It’s not the only one of its kind, but it’s definitely makes no sense without dark matter.
Throughout the Universe, galaxies and star clusters come in all different sizes and masses.
While the brightest ones are always the easiest to see, the faint ones are far greater in number.
Even within our Local Group, with just 2 or 3 large galaxies, some 60 dwarf galaxies and hundreds of globular clusters abound.
Only the stars emit visible light, but all forms of matter exert their gravity.
The smallest independent structures known today are Segue 1 and Segue 3.
Both of these are small, dwarf galaxies, occupying space in the Milky Way’s galactic halo.
They’re both incredibly intrinsically faint: only a few hundred times as luminous as the Sun.
With no more than 1000 stars apiece, spread out over just a few light-years in space, they’re the most extreme galaxies known today.
But we can track the motions of the individual stars inside, inferring each galaxy’s total mass.
Segue 1 holds the extreme record, requiring 600,000 solar masses total for just ~175 solar masses in stars.
Star formation can eject the excess normal matter, explaining why there’s no gas seen inside.
All that gravitation must come from somewhere. Only the dark matter explanation fits perfectly.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.