why the biggest galaxies have the fewest new stars

Some of the largest galaxies in the cosmos aren't creating new stars. Now scientists finally think they know why.

Some of the biggest known galaxies in the universe like IC 1101 and Alcyoneus measure millions of light years across, and they got this way through eons of collisions and mergers. Our galaxy is just a small fraction of these cosmic titans at a roughly average 130,000 light year across, as is Andromeda next door, which stretches about 220,000 light years. When the pair collides in four billion years, the end result will be an elliptical middleweight close to 400,000 light years across and containing approximately 700 billion stars.

And after massive bursts of activity lasting for hundreds of millions of years, Milkomeda, as it's already being called, will go quiet and stop producing new stars, just like its large counterparts across the universe. But why are products of violent collisions and cannibalism so quiet? Why are they not producing new stars at any meaningful rate? According to new research combining data from the ALMA radio observatory, the Sloan Digital Sky Survey, and pictures from Hubble, the answer lies in the gravitational forces involved in galactic interactions.

Just consider that the Milky Way and Andromeda both tip the scales at around 1.5 trillion solar masses. As their combined 1.3 trillion stars fly by each other at 330 times the speed of sound in the gravitational waves emitted by the galaxies' supermassive black holes, vast tails of gas and dust necessary to form new stars will be ejected from both cosmic objects. Also floating along these tails would be hundreds of billions of suns which would've ordinarily died and provided plenty of additional fuel for new generations of stars and solar systems.

In other words, galactic collisions are so fast and violent, they eject the material necessary for dozens of generations of star formation into vast cosmic voids. After the chaos of the merger settles down, the tails of gas, dust, and stars disperse and fade from view, or end up coalescing into galactic dwarves, depriving the new galaxy of fuel for new stellar populations. This is why models for Milkomeda show the end result containing just over half the stars contained in both galaxies and predict a suspiciously calm elliptical object in just six billion years.

Of course, this doesn't mean large galaxies are just waiting to wither away. Close brushes with their smaller counterparts, major supernovae, and tidal interactions with other giants can and will trigger new bursts of star formation. It's just that without these events, they've entered a comfortable, stable state in which countless red dwarves will burn for quadrillions of years as small pockets of larger stars around them flame out one by one. Think of it as reaching galactic adulthood after a wild, explosive youth.

See: Spilker, J. et al, (2022) Star formation suppression by tidal removal of cold molecular gas from an intermediate-redshift massive post-starburst galaxy, ApJL. DOI: 10.3847/2041-8213/ac75ea