A group of researchers seems to have solved an old murder mystery: do galaxies die suddenly, when their gas supply is expelled at once? Or do they die little by little, after their gas supply from outside is cut off? “Die” here refers to the galaxy’s ability to form new stars. They'll continue to exist for a long time after this death, but move into the population of “quiescent” galaxies with old stars that can expect few siblings to join them.
Stars are formed by the contraction of gas, so if a galaxy has little gas to contract, it can’t make any new stars. If some process forced all of a galaxy’s gas out suddenly, then star production would come to a sudden halt. If the galaxy's outside gas supply cut off, it would still have gas left within the galaxy to work with, but star formation would slowly grind to a halt because of this strangulation.
So the difference between the methods of dying is a bit like having the air violently sucked out of your lungs in outer space (gas ejection) or holding your breath underwater (strangulation).
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