Dense Gas Pretends to Be a Metal-Poor Galaxy ✛
The light from a distant galaxy is like a letter with some lines erased. Astronomers read from the spectrum how much oxygen and other elements heavier than helium a galaxy contains. If the oxygen lines are weak, it seems there are few metals.
But weak lines might not be due to a lack of oxygen, but rather to gas density. When the gas has a huge number of free electrons, they collide with oxygen ions and prevent them from glowing. The gas is rich in oxygen, but it looks poor.
Now astronomers will check the density before declaring a galaxy metal-poor. This will help us understand more precisely how quickly the Universe enriched with heavy elements.
The electron density in some of these distant galaxies reaches a million particles per cubic centimeter — hundreds of times denser than typical star-forming regions in our Galaxy, comparable to the gas density in the immediate vicinity of young massive stars.