A galaxy so distant that we see it practically at the edge of the observable universe has been seen to behave like a fully developed galaxy: Observations show that it rotates and rotates similarly to our own Milky Way, even though we see it as it was. Just 500 million years after the big bang!
The galaxy is called MACS1149-JD1 — or just JD1 for short — and it took about 13.3 billion years to reach us. Since the universe is only about 13.8 billion years old, we see this galaxy as it was when it was very young.
It is so far away that it would generally be too faint to be seen, but it lies behind a massive galaxy cluster called MACSJ1149.5+223. This cluster is only about 5 billion light-years away, much closer than JD1, and that’s lucky: The enormous gravity of the combined galaxies in the group acts like a lens, bending and amplifying the light from JD1, so it’s bright enough to see. It also magnifies the JD1 image, allowing us to see it in more detail, just like a lens.
JD1 has been studied before, and one paper surprisingly showed that stars in the galaxy formed just 250 million years after the universe’s birth. In another study, astronomers used the Atacama Large Millimeter/submillimeter Array, or ALMA, to observe the galaxy at a wavelength of about 88 microns and saw the light from the oxygen atoms glowing brightly. Oxygen forms inside massive stars and is expelled when they explode at the end of their lives, so some of these stars have already been born, aged, and died by the time we see this galaxy, which is impressive.
The new work expands on these observations [link to article]. Astronomers used ALMA to observe oxygen in JD1 again at 88 microns, but this time it took a total of almost 10 hours of observation, compared to just 2 hours earlier. The longer the observation, the more light the telescope collects, making structure and detail more visible. While that’s still a reasonably low resolution—after all, we’re talking about a galaxy that’s very far away—they can see structures roughly 1,000 light-years across in JD1.
They found a clear signal that some of the light emitted by oxygen is emitted by material moving towards us and some by material moving away from us. That’s a pretty good indication of rotation: An object spinning will be half moving toward us and half out, creating a Doppler shift in its light. It is this shift that astronomers have seen (after accounting for the massive redshift of the galaxy itself, which the universe’s expansion has swept away from us).
The galaxy is small, only about 3,000 light-years across, which isn’t surprising. At that time in the universe, galaxies were beginning to form through gas accumulation, so they did not have enough time to grow to a size such as our Milky Way, which is about 120,000 light-years wide. It is also much less massive, only about 650 million times the mass of the Sun; The Milky Way, by comparison, is more than 1000 times more massive than JD1.