If you want your galaxy to be neat and clean, you need a cosmic cleaning service made of supermassive black holes and supernova explosions. A warning, though: cleaned galaxies are also described as dead because they lack new star formation; star formation thrives on mess.
One of the great questions in astronomy today is that of why some galaxies stop forming stars while others continue to renew themselves. The presence of molecular gas is essential for star formation but the debate about why some galaxies are rich in gas while others are not remains a closely studied topic.
Part of the process of galactic death has been attributed to winds from supermassive black holes at the galactic core. These are thought to drive gas out of the galaxy into regions where it becomes so diffuse that it cannot form new stars.
However, debate rages over whether this is enough. "Our previous research had shown that black-hole outbursts can limit star formation in massive galaxies, but they can't completely shut it off," says Michigan State University's Professor Mark Voit. "Something else needs to keep sweeping out the gas that dying stars continually dump into a galaxy."
In The Astrophysical Journal, Voit and his coauthors produce evidence that this "something" is supernova explosions. They examine five massive elliptical galaxies in which star formation is still occurring and five where it has ceased. Counterintuitively, galaxies with hot young stars being formed are cooler than those without.
One of the dead galaxies chosen, NGC 4261, proved very revealing, with jets from the black hole 50 times more powerful than those in any of the other nine. The authors attribute this to a temporary accretion of gas.
The authors found that black holes warm and stir gas clouds, temporarily preventing star formation, but not shutting it down permanently. This is where Type Ia supernovae take over. "We propose that the difference [between galaxy states] arises because [supernovae] are successfully sweeping gas ejected by the old stellar population out of the [galaxies]," the authors write. In living galaxies the rate at which gas cools and condenses is faster than that at which the supernovae sweep it out.
The work is done by Type Ia supernovae, rather than other sorts, because other types of supernovae occur in large stars with short life spans, and therefore cease a short time (astronomically speaking) after a galaxy stops forming stars.
What, then, determines whether there are enough supernovae to keep the gas from cooling? "Intriguingly," the authors observe, their findings suggest that the outcome depends "more directly on the concentration of stellar mass toward the center of a galaxy than on the total stellar mass itself." It has been observed previously that galaxies with densely packed centers are more likely to have ceased forming new stars, and this paper suggests that this is because they have enough stars that become supernovae to keep the gas from settling.