The early bird gets the one ring to rule them all

Was the early Universe much different from the modern one ? The answer is definitely yes : there are far more quasars, more merging galaxies, higher levels of star formation, and galaxies tended to look a lot messier. But there are some anomalies.


This first paper notes the discovery of a mostly-normal disc galaxy formed just 1.5 Gyr after the Big Bang. Using high resolution observations of ionised carbon with the ALMA radio telescope, the authors were even able to measure its rotation curve... and it looks perfectly normal. Overall, it looks like a galaxy that's a bit bigger than the Milky Way that's somehow travelled back in time to the early universe. The only major difference seems to be that it's forming stars an order of magnitude faster than contemporary galaxies, which is normal for galaxies in this era.

Interestingly, the molecular mass of the galaxy is very similar to its dynamical mass. Considering that its rotation curve is nicely flat, one wonders about the dark matter content - which, oddly, is barely mentioned. Especially since there was much-popularised result a few years ago showing that galaxies in the early Universe show declining rotation curves, which this one clearly doesn't, but they don't cite that. Though, that result probed the rotation out to much greater distances, with this new result only being able to examine the inner regions of the galaxy's disc. Still, it's a bit weird that it isn't even mentioned.


If boring normal galaxies that travel through time aren't exciting enough, how about one with a ring ? This second paper describes a ring galaxy found just a bit later at 3 Gyr after the Big Bang. It's a very small ring compared to modern galaxies, in the bottom 10% of the distribution. Either they spotted it just 40 Myr after it formed, if it was produced in a collision like many contemporary rings, or it was formed by a different mechanism. While most modern ring galaxies show higher star formation rates than their counterparts, this one has about the same activity as typical non-ring galaxies of its era.

The galaxy also has a large, extended stellar disc outside the ring. If this is a collisional object, they say, then the timescales for the formation of both structures (which I assume they get from the kinematics) are mutually exclusive : one must be more than 80 Myr and the other less than 50 Myr. They say this could be explained if the ring is actually the second formed from the collision, with the disc being the remnant of the first ring which has now been smoothed out (I didn't know multiple rings were much of a thing but apparently this is possible).


The strange thing about both of these galaxies is how they were able to form so quickly after the Big Bang. Back then the Universe was much smaller, so mergers were much more common and melodramatic, so even forming a nice stable disc galaxy ought to be a problem. And then to get one to form a ring as well implies things have really settled down quite quickly. One the other hand, rings in general should be more common due to all the merger, but the second paper says their estimates are that rings were no more common back then than they are now. Apparently the higher merger rate is neatly balanced out by the fraction of disc galaxies available for collision.

I'm pretty sure by time-travelling galaxy theory is the most sensible explanation for this, and I expect to be quoted on it. If I don't see headlines like, "MILKY WAY IN DANGER OF TRAVELLING THROUGH TIME, SAYS SCIENTIST", I'm going to be disappointed.