One of the things that will never stop taking my breath away is the discovery of new, large objects that are shining faintly in plain sight. From the excitement of a mega comet or a melty minor planet, we now turn to a massive molecular cloud of cold gas lurking in the Milky Way. 

Cold gas gives off no optical light of its own. If it is diffuse enough, it also doesn’t significantly block background light. And if that cold gas is mostly made of molecular hydrogen, it is going to be basically invisible even to radio telescopes. Luckily, clouds of molecular gas aren’t generally made of just one atom, and mixed in with that molecular hydrogen should be other gases. 

Over the years, astronomers mapping the molecular clouds in our galaxy have generally used easy-to-observe carbon monoxide as a tracer of where atomic hydrogen is located, but in 2012, astronomer Ron Allen found radio omissions from hydroxide (OH) in a region of the sky where no carbon monoxide was found. Hydroxide and carbon monoxide both usually trace atomic hydrogen, and the question became: Can there be carbon-monoxide-dark molecular clouds? Early computer models said yes, and observation after observation also seemed to say yes.

A single, curious, region of the sky was found to be emitting cosmic rays that seemed to indicate something was out there. It also was glowing faintly in OH and patchily in carbon monoxide. This signal was observed in scope after scope and didn’t go away with better observations; it just didn’t ever particularly stand out. Much to everyone’s frustration, this is a massive blob of cold gas that is cold enough to be largely invisible, but it shined ever so faintly in the Green Bank Observatory’s twenty-meter telescope and appears to be a source of cosmic rays.

These results will now need to be factored in when folks try to model future star formation and the ratio of dark matter to luminous matter in our solar system.

And this is your reminder that the total amount of dark matter needed to understand our galaxy has gone down ever so small an amount as we’ve gotten better and better at finding cold gas. While we remain certain that some strange stuff that refuses to interact with light will one day be found, this discovery tells us that we might need to find a snert less of that stuff than we were expecting.

More Information

Green Bank Observatory press release

“Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy,” Michael P. Busch, Philip D. Engelke, Ronald J. Allen, and David E. Hogg, 2021 June 16, The Astrophysical Journal