What's going on with these mysterious, ultra-red galaxies?

Four newly discovered galaxies are so dim, so dusty, so impossibly distant that even the Hubble telescope couldn't spot them. But that's not all: these galaxies are so insanely red that astronomers are declaring them a new "species" of galaxy.

NASA needed the Spitzer Space Telescope and its incredibly powerful infrared telescope to spot these four galaxies at all. Located about 13 billion light-years away - just a billion or so years after the Big Bang itself - the galaxies have been dimmed by the vast distances their light has to travel to reach us, and the fact that they're surrounded by huge clumps of dust doesn't help.

But what really sets these galaxies apart is just how red they are. That's what infrared light is, essentially - light so red that it can't even be seen on the visible spectrum. These galaxies clock in at over 60 times brighter than the reddest colors that the Hubble telescope is even capable of detecting. And while astronomers can describe these new galaxies just fine, explaining what causes this ultra-red look is another matter entirely.

There are three basic reasons why a galaxy might appear red to us. First, it might feature an unusual concentration of old, red stars. Its distance can be another factor, as the expansion of the universe actually stretches out the light and makes it appear redder, which is known as the redshift. Finally, the galaxy could be stuffed full of dust.

No single one of these is enough to explain just how ultra-red these galaxies are, but a mix of all three explanations might just do it. We already know that they're extreme far away from us, and we're pretty sure the dust is there too. That just leaves the red stars, which we can't confirm but does seem like the most logical assumption to account for all this redness.

The galaxies appear to be in actual close physical proximity to one another. That suggests they really do occupy roughly the same place in space and time, and this isn't just a coincidental lineup created by our viewpoint. That bolsters the possibility that these actually share some particular qualities that are perhaps unique to extremely ancient galaxies. NASA astronomer Giovanni Fazio explains:

Hubble has shown us some of the first protogalaxies that formed, but nothing that looks like this. In a sense, these galaxies might be a 'missing link' in galactic evolution. There's evidence for others in other regions of the sky. We'll analyze more Spitzer and Hubble observations to track them down."

The next step will be to enlist other powerful telescopes, such as those at the European Southern Observatory in Chile, to start searching for more of these enigmatic, ultra-red galaxies.

Via the Astrophysical Journal. Artist's conception by David A. Aguilar (CfA).