What is a galaxy?
A galaxy is a discrete group of stars bound together by gravity (dominated by the dark matter clump in which they live) that is able to form stars over long time periods. While the first stars formed in similar environments to galaxies – dark matter clumps – their star formation was a fleeting event, as the light and supernovae generated by the stars themselves were easily able to expel gas from the clumps shortly after star formation began. Galaxies require much stronger gravity – and hence more mass – to remain stable to this kind of feedback.
Image: The Andromeda galaxy, the most massive in the Local Group of galaxies (which encompasses our own Milky Way). This galaxy is much bigger than the first structures!
Credit: JPL, Caltech, NASA Galaxy Evolution Explorer
How did the matter clumps hosting the first stars grow and evolve?
Dark matter clumps are bound by their gravity, which is also strong enough to attract more material from their surroundings. While the stars themselves live only a few million years before ending their lives, this accretion of dark matter is an ongoing process, and the clumps grow rapidly over time. Eventually, a clump accumulates enough dark matter that it is able to retain gas even in the presence of disruptive feedback generated by stars (such as supernovae). At this point, the clump is able to form stars relatively steadily, and we can call it a galaxy.
Image: Cassiopeia A is the debris field left behind after a massive star exploded.
Credit: Chandra X-Ray Observatory
What were the first galaxies like?
The first galaxies have not yet been observed, and the transitionary period during which long-lived star formation becomes possible is of exceptional interest to astronomers. But we can make some educated guesses about many of their properties. Most importantly, they were small – likely at least thousands of times less massive than our own Galaxy, the Milky Way. The galaxies were also much more “primitive” in their composition than modern galaxies. In large galaxies, repeated generations of stars processed gas into heavy elements. But the very first galaxies are closer to primordial composition – the mixture of hydrogen and helium generated early in the Universe’s history – because they have not produced enough stars to create heavier elements. There are many other differences as well: these galaxies are denser, evolve much more quickly, and may have larger stars than nearby galaxies.
Image: In this Hubble Space Telescope image, a firestorm of star birth is lighting up one end of the dwarf galaxy Kiso 5639.
Credit: NASA, ESA, HST
How common were these tiny galaxies?
Depending on your perspective, the first galaxies were both rare and very common. They were rare in that they only contained a small fraction – less than 1% - of all the matter in the Universe. But, because they were so small, there were nevertheless many of them. For comparison, our Milky Way is a very average galaxy, and today similar galaxies have a typical separation of about 10 million light years. During the Cosmic Dawn, the faintest galaxies so far observed are about ten times more abundant than Milky Way-like systems are today. However, because nearly all of the galaxies are so small, systems as large as the Milky Way were billions of times less common! Because they were so much more abundant, most of the star formation occurred in these tiny systems – and most of the light comes from them too, even though the individual galaxies are too small to be detected directly by our telescopes.
Image: Hubble Ultra Deep Field. Among the 10,000 or so galaxies pictured here are newborns, adolescents, adults, and retirees.
Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team
