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Title: Why are UV-bright galaxies at redshift above 10 so abundant?
Plain: JWST sees many more bright galaxies in the first 500 million years than models built before its launch predicted. Either early galaxies made stars much more efficiently, or something else is different.
Precise: The number density of galaxies with $M_{\mathrm{UV}} < -20$ at $z \sim 10\ \text{to}\ 15$ exceeds pre-JWST models by factors of 10 to 100 (the spectroscopically confirmed MoM-z14 at $z = 14.44$ implies a factor $\sim 100$ at $z \sim 14$). Identify the mechanism: high star-formation efficiency (feedback-free starbursts), bursty star formation, a top-heavy initial mass function, negligible dust, active-galactic-nucleus light, or enhanced small-scale primordial power. An answer reproduces the UV luminosity function at $z = 10\ \text{to}\ 15$ together with clustering and spectral constraints.
Settled by: Clustering measurements of $z > 10$ galaxies giving their halo masses, combined with spectroscopic constraints on stellar populations and burstiness.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: 2025 confirmation of MoM-z14 at $z = 14.44$ extended the excess to 280 million years after the Big Bang.
Posed since: 2022