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Starts With A Bang

What Was It Like When Galaxies Formed The Greatest Number Of Stars?

For more than 10 billion years, the star-formation rate across the Universe has been plummeting. Here’s the story.

Take a look at a wide variety of galaxies in the Universe, and you’ll find a vastly different set of stories. The largest ones are giant ellipticals, many of which haven’t formed any new stars over the latter half of our entire cosmic history. Many spiral galaxies are like our own Milky Way, with a small number of regions forming new stars, but where the overall galaxy is largely quiet. And a few galaxies are undergoing rapid, intense periods of star-formation, from interacting spirals that are littered with millions of new stars to irregular starburst galaxies, where the entire galaxy transforms into a star-forming region.

But on average, the rates of new star formation today are the lowest they’ve been since the extreme early stages of the Universe. The majority of stars in the Universe formed only in the first 1-to-3 billion years, and the star-formation rate has plummeted ever since. Here’s the cosmic story behind it.

A Hubble/Spitzer composite image of galaxy cluster SpARCS1049+56 shows how a gas-rich merger (center) can trigger the formation of new stars. (NASA/STSCI/ESA/JPL-CALTECH/MCGILL)

In the early days of the Universe, matter is far denser than it is today. There’s a very simple reason for this: there’s a fixed amount of material in the observable Universe, but the fabric of space itself is expanding. So you’d expect, when the Universe was younger, that there’d be more star formation, since more matter would be closer together to clump and form stars.

But also in the early days, the Universe was more uniform. At the moment of the hot Big Bang, the densest regions of all were only about 0.01% denser than a typical, average region, and so it takes a long time for those overdense regions to grow and collect enough matter to form stars, galaxies, and even larger structures. Early on, you have factors working both for you and against you.

Galaxies that are currently undergoing gravitational interactions or mergers are almost always also forming new, bright, blue stars. Simple collapse is the way to form stars at first, but most of the star formation we see today results from a more violent process. The irregular or perturbed shapes of such galaxies are a key signature that this is what’s occurring. (NASA, ESA, P. OESCH (UNIVERSITY OF GENEVA), AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES))

The way you form stars is pretty straightforward: get a large amount of mass together in the same spot, let it cool and collapse, and you get a new star-forming region. Often, a large, external trigger, like tidal forces from a large, nearby mass or rapidly-ejected material from a supernova or gamma-ray burst, can cause this type of collapse and new star-formation as well.

We see this in the nearby Universe, both in regions within a galaxy, like the Tarantula Nebula in the Large Magellanic Cloud, as well as on the scales of entire galaxies themselves, like in Messier 82 (the Cigar galaxy), which is being gravitationally influenced by its neighbor, Messier 81.

The starburst galaxy Messier 82, with matter being expelled as shown by the red jets, has had this wave of current star-formation triggered by a close gravitational interaction with its neighbor, the bright spiral galaxy Messier 81. (NASA, ESA, THE HUBBLE HERITAGE TEAM, (STSCI / AURA); ACKNOWLEDGEMENT: M. MOUNTAIN (STSCI), P. PUXLEY (NSF), J. GALLAGHER (U. WISCONSIN))

But the greatest trigger for star-formation of all is during what astronomers call a major merger. When two comparably galaxies collide and merge together, a huge wave of star-formation can envelop the entire galaxy, causing what we call a starburst. These are the largest instances of star-formation in the Universe, and some of them are occurring even today.

Does that mean that star-formation is continuing to occur at the same rates, or near them, as at its peak? Hardly. Most of these major merger are already far in the rear-view mirror of the Universe’s history. The expansion of the Universe is a relentless phenomenon, just like gravitation. The problem is that there’s a competition going on, and gravitation lost a long time ago.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy fights against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it. (E. SIEGEL / BEYOND THE GALAXY)

If the Universe were made 100% of matter, and the initial expansion rate and the matter density balanced one another perfectly, we’d live in a Universe that would always have major mergers in its future. There would be no limit to the size of the large-scale structure that formed:

  • star clusters would merge into proto-galaxies,
  • proto-galaxies would merge into young, small galaxies,
  • those galaxies would merge into the large spirals we have today,
  • spirals would merge together to form giant ellipticals,
  • spirals and ellipticals would fall into clusters,
  • clusters would collide and form superclusters,
  • and superclusters themselves would form together, leading to megaclusters,

and so on. As time continued to pass, there would be no limit to the scale at which the cosmic web grew and grew.

The cosmic web of dark matter and the large-scale structure it forms. Normal matter is present, but is only 1/6th of the total matter. The other 5/6ths is dark matter, and no amount of normal matter will get rid of that. If there were no dark energy in the Universe, structure would continue to grow-and-grow on larger-and-larger scales as time went on. (THE MILLENIUM SIMULATION, V. SPRINGEL ET AL.)

Unfortunately, for all you fans of new stars, that isn’t our Universe. Our Universe has far less matter than that, and most of the matter we do have isn’t star-forming material at all, but rather some form of dark matter. In addition, most of the Universe’s energy comes in the form of dark energy, which only serves to drive the unbound structures apart.

As a result, we don’t get any large-scale structures that are bound beyond the sizes of galaxy clusters. Sure, some galaxy clusters will merge together, but there’s no such thing as a supercluster; those apparent structures are mere phantasms, to be destroyed as the Universe continues to expand.

The Laniakea supercluster, containing the Milky Way (red dot), on the outskirts of the Virgo Cluster (large white collection near the Milky Way). Despite the deceptive looks of the image, this isn’t a real structure, as dark energy will drive most of these clumps apart, fragmenting them as time goes on. (TULLY, R. B., COURTOIS, H., HOFFMAN, Y & POMARÈDE, D. NATURE 513, 71–73 (2014))

Given the Universe we have, then, what does our star-formation history look like? The first stars form after perhaps 50–100 million years, when the small-scale molecular clouds can accrue enough matter to collapse. By time the Universe is around 200–250 million years old, the first star clusters have merged together, triggering a new wave of star-formation and forming the earliest galaxies. By time the Universe is 400–500 million years old, the largest galaxies have already grown to a few billion solar masses: around 1% the mass of the Milky Way.

A little bit later than this, the first galaxy clusters start to form, major mergers become common, and the cosmic web starts to get more and more dense. For the first 2-to-3 billion years of the Universe, the star formation rate only continues to rise.

A stellar nursery in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. This new, nearby sign of star-formation may seem ubiquitous, but the rate at which new stars forms today, across the entire Universe, is only a few percent of what it was at its early peak.(NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA)-ESA/HUBBLE COLLABORATION)

This rise, however, doesn’t continue beyond this point. After about 3 billion years of age, the star-formation rate begins to drop, and drops precipitously and continuously thereafter.

What’s going on to cause that?

A number of factors, all working in tandem. Stars form out of (mostly) hydrogen and helium gas, which collapse and ignite nuclear fusion. This fusion increases the internal pressure, working to expel much of the potentially star-forming material. As galaxies clump together to form groups and clusters, the gravitational potential gets greater, but the intergalactic medium also gets more material inside it. This means, as galaxies speed through denser regions of space, much of this potentially star-forming material gets stripped away.

One of the fastest known galaxies in the Universe, speeding through its cluster (and being stripped of its gas) at a few percent the speed of light: thousands of km/s. Trails of stars form in its wake, while the dark matter continues on with the original galaxy. (NASA, ESA, JEAN-PAUL KNEIB (LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE) ET AL.)

Additionally, more and more of the material found in these galaxies is processed as time goes on: filled with heavier and heavier elements. In a new study by UC Riverside scientists, they found that the older a star-forming galaxy is, the slower it forms stars.

Using some of their own newly discovered SpARCS clusters, the new UCR-led study discovered that it takes a galaxy longer to stop forming stars as the universe gets older: only 1.1 billion years when the universe was young (4 billion years old), 1.3 billion years when the universe is middle-aged (6 billion years old), and 5 billion years in the present-day universe.

In other words, new stars form at a faster rate early on, and at a slower rate today. Add in dark energy, which restricts additional structure from forming, and you’ve got a recipe for a very quiet Universe.

The Pandora Cluster, known formally as Abell 2744, is a cosmic smash-up of four independent galaxy clusters, all brought together under the irresistible force of gravity. Thousands of galaxies may be evident here, but the Universe itself contains perhaps two trillion of them. (NASA, ESA, AND J. LOTZ, M. MOUNTAIN, A. KOEKEMOER, & THE HFF TEAM)

Let’s put it all together, now. Early on, there was plenty of pristine (or more pristine) material, and many more mergers of comparably-sized galaxies occurring. When large galaxies merged in clusters, they were first forming clusters back then, meaning there was less mass-stripping and more starbursting when galaxies interacted. And even though galaxies are larger today than they were back then, they were still substantial after a few billion years, and mergers were far more common.

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All told, according to the most comprehensive studies ever undertaken, the star-formation rate has declined by a whopping 97% since its maximum, 11 billion years ago.

The star-formation rate peaked when the Universe was approximately 2.5 billion years old, and has been declining ever since. In the recent past, the star formation rate has actually plummeted, corresponding to the onset of dark energy dominance. (D. SOBRAL ET AL. (2013), MNRAS 428, 2, 1128–1146)

The star-formation rate declined slowly and steadily for a few billion years, corresponding to an epoch where the Universe was still matter-dominated, just consisting of more processed and aged material. There were fewer mergers by number, but this was partially compensated for by the fact that larger structures were merging, leading to larger regions where stars formed.

But right around 6-to-8 billion years of age, the effects of dark energy began to make their presence known on the star formation rate, causing it to plummet precipitously. If we want to see the largest bursts of star formation, we have no choice but to look far away. The ultra-distant Universe is where star formation was at its maximum, not locally.

Hubble’s advanced camera for surveys identified a number of ultra-distant galaxy clusters. If dark energy is a cosmological constant, all of these clusters will remain gravitationally bound themselves, like all galaxy groups and clusters, but will accelerate away from us and one another over time as dark energy continues to dominate the Universe’s expansion. These ultra-distant clusters display star-formation rates far greater than the clusters we observe today. (NASA, ESA, J. BLAKESLEE, M. POSTMAN AND G. MILEY / STSCI)

As long as there is gas remaining in the Universe and gravitation is still a thing, there will be opportunities to form new stars. When you take a cloud of gas and allow it to collapse, only about 10% of that material winds up in stars; the remainder goes back into the interstellar medium where it will get another chance in the distant future. Although the star-formation rate has plummeted since the Universe’s early days, it’s not expected to drop off to zero until the Universe is many thousands of time its present age. We will continue to form new stars for trillions upon trillions of years.

But even with all that said, new stars are much more of a rarity now than they have been at any point in our past since the Universe was in its infancy. We should be able to find out how star formation rose to its peak, and what the factors were that shaped the star-formation rate in the early days, with the advent of the James Webb Space Telescope. We already know what the Universe looks like, and how it’s declining today. The next great step, that’s almost upon us, is to learn just how it grew up to be the way it was at every step in our past.

Further reading on what the Universe was like when:

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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