We consider galaxies to be old. Our own galaxy, the Milky WayFormed 13.6 billion years ago, and the James Webb Space Telescope has allowed us to look back at some of the first galaxies in the early universe. But are galaxies still being born today?
It’s a fun question to tackle because it gives us the opportunity to delve deeper into the messy, complicated, and beautiful process of formation of galaxiesLet’s take a 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 distinguishable from each other; a typical galaxy is about 100,000 light-years across, while the typical distance between galaxies is about 1 million light-years.
Sometimes galaxies merge or clump together in clusters, but with a few exceptions we can largely separate one galaxy from another. They are like cities in the countryside: the distance between the cities is greater than the cities themselves, so they are easy to spot and define. Sometimes cities collide with each other, and sometimes a sprawling city swallows up its neighbors. But for the most part, a city is just a city.
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Defining the beginning of a galaxy is another story, however. Galaxies formed in the early universe through a gradual process that went all the way back to the first second of the Big Bang. At that time, small pockets of higher than average density emerged, which grew steadily over the next few hundred million years. At first, only the dark matter could flow in, because ordinary matter was busy entangling itself. But as soon as the pockets of dark matter grew large enough, they attracted the surrounding ordinary matter.
As the normal matter gathered, it compressed, fragmented, and gave light to the first starsThese protogalaxies then consumed more gas, merged with neighboring galaxies, and grew into the fully formed galaxies we see today.
So in many ways, no new galaxies are appearing today. The process of building them — seeding them as tiny differences in density or the first collection of dark matter — is over and done, an act that happened in the old cosmos and never again. There are no more protogalaxies — no more gas clouds just waiting for the chance to collapse and create a new galaxy — in the present universe.
When it comes to galaxies, what we see is what we get.
Second answer: yes
But that’s just one way to define the beginning of a galaxy. We can also look at another crucial step: the appearance of the first stars. Returning to the city analogy: there is a difference between the moment a city is first planned — its contours are determined with boundary markers and measurement lines — and the moment the first people start living there.
If we focus on star formation alone, we see that it is an ongoing process that continues even in the modern universe. In recent years, astronomers have built up a detailed understanding of a measure called the stellar mass function. This is a basic demographic census that maps out how many stars are lighting up in each galaxy — or, put another way, how much mass there is in the form of stars in each galaxy at different times in the universe.
Stars make up only a small percentage of a galaxy’s mass; the rest goes to dark matter and random clumps of gas. But stars are what make a galaxy what it is, and they’re much easier to observe than any other galactic component.
With new studies that sample galaxies across the universe, astronomers have recently discovered that the stellar mass function is going up across the board. This means that there are more small galaxies, medium-sized galaxies, and large galaxies than there were billions of years ago.
The 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. The larger galaxies, on the other hand, are driven primarily by the ongoing merger of smaller galaxies.
It won’t last forever
So, in at least one important way, new galaxies keep bursting onto the cosmic scene as they light up with new rounds of star formation. They’ve been around forever, hanging around for billions of years, but they’re only just now making themselves visible. This process is viable because star formation is incredibly inefficient. Most of the gas in a galaxy will never turn into stars, and it can go on for a very long time without using up much material — and it can take a galaxy a very long time to get started in the first place.
But alas, the party won’t last forever. The problem is that not only universe expandingbut its expansion accelerates – an effect known as dark energyWhile astronomers still don’t understand what powers dark energy, they can observe its effects on the rest of the universe: it spreads everything.
As the universe ages, it becomes increasingly difficult for material to clump together to form new galaxies and fuel continued star formation. In fact, the peak of star formation passed billions of years ago. While new galaxies continue to light up, the rate of formation slows, with fewer and fewer new galaxies appearing each year.
We still have time – galaxies will continue to form stars for hundreds of billions of years – but we should still enjoy the party while it lasts.