Original title: RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization
Yunwei Deng, Hui Li, Ewald Puchwein, Boyuan Liu +6 · September 2026
Ultra-faint dwarf galaxies are among the oldest and tiniest galaxies in the universe. Scientists want to know how cosmic reionization—a period when intense radiation swept through space—stops their star formation and creates their diversity. Researchers ran eight detailed computer simulations of isolated dwarf galaxies to find out. They learned that reionization shuts off star birth in two distinct steps. First, an incoming wave of radiation quickly heats up thin gas outside the galaxy, which stops fresh gas from falling in. Second, the galaxy keeps making stars for several hundred Myr using dense gas that shields itself from the heat. Star formation halts only after the galaxy uses up or scatters this remaining fuel. Within 500 Myr after reionization, less than 40% of the starting gas remains, primarily lost through photoevaporation. Halos that were more massive during reionization hold gas longer, producing larger final stellar masses.
Imagine filling a bathtub with the drain open. You turn on the tap, but the water level barely rises because water escapes as fast as it arrives. Then someone pulls the plug: no new water comes in, and the water already in the tub slowly drains away. In this paper, the galaxy is the bathtub, the tap is gas falling in from the universe, and the plug is the arrival of the universe's first light, which heats the surrounding gas. The light stops new gas from entering, but the gas already inside keeps making stars for a while until it is used up or blown away.
Ultra-faint dwarf galaxies are living fossils from the universe's first billion years. Understanding why some kept making stars longer than others tells us how the first light in the universe switched off galaxy formation — and why the smallest galaxies around us look so different from each other.
The abstract doesn't say. It does specify that the simulations are isolated UFDs with present-day halo masses around 10^9 solar masses, so the results may not apply to UFDs in denser environments or with different masses.
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RIGEL: Ultra-faint dwarf galaxy diversity shaped by inhomogeneous cosmic reionization — Yunwei Deng, Hui Li, Ewald Puchwein et al.
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.