Revision 1

Accepted · first revision · record

Title: How did massive galaxies stop forming stars within 700 Myr?
Plain: JWST found massive galaxies that had already finished forming stars when the universe was under a billion years old. Making that many stars and then stopping so quickly is hard to reproduce in models.
Precise: JWST has confirmed massive quiescent galaxies up to $z = 7.29$ (RUBIES-UDS-QG-z7, $M_{*} >\sim 10^{10} M_{\mathrm{sun}}$); quiescent number densities exceed most models by up to $\sim 1\,\mathrm{dex}$ at $z \sim 3-5$, and the abundance implied at $z \sim 7$ is more than 100 times any model prediction (Weibel et al. 2025). Determine whether standard Lambda-CDM halo abundances with physical feedback can produce such galaxies, which requires star formation rates of hundreds of $M_{\mathrm{sun}}/\mathrm{yr}$ followed by quenching within $\sim 10^{8}\,\mathrm{yr}$, or whether stellar masses and ages are biased by the assumed initial mass function or AGN light.
Settled by: Spectroscopic stellar masses, ages and number densities for a complete sample at $z > 4$ compared with simulations that use the same selection, giving agreement or a quantified excess.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: As of 2026 initial mass function variation, AGN contamination of the light and small-sample statistics are proposed explanations; none is established.
Posed since: (empty)