Selections from 2024: Which Came First, Supermassive Black Holes or Galaxies?

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Which Came First: Supermassive Black Holes or Galaxies? Insights from JWST

Published January 2024

Main takeaway:

Joseph Silk (Sorbonne University; The Johns Hopkins University; University of Oxford) and collaborators have theorized that supermassive black holes are responsible for the high rate of star formation seen in many young galaxies. The proposed framework flips the conventional narrative of black hole and galaxy formation, suggesting that supermassive black hole growth spurred the formation of new stars rather than lagging behind the formation of stars.

Why it’s interesting:

How and when supermassive black holes formed in the early universe is one of the key unanswered questions in astronomy. The conventional theory of supermassive black hole formation suggests that galaxies formed first: gas clouds collapsed to form the first stars, which left behind stellar-mass black holes when the stars expired. A series of collisions between these stellar-mass black holes slowly built the first supermassive black holes while star formation continued busily in the background. The team’s new theory suggests that black holes and galaxies grew in tandem instead, with black hole growth playing an important role in the formation of new stars.

Solving a cosmic chicken-and-egg problem:

Observations from JWST have revealed the presence of extremely bright galaxies in the early universe, leading astronomers to wonder how these galaxies became so brilliant so quickly. Within the framework proposed by Silk’s team, the extraordinary brightness of these young galaxies is a natural consequence of the supermassive black holes at their centers; as the growing supermassive black holes accreted gas from their surroundings, they shot out powerful outflows that slammed into the surrounding gas, compressing it and triggering an explosive burst of star formation. This theorized powerful burst of star formation doesn’t last forever, though; about 1 billion years into the universe’s history, a shift in the outflowing winds of the supermassive black holes cast out the gas that fueled star formation, bringing it to a halt. Testing the predictions of this theory is likely to be difficult, though sensitive observations and intricate simulations may provide a path forward.

Citation

Joseph Silk et al 2024 ApJL 961 L39. doi:10.3847/2041-8213/ad1bf0