Star formation in the first billion years: puzzles and insights from JWST

by Dr Viola Gelli (NBI/DAWN, UCPH, Denmark)

Wednesday 22 Apr 2026, 13:30 → 14:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

The first billion years after the Big Bang mark a critical phase in cosmic history, when the first galaxies formed and began shaping the Universe we observe today. The James Webb Space Telescope (JWST) has revealed a surprising number of UV-bright galaxies at z>10 as well as the first quiescent, low-mass galaxies already in place by z∼6. The coexistence of these extreme sources challenges standard models of galaxy formation and raises fundamental questions: was star formation different in the early Universe? What physical processes regulated it? In this talk, I will discuss how JWST observations are challenging our understanding of star formation regulation, but provide critical opportunities to gain insights into the underlying physical processes.

Cosmological simulations predict that high-z galaxies exhibit bursty, time-variable star formation histories, with temporary quiescent phases dominating at low masses (M* < 10^8 Msun) and faint luminosities (M_UV > –17). However, they underpredict JWST observed fractions, suggesting stronger feedback mechanisms at z>6.
I will also show that galaxies undergoing strong bursts can naturally explain luminosity functions up to z~12, but fail to reproduce those at z~14, implying additional physical processes must enhance the UV luminosity in the highest redshift galaxies.

I will discuss how bursty SFHs are expected to affect other key quantities, including galaxy clustering, reionization history, and the distributions of spectral features such as UV slopes and emission line strengths. These predictions will help us constrain the stochasticity in high-z galaxies and unveil the physical processes regulating their evolution.