Dwarf galaxies are different beasts. They are diminutive and dim, sometimes only a diffuse gaggle of stars.

The stars are enveloped in a big blob of dark matter

While dwarf galaxies can be isolated, floating in space alone, they are also thought to hide around their bigger, more majestic counterparts, orbiting them like moons around planets.

The Milky Way, for instance, has nearly 50 such companions. Also called satellite galaxies, they are relatively tiny, weighing roughly 10,000 times less than the Milky Way, with some containing only a few thousand stars.

Like in all galaxies, the stars are enveloped in a big blob of dark matter, the unknown stuff that comprises around a quarter of the Universe. The dark matter accounts for most of the dwarf galaxy’s mass, forming the gravitational glue that binds the galaxy together.

The biggest of the Milky Way’s satellites are the Large and Small Magellanic Clouds, which appear as two fuzzy patches to the naked eye. The other satellite galaxies, however, are small and faint, detectable only with telescopes.

Astronomers found a handful by the mid-20th Century, but the majority remained unseen until the last decade, when modern astronomical surveys discovered dozens.

Cold dark matter fundamentally is a theory that says little things formed first

This was a remarkable revelation. In the late 1990s, before so many were found, astronomers were in a crisis, Treu says. Computer simulations were revealing a glaring discrepancy between how many satellite galaxies astronomers saw and what was predicted by their prized theory of cosmology: cold dark matter (CDM).

Dark matter, the theory posits, is cold: its particles move around space slowly (in this case, much slower than light). Because they were not zipping around too fast in the early Universe, their mutual gravity could corral them into small, dense clumps. These clumps eventually attracted the gas needed to form stars and become dwarf galaxies. Over time, these dwarfs merged together and became big galaxies like the Milky Way.

“Cold dark matter fundamentally is a theory that says little things formed first,” says James Bullock, an astrophysicist at the University of California, Irvine. “They merged together to form bigger things over time.”

Source: BBC – Earth – The hunt for invisible dwarf galaxies

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