A distant galaxy birthing hundreds of stars should glow in oxygen. That's what astronomers thought: hot young stars heat the gas, which then emits a characteristic infrared glow at a wavelength of 63 microns. It's like steam rising from a boiling pot — a sure sign of vigorous activity. But when the ALMA telescope aimed its antennas at twelve galaxies located 12–13 billion light-years away, the picture was the opposite.
Instead of bright patches — dark troughs. The gas wasn't emitting; it was absorbing light coming from dust behind it. It's like how thick fog doesn't glow itself but dims a flashlight beam. In ten of the twelve galaxies, the oxygen line is completely invisible in emission, yet clear absorption lines are seen. And the absorption is extremely strong: in some places, the gas is so dense that its glow falls below the level set by the cosmic microwave background itself — the cooled light of the Big Bang that fills the entire universe.
But there are also bright 'windows'—narrow channels where radiation does escape. These occur where there's less dust, allowing the beam to break through. So oxygen in such galaxies lives a double life: in some places it absorbs, in others it barely shines. Previous estimates of the number of stars being born, based on the brightness of this line, could be off by a factor of several.
The cosmic microwave background is light left over from the Big Bang. It's everywhere, and its temperature is just 2.7 degrees above absolute zero. In distant galaxies, this background is so 'warm' compared to the cold gas that it can itself illuminate the clouds.