JWST Solves Mystery Of Early Galaxies That Live Fast And Die Young

Astronomers may have finally pinned down why some galaxies stop making stars. At a specific mass, they appear to build a self-supporting halo of hot gas that cuts off the cool fuel needed for new star formation, which results in the galaxy slowly starving itself.

Galaxies do not all age the same way. Some keep forming stars for billions of years, while others fade into quiescence after crossing a threshold that appears to sit near a total mass of about 10^12.5 solar masses, according to a study led by Preetish Mishra and colleagues of the Korea Institute for Advanced Study. Using the Horizon Run 5 simulation, the team tracked about 20,000 massive central galaxies and found that the stellar-to-total mass ratio peaks in a narrow mass range around 10^12.4 to 10^12.7 solar masses before dropping sharply. And that turnover is what experts believe is the point where galaxies become much less efficient at turning their raw material into stars.

The belief here is that once a galaxy grows large enough, infalling gas gets shock-heated into a stable halo that can stay hot for billions of years. Instead of cooling and flowing inward to feed star birth, the gas remains suspended, effectively locking the galaxy in a long starvation cycle. Mishra and the team argue that this change in gas accretion drives the change, because the fraction of baryons retained by the galaxies varies by no more than 30%. That's far too little to explain the sharp fall in star formation efficiency. 

The new study adds weight to that picture by showing the transition may be tied to a very specific scale rather than a vague evolutionary stage. It also suggests a second, smaller tipping point around 10¹¹ solar masses, where galaxies begin to show changes in gas retention and star-forming efficiency.