We think of galaxies as ancient. Our own galaxy, Milky Wayformed 13.6 billion years ago, and James Webb Space Telescope allowed us to peek back at some of the first galaxies in the early universe. But are galaxies still being born today?
The question is fun because it allows us to delve into the messy, complex, and beautiful process of galaxy formation. Let’s look at the possibilities.
First answer: No
Galaxies are fairly easy to identify. They are large collections of stars, gas and dark matter. They are largely different from each other; a typical galaxy is approximately 100,000 light-years in diameter, while the typical distance between galaxies is approximately 1 million light-years.
Sometimes galaxies merge or cluster together, but with a few exceptions we can pretty much tell one galaxy from another. They are like towns in the countryside: the distance between the towns is greater than the towns themselves, so they are easy to spot and define. Sometimes cities collide with each other, and sometimes a growing city swallows up its neighbors. But in general, the city is just a city.
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Determining the origin of a galaxy, however, is a different matter. Galaxies appeared in the early universe through a gradual process beginning as early as the first second of the Big bang. At this time, small pockets of higher-than-average density appeared and grew steadily over the next several hundred million years. Only at first dark matter it could be poured out as ordinary matter was busy entangling itself. But once pockets of dark matter grew large enough, they pulled in the surrounding normal matter.
As ordinary matter gathered, it compressed, fragmented, and gave light to the former stars. These protogalaxies continued to consume more gas, merged with their neighbors, and grew to become the fully formed galaxies we see today.
So, in many ways, no new galaxies are appearing today. The process of building them up—of seeding them as tiny density differences or the initial accretion of dark matter—is over and done with, an act that happened in the ancient cosmos and never again. There are no more protogalaxies—no more clouds of gas just waiting for a chance to compress and create a new galaxy—in today’s universe.
When it comes to galaxies, what we see is what we get.
Second answer: yes
But this is only one way to determine the origin of a galaxy. We can also look at another key step: the appearance of the first stars. Returning to the city analogy, there is a difference between when a city is first planned—its outline defined by boundary markers and geodesic lines—and when the first people begin to settle.
If we focus on star formation alone, we see that it is a continuous process that continues even in the modern universe. In recent years, astronomers have built a detailed understanding of a measure called the stellar mass function. It’s basically a demographic census that maps how many stars shine in each galaxy—or, to put it another way, how much mass there is in the form of stars in each galaxy at different epochs in the universe.
Stars make up only a small percentage of the galaxy’s mass; the rest goes into dark matter and random clumps of gas. However, stars make a galaxy what it is, and they are much easier to observe than any other galactic component.
With new studies that have sampled galaxies across the Universe, astronomers have recently found that the stellar mass function is increasing everywhere. This means that there are more small galaxies, medium-sized galaxies, and large galaxies than there were billions of years ago.
New small galaxies do not come from the emergence of protogalaxies in dark matter seeds; they are pre-existing clumps of material that are just beginning to form stars. Larger galaxies, on the other hand, are driven mostly by the ongoing merger of smaller galaxies.
It won’t last forever
So, in at least one important way, new galaxies continue to appear on the cosmic stage as they light up with new rounds of star formation. They’ve always been there, hanging around for billions of years, but only now are they becoming visible. This process is viable because star formation is incredibly inefficient. Most of the gas in a galaxy will never become stars, and it can go for very long periods without using much material – and a galaxy can take a really long time to start.
But unfortunately, the party won’t last forever. The problem is that not only expanding universebut its expansion is accelerated – an effect known as dark energy. Although astronomers still don’t understand what drives dark energy, they can observe its effects on the rest of the universe: it permeates everything.
As the universe ages, it becomes harder and harder for material to clump together to form new galaxies and drive continued star formation. In fact, the peak of star formation passed billions of years ago. While new galaxies continue to shine, the rate of formation is slowing, with fewer and fewer new galaxies appearing each year.
We still have a long time—galaxies will continue to form stars for hundreds of billions of years to come—but we should still enjoy the party while it lasts.
