The universe seems like a well-illuminated place with over 100 billion galaxies, each containing billions of stars. However, some galaxies don’t have nearly as many stars as models predict there should be. A new paper published in Nature suggests that overall temperatures of interstellar gas are simply too high to facilitate the formation of these stars.
“It would be like putting an ice cube in a boiling pot of water — the average temperature is pretty much still boiling,” senior author Michael McDonald from Massachusetts Institute of Technology said in a press release. “At super-high temperatures, conduction smooths out the temperature distribution so you don’t get any of these cold clouds that should form stars.”
Galaxies rely on black holes to regulate temperatures that are responsible for the formation of stars. In order for new stars to form, interstellar gas needs to cool and condense. However, even when certain areas begin to cool, conduction from surrounding areas heat it back up. This precipitation feedback cycle controlled by activity from the black hole does allow a few stars to form, but not very many.
Cool core cluster galaxies are as their name implies, and have galactic cores that cool readily enough to facilitate star formation. Non-cool core clusters, on the other hand, do not. Even so, scientists are not seeing the amount of stellar formation from cool core galaxies that they believed they would be seeing.
“The amount of fuel for star formation outpaces the amount of stars 10 times, so these clusters should be really star-rich,” McDonald continued. “You really need some mechanism to prevent gas from cooling, otherwise the universe would have 10 times as many stars.”
The researchers were able to establish the temperature threshold for stellar formation through a number of the galaxy’s attributes, including mass, temperature, and density. Galaxies close to the line still experience conduction that will affect star formation, though it does occur. Other galaxies much higher than this line have limited star formation due to the high levels of conduction.
“For these hotter clusters, they’re stuck in this hot state, and will never cool and form stars,” McDonald explained. “Once you get into this very high-temperature regime, cooling is really inefficient, and they’re stuck there forever.”
Moving forward, the researchers will continue to track certain galaxies and predict how others will change in the future. This will be important in understanding how the shape of galaxies, and ultimately the universe as a whole, literally comes to light.
“If we can use all this information to understand why or why not stars form around us, then we’ve made a big step forward,” McDonald concluded.