by SDSS (L) and Gemini (R). Image credit: Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey.
If their normal matter was stripped away, perhaps these “dark galaxies” weren’t always so dark.
Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.
