Why does this galaxy look older than its years?
Astronomers have found a mightily surprising early-universe galaxy that appears more mature than it should for its age. They announced this result on February 11, 2021, pointing out that galaxy ALESS 073.1 is located about 12 billion light-years away. That means we see this galaxy as it was when the universe was only 1.2 billion years old, or 10% of its current age. At such a time, according to astronomers’ theories, this galaxy would be considered young. It should be relativity unformed. But ALESS 073.1 already has a large inner bulge, a rotating disk and potential spiral arms. How did it get those features – all signs of relative maturity – at such an early age?
Observations using the ALMA telescope show that ALESS 073.1 already has all the features we would expect from a mature galaxy, which challenges our current understanding on how galaxies form. This makes astronomers wonder how it grew so fast, compared to what we would expect that close to the beginning of the universe. Note that, rather conveniently, the farther away you look, also the farther back in time you look (like a time machine). So a galaxy observed at that distance also automatically shows us how it looked like that long time ago, not how it would look like now.
Lead author of the paper, Dr. Federico Lelli, who conducted the research while at Cardiff University’s School of Physics and Astronomy (he is now at Arcetri Astrophysical Observatory in Florence) said:
We discovered that a massive bulge, a regular rotating disk, and possibly spiral arms were already in place in this galaxy when the universe was just 10% of its current age, In other words, this galaxy looks like a grown adult, but it should be just a little child. It defies our understanding of galaxy formation.
Galaxies in the very early universe are thought to go through massive star formation and have violently turbulent things going on in them. This would cause them to look chaotic, without any particular structure, and especially not any well-organized spiral arms. Then, as time would pass, over billions of years, things would calm down and the galaxies would not be so dynamically violent anymore but turn into more stable entities with the recognizable shapes and structures we see in galaxies of today.
An inner galaxy bulge (resembling the yolk of a fried egg) contains mainly old stars and does not have much new star formation going on. Until now, we believed that bulges form slowly, as smaller galaxies come by a larger galaxy and merge with it, and via inner processes in the galaxy itself as time passes and stars grew old. (Much older galaxies do not have disks but have evolved into elliptical galaxies, consisting mainly of a bulge alone.)
In other words, according to our understanding, a young galaxy should not have a large bulge and a regular disk yet for a long time. But ALESS 073.1 has both, with around half of all its stars within a bulge and maybe even spiral arms superposed on its unexpected disk.
This means that galaxy formation may be a much faster process than previously thought and that a huge bulge can form a lot quicker than believed.
Now, how can we see that ALESS 073.1 has a disk, you may ask, just by looking at the images in this article?
When we think of images that go really far back in space and time we often think of images like the Hubble Deep Fields that were observed in optical and infrared light. But these images were observed with ALMA, short for Atacama Large Millimeter/submillimeter Array, which is located in the Andes in northern Chile. ALMA looks in the microwave region of the electromagnetic spectrum, between radio and infrared, that is, at longer wavelengths than optical light. This is useful because different types of telescopes can figure out different things about what they observe. For example, you can measure how the gas moves in the galaxies with this type of telescope. So these astronomers measured the movement of gas and dust in the galaxy to find the rotating disk and then modeled the mass distribution within the galaxy to measure the massive stellar bulge.
If you look at the image above, one side is red and the other is blue. These are not the actual colors but a representation of how the galaxy rotates: the red area shows gas whose emission is redshifted, which means that it moves away from us, and the blue area indicates gas that is blueshifted (same phenomenon as redshift but in the other direction), meaning it moves towards us. With such movement going on, we can see that the galaxy is rotating. And when there is rotation, physics tells us there will also be a disk.
This is, however, not the first study showing that primordial galaxies are more mature than believed. Last October, another ALMA project, called ALPINE, corroborated this idea. The astronomers involved in that study were actually looking for something different; an early universe growth spurt in galaxies. They were equally surprised when they found that as many as 20% of the 118 galaxies they studied were much more mature than expected!
In this case, the conclusion came via measuring the presence of a lot more dust and heavy elements than expected in these galaxies. You see, dust and heavy elements (atoms heavier than hydrogen and helium) don’t show up until the end of the life of stars, where they are formed. At that early age, the universe had supposedly not had enough time to build up that many stars yet, so there shouldn’t be that much dust and heavy elements around! And yet there was, showing that the galaxies were more mature than we thought.
This video explains the findings of the ALPINE team:
Bottom line: Astronomers have observed galaxies in the early universe, when the universe was only 10% of its current age, that are far more evolved than expected. This changes our current understanding of galaxy formation radically and indicates that galaxies evolve much faster than previously believed.