The Confinement of Star-Forming Galaxies into a Main Sequence through Episodes of Gas Compaction, Depletion, and Replenishment

TL;DR

This study uses cosmological simulations to explore how gas dynamics like compaction, depletion, and replenishment confine high-redshift star-forming galaxies to the main sequence, revealing mechanisms behind their evolution and quenching.

Contribution

It provides a detailed simulation-based model linking gas processes to galaxy evolution along the star-forming main sequence, highlighting the roles of compaction, depletion, and replenishment.

Findings

High-SFR galaxies are compact with high gas fractions ('blue nuggets').

Galaxies oscillate around the MS on ~1 Gyr timescales due to gas processes.

Replenishment timescales influence the bending of the MS at high masses.

Abstract

Using cosmological simulations, we address the properties of high-redshift star-forming galaxies (SFGs) across their main sequence (MS) in the plane of star-formation rate (SFR) versus stellar mass. We relate them to the evolution of galaxies through phases of gas compaction, depletion, possible replenishment, and eventual quenching. We find that the high-SFR galaxies in the upper envelope of the MS are compact, with high gas fractions and short depletion times ("blue nuggets"), while the lower-SFR galaxies in the lower envelope have lower central gas densities, lower gas fractions and longer depletion times, consistent with observed gradients across the MS. Stellar-structure gradients are negligible. The SFGs oscillate about the MS ridge on timescales $\sim0.4~t_{\mathrm{Hubble}}$ ($\sim1$ Gyr at $z\sim3$). The propagation upwards is due to gas compaction, triggered, e.g., by mergers, counter-rotating streams, and/or violent disc instabilities. The downturn at the upper envelope is due to central gas depletion by peak star formation and outflows while inflow from the shrunken gas disc is suppressed. An upturn at the lower envelope can occur once the extended disc has been replenished by fresh gas and a new compaction can be triggered, namely as long as the replenishment time is shorter than the depletion time. The mechanisms of gas compaction, depletion and replenishment confine the SFGs to the narrow ($\pm0.3$ dex) MS. Full quenching occurs in massive haloes ($M_{\mathrm{vir}}>10^{11.5}~M_\odot$) and/or at low redshifts ($z<3$), where the replenishment time is long compared to the depletion time, explaining the observed bending down of the MS at the massive end.