Astronomers Discover “Invisible” Galaxy

Astronomers have found something bizarre: a galaxy that appears to be made only of gas.

The galaxy, J0613+52, turned up in a survey of the neutral hydrogen gas in some 350 dim, diffuse galaxies called low surface brightness galaxies. These are systems at least one magnitude fainter than the ambient glow of the night sky. They have very few stars, making them challenging to spot with visible-light telescopes — which is why astronomers look for the faint glow of their gas at radio wavelengths instead.

But thanks to a fortuitous coordinate mistake, Karen O’Neil (Green Bank Observatory) and her colleagues found something even weirder: a disk of gas without any stars.

The dark galaxy lies 270 million light-years away, just above the loop of the constellation of Auriga, and appears as a blank patch to the eye. But seen in radio, it contains more than 1 billion Suns’ worth of hydrogen rotating quickly in an organized way — hallmarks of what should be a fairly normal, massive spiral galaxy.

Except astronomers don’t detect any stars.

“What we might have here — might — is the discovery of a primordial galaxy, a galaxy that so diffuse, it hasn’t been able to form stars readily,” O’Neil said January 8th during a press conference at the winter American Astronomical Society meeting in New Orleans.

Notably, the dark galaxy is a loner: No other galaxies huddle nearby. With no neighbors to harass it gravitationally, there would have been nothing to stir and compress the galaxy’s gas and trigger star formation. That in itself is unusual, she said, because galaxies tend to come in groups — rarely do you find one sitting off by itself.

The galaxy may in fact have some stars, but they are so few that they’ve so far evaded detection. The team hopes in the future to push a few magnitudes deeper to look for starlight.

Galaxies like these are important, O’Neil said, because they stress-test our theories about star formation and galaxy evolution. “This might be our first chance to really take a look at what happens to star formation in a diffuse environment where nothing has come along to tweak it.”

Objects like J0613+52 must be rare, because previous large radio surveys have turned up none. To find more, astronomers will likely need a deep, full-sky survey, perhaps with the GBT’s planned Advanced L-Band Phased Array Camera for Astronomy (ALPACA) instrument. Even so, studying dim, diffuse galaxies is challenging, because it takes so much telescope time, O’Neil cautioned — detecting just one low surface brightness galaxy can require more than 100 hours on the Very Large Array at its largest configuration.