From FFB Starbursts at Cosmic Dawn to Quenching at Cosmic Morning: Hi-z Galaxy Bimodality

Abstract

We propose a mass-dependent bimodality in the early evolution of galaxies. The massive track connects the super-bright galaxies at cosmic dawn ($z>8$) to the super-massive quiescent galaxies and black holes (BHs) at cosmic morning ($z\sim 4-7$). The dark-matter halos $>10^{10.5}{\mathrm{M}}_{\odot }$ at $z=10$ are expected to undergo feedback-free starbursts (FFB) with high star-formation efficiency in dense star clusters within compact galaxies. The less massive halos avoid FFB and form stars gradually under stellar feedback, possibly leading to the peak star-forming galaxies at cosmic noon ($z\sim 1-3$). The FFB and non-FFB halos originate from $>4\sigma$ and $2-3\sigma$ density peaks, respectively. The post-FFB galaxies quench their star formation soon after the FFB phase and remain quiescent due to (a) gas depletion by the FFB starbursts and outflows, (b) compaction events driven by angular-momentum loss in colliding streams within the high-sigma-peak FFB halos, (c) turbulent circum-galactic medium (CGM) that suppresses feeding by cold streams, and (d) BH feedback, being a key for complete quenching. BH feedback is enhanced by FFB-driven BH seeding and growth. It seems capable of disrupting the streams by generating CGM turbulence or photo-heating, but this remains an open challenge. The cosmic-morning quiescent galaxies are expected to be massive, compact, showing signatures of compaction, outflows and AGN, with a comoving number density $\sim 10^{-5}{\mathrm{Mpc}}^{-3}$, comparable to the super-bright galaxies at cosmic dawn and the AGN at cosmic morning. Their UV luminosity function is predicted to peak about $M_{\mathrm{uv}}\sim -22$ and contribute $\sim 10\%$ of the galaxies there.

22 pages, 13 figures