The earliest galaxies assembled most of their stars remarkably fast — in tens of millions of years, not hundreds. That is the conclusion of a team that modelled star formation histories from cosmic dawn to the end of the Epoch of Reionization.
The researchers combined two tools. The first is the GUREFT simulation suite, which tracks how dark matter halos (invisible clumps of matter where galaxies form) grew over time. The second is the Santa Cruz semi-analytic model, which describes how stars are born inside those halos. Together they trace individual galaxies from about z 14 to about z 6.
On average, the star formation rate in galaxies of all masses rose steadily and rapidly. But individual galaxies showed diverse paths: bursts of star birth and brief pauses (so-called mini-quenching). This matches what astronomers see in observations.
The key figures concern timescales. For galaxies at z above 12, half of their stars formed in under 30 million years, and 90% in under 70 million. That is 3 to 4 times faster than for similar galaxies near the end of the Epoch of Reionization, at about z 6.
The consequence is practical. The stellar populations of ultra-high-z galaxies are dominated by young stars. So, to interpret James Webb images correctly, one must carefully model these young stars — otherwise estimates of galaxy mass and age will be off.