Efficient formation of massive galaxies at cosmic dawn by feedback-free starbursts

ABSTRACT JWST observations indicate a surprising excess of luminous galaxies at z ∼ 10 and above, consistent with efficient conversion of the accreted gas into stars, unlike the suppression of star formation by feedback at later times. We show that the high densities and low metallicities at this epoch guarantee a high star formation efficiency (SFE) in the most massive dark-matter haloes. Feedback-free starbursts (FFBs) occur when the free-fall time is shorter than ∼1\, Myr, below the time for low-metallicity massive stars to develop winds and supernovae. This corresponds to a characteristic density of ∼3\! \!10³\, cm^-3. A comparable threshold density permits a starburst by allowing cooling to star-forming temperatures in a free-fall time. The galaxies within ∼1011 M⊙ haloes at z ∼ 10 are expected to have FFB densities. The halo masses allow efficient gas supply by cold streams in a halo crossing time ∼80\, Myr. The FFBs gradually turn all the accreted gas into stars in clusters of ∼104–7 M⊙ within galaxies that are rotating discs or shells. The starbursting clouds are insensitive to radiative feedback and are shielded against feedback from earlier stars. We predict high SFE above thresholds in redshift and halo mass, where the density is 10^3\!-\!4\, cm^-3. The z ∼ 10 haloes of ∼1010. 8 M⊙ are predicted to host galaxies of ∼1010 M⊙ with star formation rate ∼65\, M_ \, yr^-1, blue colours, and sub-kpc sizes. The metallicity is ≤0. 1 Z⊙ with little dust, gas, outflows, and hot circumgalactic gas, allowing a top-heavy initial mass function but not requiring it. The compact galaxies with thousands of young FFB clusters may have implications on reionization, black hole growth, and globular clusters.