Dwarf Galaxies

Dwarf galaxies are small galaxies containing anywhere from a few million to a few billion stars. Compare that to spiral galaxies like the Milky Way with 200 to 400 billion stars, and it’s clear why they’re called dwarfs. Despite their modest stellar populations, dwarf galaxies vastly outnumber their larger cousins and play crucial roles in galaxy evolution and cosmology.

The Local Group contains about 80 known galaxies, and the vast majority are dwarfs. Only three are large spirals - the Milky Way, Andromeda, and Triangulum. The rest are dwarf spheroidals, dwarf irregulars, and dwarf ellipticals orbiting the larger galaxies like stellar satellites. Many more probably remain undiscovered, too faint and diffuse to detect against the background sky.

Dwarf spheroidal galaxies are the most common type in the Local Group. They’re roughly spherical, contain old stars, and show little ongoing star formation. Sculptor and Fornax are examples orbiting the Milky Way. These galaxies are extremely faint - their surface brightness is so low they’re nearly invisible. Sculptor has about 4 million stars spread across a volume larger than most globular clusters, giving it an average density lower than the solar neighborhood.

The faintness of dwarf spheroidals raises a puzzle. How did they form and survive? They contain far more dark matter than normal matter - mass-to-light ratios reaching 100 or more times the sun. This dark matter provides the gravitational binding preventing tidal forces from the parent galaxy from ripping them apart. Without it, they’d have been shredded long ago.

Dwarf irregular galaxies are more active. The Magellanic Clouds orbiting the Milky Way are the most famous examples. The Large Magellanic Cloud contains about 30 billion stars with significant ongoing star formation. The Small Magellanic Cloud has roughly 3 billion stars. Both show irregular structures shaped by tidal interactions with the Milky Way. The Magellanic Stream, a ribbon of hydrogen gas trailing behind them, was stripped away by these gravitational encounters.

Ultra-faint dwarf galaxies push the definition of galaxy to its limits. These systems contain just thousands to hundreds of thousands of stars - comparable to a globular cluster. But their large physical sizes and extremely high dark matter content mark them as true galaxies rather than clusters. Segue 1 has fewer than 1,000 stars spread across 300 light-years, making it one of the faintest galaxies known.

BTW dwarf galaxies are dominated by dark matter even more than large galaxies. Some have mass-to-light ratios exceeding 1,000, meaning they’re 99.9 percent dark matter. This makes them excellent laboratories for studying dark matter properties. If dark matter particles can annihilate and produce gamma rays, ultra-faint dwarfs should be detectable sources - though no clear signal has been found yet.

Dwarf galaxies are building blocks of larger galaxies. Hierarchical structure formation in the standard cosmological model predicts that large galaxies assemble through mergers of smaller systems. The Milky Way shows evidence of having consumed dozens of dwarf galaxies over its history. Stellar streams - tidal debris from disrupted dwarfs - crisscross the galactic halo, fossil records of past cannibalism.

The missing satellites problem challenges cosmological models. Simulations predict hundreds of dark matter halos massive enough to host dwarf galaxies should orbit the Milky Way, but only about 60 are observed. Either something prevents star formation in most halos, or our understanding of dark matter needs revision. Reionization after the first stars formed may have heated gas and prevented it from cooling and forming stars in small halos.

Dwarf starburst galaxies undergo intense episodes of star formation despite their small sizes. NGC 1569 is forming stars at a rate that would consume its entire gas supply in just 100 million years. Supernovae from these massive stars blast metal-enriched gas into intergalactic space, possibly stripping the galaxy of future star-forming material. These bursts may be triggered by interactions with other galaxies or by gas falling in from the intergalactic medium.

Dwarf galaxies preserve information about the early universe. Their low masses mean they’ve experienced less chemical enrichment than large galaxies. Some contain very metal-poor stars formed from nearly primordial gas. Studying these ancient stellar populations reveals conditions in the early universe when the first galaxies were forming, just hundreds of millions of years after the Big Bang.