LARGER AND
LARGER GALAXIES
The first galaxies were more than one thousand times smaller than the Milky Way. But these galaxies formed stars hundreds of times faster than the Milky Way does, and over a few hundred million years the galaxies could grow hundreds of times larger. It is inside these galaxies that the transition to “normal” star formation occurred. The light and supernovae produced by these stars made these early galaxies violent places, driving massive “winds” into their surroundings and possibly growing supermassive black holes inside them. By about a billion years after the Big Bang, the largest of these galaxies exceeded the mass of the Milky Way.
This ultraviolet image from NASA Galaxy Evolution Explorer is of the interacting group of galaxies known as Stephan Quintet.
Image Credit: NASA/JPL-Caltech/SSC
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How do these galaxies grow over time?

The simplest way to view a galaxy is as a “machine” that transforms incoming matter into stars. Gas flows onto the galaxy because of the dark matter’s gravity. As it accumulates, the gas fragments into clouds, which eventually form stars. But shortly after star formation begins, supernova explosions and other forms of feedback help stir up the clouds, eventually destroying them. Meanwhile, other clouds are collapsing out of the galaxy’s gas and forming more stars. In “normal” galaxies, a balance occurs so that clouds form stars just fast enough so that the resulting supernovae and other feedback can support the remainder of the gas (although other processes may contribute as well).

But the feedback from these supernovae has another important effect, in addition to regulating the amount of star formation: it can be so strong that it ejects some of the gas entirely out of the galaxy! This removes the fuel for future star formation, reducing the overall rate at which stars can form. These “winds” of ejected gas have the most dramatic effects in small galaxies, because their gravity is weaker and cannot retain the gas as effectively. Thus, we expect the efficiency with which stars form to increase as galaxies grow.

What do we know about these galaxies already?

The Hubble Space Telescope has imaged hundreds of galaxies from the first billion years of the Universe’s history – but they are, for the most part, at the limit of its capability. Many of these galaxies came from a set of programs observing the Ultra-Deep Field – a small patch of sky (about one-tenth the diameter of the full Moon) that the Hubble telescope has observed for more than 23 days! With such a long exposure, the image captures the faintest objects ever observed. In this, and other fields with extensive observations, astronomers have measured the abundances of some of the larger galaxies during the first billion years of cosmic history.

These galaxies are so faint that their detailed properties are hard to study – astronomers hope to learn much more with future telescopes. But, during this early time period, two things are already clear: galaxies are smaller and growing more rapidly than those around us today. Structures, including dark matter clumps, grow from small to large over time, so it is not surprising that galaxies do as well. During the first billion years, an average galaxy was more than 100 times smaller than the Milky Way, the galaxy in which we live. Larger galaxies were very, very rare. However, these small galaxies were also growing much more rapidly than their counterparts today, mostly because the Universe was denser and expanding more rapidly at that time. The rapid growth meant that gas was also falling into these galaxies at a much larger rate (nearly 100 times as rapidly as today, on average), so there was much more fuel available to form stars.

Image: Galaxy MACS2129-1 seen by NASA's Hubble Space Telescope, the first example of a compact yet massive, fast-spinning, disk-shaped galaxy.
Credit: NASA, ESA, M. Postman (STScI), and the CLASH team

What kind of stars and other matter did these larger galaxies contain?

All galaxies have three basic components: the dark matter whose gravity binds them together, stars, and an interstellar medium of gas and dust, which fills the space between the stars (and from which the stars form). Early galaxies had the same basic components, but with a few key differences, including:

  • Irregular Structure: Almost all large galaxies today have one of two shapes: disks (which often form beautiful spiral patterns) or ellipticals. But early galaxies were growing so rapidly that they had not yet settled into these configurations. Instead, they were irregular galaxies, much more “messy” than their modern counterparts.

Image: Hubble Ultra Deep Field 2009-2010, showing one of the farthest and eariest galaxies in the universe.
Credit: NASA, ESA, G. Illingworth, R. Bouwens (UCSC) and the HUDF09 Team

  • Simple Composition: The Big Bang created a Universe with a very simple elemental composition: hydrogen and helium. Stars are responsible for all of the other elements in the Universe, but it takes time for stars to make them. Galaxies during the Cosmic Dawn have experienced many fewer generations of stars, so their gas is much less enriched with these heavy elements.

Image: Farthest confirmed galaxy of 2017, and one of the brightest and most massive sources at that time.
Credit: NASA, ESA, P. Oesch (Yale U.)

  • Activity: The Universe is evolving rapidly during the Cosmic Dawn, and so are galaxies. Early galaxies are much denser than those around us today, which means gravity – the ultimate driver of star formation – acts more rapidly. These galaxies form stars more quickly, and that likely means that they are also more “bursty,” with their star formation rates varying rapidly (at least by astronomical time scales).

Image: The most active star-making galaxy in the very distant universe.
Credit: NASA/JPL-Caltech/Subaru/STScI

What are some signatures of new astrophysical phenomena in these galaxies?

So far, it is difficult to study these early galaxies in detail because they are so faint. While the Hubble Space Telescope has found many of them, their detailed properties remain largely unknown. Astronomers hope that future facilities, like the James Webb Space Telescope, will probe them in more detail. In particular, is there evidence that these early galaxies have different properties than later generations? One possibility is to search for “primordial stars” – though the very first such stars formed in tiny systems beyond the reach of planned telescopes, some of the gas raining onto later galaxies may still have primordial composition. If stars form quickly enough, these exotic stars may continue to form even in large systems, and we might be able to detect their presence by looking at the colors of light generated by the galaxies. Another key question is how the “burstiness” of these galaxies affects their growth. Does star formation occur so violently that the resulting supernovae tear apart their gas reservoirs? Or does it occur in isolated clumps? Only future observations can answer these questions.

Image: Hubble Ultra Deep Field.
Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

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