Galaxies did not exist at the very beginning. The early universe was a nearly smooth hot gas. Tiny differences in density, visible today in the cosmic microwave background, grew over hundreds of millions of years into the galaxies we see.
Dark matter first
According to the standard model of cosmology, dark matter clumped first, because it does not interact with light and was not pushed apart by radiation. These clumps, called halos, attracted ordinary gas. The gas cooled, sank to the center and formed the first stars and small galaxies, see what dark matter is.
Growing by mergers
Galaxies grow in two ways:
- Accreting gas from the cosmic web, the network of filaments connecting galaxies.
- Merging: small galaxies combine into larger ones. This “hierarchical” growth continues today. The Milky Way is absorbing small satellite galaxies, and it may one day merge with Andromeda, although recent studies suggest that collision is less certain than once thought.
Major mergers between similar-sized spirals can scramble their disks into an elliptical galaxy and feed the central black hole, briefly creating a quasar, see quasars.
Why some galaxies stop forming stars
Many massive galaxies are “red and dead”: they stopped forming stars billions of years ago. Astronomers call this quenching. Likely causes include energy from the central black hole heating or expelling gas, gas being stripped as galaxies fall into clusters, and gas supplies simply running out.
Looking back in time
Light from distant galaxies left them long ago, so deep images show galaxies as they were in the past. Studying galaxies at many distances lets astronomers reconstruct their evolution:
- The Hubble Deep Fields revealed thousands of small, irregular young galaxies.
- The James Webb Space Telescope, observing in infrared, has found galaxies from less than 400 million years after the Big Bang. Some are brighter and more numerous than models predicted, which is prompting astronomers to refine their theories, see the role of the James Webb Space Telescope.
- Star formation across the universe peaked around 10 billion years ago and has been declining since.
Tools of the trade
Astronomers combine observations at many wavelengths, spectroscopy to measure distances and chemical makeup, and large computer simulations of the universe, see spectral lines. For the types of galaxies these processes produce, see the different types of galaxies.
