Ancient galaxies possess gigantic, mysterious ultraviolet rings

Some of the universe's oldest galaxies appear to be going through the cosmic equivalent of a mid-life crisis, surrounding themselves with immense ultraviolet halos. These highly energetic rings of light make galaxies look young again, and astronomers are totally baffled.

Using NASA's Galaxy Evolution Explorer and Hubble Space Telescope, astronomers closely examined thirty galaxies that were all at least ten billion years old. Although all visible signs suggested these galaxies were well past their period of new star formation and had settled into the duller reddish glow associated with aging stars, these galaxies were emitting massive amounts of ultraviolet light. That didn't make any sense, considering intense ultraviolet energy is something young, star-forming galaxies emit, not ones quietly entering their final years.

But Hubble was able to reveal the answer - it wasn't the galaxies themselves that were emitting the ultraviolet light, but rather immense rings of light that surround 75% of the galaxies in the study. Some of the rings could fit our galaxy inside them several times over, and they featured ripples and perturbations that measured a quarter of a million light-years across.

As UCLA astronomer Michael Rich explains, this discovery just doesn't fit into what we thought we knew about galaxies:

We haven't seen anything quite like these rings before. These beautiful and very unusual objects might be telling us something very important about the evolution of galaxies."

So now we know what's emitting all that ultraviolet light, but that only pushes the mystery back a level. After all, if the rings are causing the ultraviolet light, what's causing the rings? We know that these older galaxies would have needed to get a fresh supply of cold gas from somewhere, but it's unclear where that could have come from. Astronomers have some preliminary hypotheses, but none of them are all that convincing just yet.

One possibility is that smaller galaxies collided with the ones we now see, providing the needed infusion of gas. But as Indiana University astronomer Samir Salim explains, the mechanics of that are highly improbable:

"To create a density shock wave that forms rings like those we've seen, a small galaxy has to hit a larger galaxy pretty much straight in the center. You have to have a dead-on collision, and that's very uncommon."

Another idea holds that the intergalactic medium, the thin bands of ionized hydrogen that lie between galaxies, might have been able to draw out some materials and building blocks from inside the galaxies and form the rings that way. That doesn't ask for quite so much suspension of disbelief as the collision theory, but it's still outside our current knowledge of galactic behavior.

Still, these rings suggest galaxies can somehow recapture a part of their youthful energy and vigor in ways we don't quite yet understand. As Salim puts it:

"In a galaxy's lifetime, it must make the transition from an active, star-forming galaxy to a quiescent galaxy that does not form stars. But it is possible this process goes the other way, too, and that old galaxies can be rejuvenated."

Whatever they are, these rings might just mean galaxies enjoy a second act, a chance to be reborn and begin again. Or they're going through one hell of a cosmic mid-life crisis and are compensating with these age-concealing, unbelievably gaudy rings.

[The Astrophysical Journal via Space]