by We think of galaxies as ancient. Our own galaxy, the The Milky Wayformed 13.6 billion years ago, and the James Webb Space Telescope which allowed us to peer back to some of the first galaxies early in the universe. But are galaxies still being born today?
It’s a fun question to tackle because it allows us to tackle the messy, complicated, beautiful process of galaxy formation. Let’s take a look at the possibilities.
The first answer: No
Galaxies which is quite easy to identify. They are vast collections of stars, gas and dark matter. They are very different from each other; a typical galaxy is about 100,000 light-years across, and the typical distance between galaxies is about 1 million light-years.
Sometimes, galaxies merge or come together within clusters, but with a few exceptions, we can separate one galaxy from another. They are like towns in the countryside: The distance between towns is greater than the towns themselves, so they are easy to identify and define. Sometimes, adjacent towns collide, and sometimes, a sprawling city consumes its neighbors. But all in all, a town is just a town.
Related: Did supermassive black holes or galaxies come first? The James Webb Space Telescope may have a surprising answer
However, defining the beginning of a galaxy is another matter. Galaxies emerged in the early universe through a gradual process starting all the way back in the first second of Big bang. At that time, tiny pockets of higher-than-average density emerged and grew steadily over the next hundreds of millions of years. At first, there was only the dark dark he could pour in, as a regular subject was busy dealing with himself. But once the pockets of dark matter grew large enough, they pulled in the surrounding regular matter.
As the regular matter gathered, it compressed, fragmented and gave light to the former stars. These protogalaxies continued to consume more gas, merging with neighbors and growing to become the fully formed galaxies we see today.
So, in many senses, there are no new galaxies to be seen today. The process of building them – seeding them as small differences in density or the initial gathering of dark matter – is over and done with, an action that happened in the ancient cosmos and has not happened again. There are no more protogalaxies – no more gas clouds just waiting for the chance to compress and form a new galaxy – in today’s Universe.
When it comes to galaxies, what we see is what we get.
Second answer: yes
But that is only one way to define the beginning of a galaxy. We can also look at another crucial step: the the appearance of the first stars. Going back to the city analogy, there is a difference between when a city is first planned — its outlines defined with boundary markers and survey lines — and when the first people start moving in.
If we focus directly on star formation, we see that this is an ongoing process that continues even in today’s Universe. In recent years, astronomers have built a detailed understanding of a measure known as the stellar mass function. This is a basic demographic census that maps how many stars are burning in each galaxy — or, put another way, how much mass is in the form of stars within each galaxy at different times of the universe.
Stars make up only a small percentage of a galaxy’s mass; the rest goes to dark matter and random gas clusters. However, stars make a galaxy what it is, and they are much easier to observe than any other galactic component.
With new surveys that sampled galaxies across the universe, astronomers recently found that the stellar mass function is going up across the board. This means that there are more small, medium 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 already clusters of matter that are just beginning to form stars. On the other hand, the larger galaxies are mainly driven by the continuous merger of smaller galaxies.
It won’t last forever
So, in at least one important way, new galaxies are still appearing on the cosmic scene as they light up with new rounds of star formation. They’ve always been there, hanging out for billions of years, but now they’re making themselves visible. This process is viable because star formation is extremely inefficient. Most of the gas within a galaxy will never become a star, and it can go on for very long periods of time without consuming much material – and it can take a long time for a galaxy to start in the first place .
Related stories:
—How do we know how far away and early in the universe’s galaxies?
—How many galaxies are there?
—How do galaxies grow when they are enveloped in the cosmic web of the universe?
But alas, the party won’t last forever. The problem is that only the an expanding universebut its expansion is accelerating – a known effect dark energy. Although astronomers still don’t understand what drives dark energy, they can see its effects on the rest of the universe: It’s spreading everything.
As the universe ages, it becomes harder and harder for matter to come together to form new galaxies and drive continued star formation. In fact, star formation peaked billions of years ago. Although new galaxies continue to shine, the rate of emergence is slowing, with fewer and fewer new galaxies appearing each year.
We still have plenty of time — galaxies will continue to create stars for hundreds of billions of years to come — but we should still enjoy the party while it lasts.
