The Rapid Onset of Stellar Bars in the Baryon-dominated Centers of Disk Galaxies

Abstract

Recent observations of high-redshift galactic disks ( z ≈ 1–3) show a strong negative trend in the dark-matter (DM) fraction f _DM with increasing baryon surface density. For this to be true, the inner baryons must dominate over DM in early massive galaxies, as observed in the Milky Way today. If disks are dominant at early times, we show that stellar bars form promptly within these disks, leading to a high bar fraction. New James Webb Space Telescope observations provide the best evidence for mature stellar bars in this redshift range. The disk mass fraction f _disk within R _s = 2.2 R _disk is the dominant factor determining how rapidly a bar forms. Using 3D hydro simulations of halo-bulge-disk galaxies, we confirm the “Fujii relation” for the exponential dependence of the bar formation time τ _bar as a function of f _disk . For f _disk > 0.3, the bar formation time declines exponentially fast with increasing f _disk . Instead of Fujii's arbitrary threshold for when a bar appears, for the first time, we exploit the exponential growth timescale associated with the positive feedback cycle as the bar emerges from the underlying disk. A modified, mass-dependent trend is observed for halos relevant to systems at cosmic noon ( $10.5\lt \mathrm{log}{M}_{\mathrm{halo}}\lt 12$ ), where the bar onset is slower for higher-mass halos at a fixed f _disk . If baryons dominate over DM within R ≈ R _s , we predict that a high fraction of bars will be found in high-redshift disks long before z = 1.

Keywords

• Disk galaxies
• Galaxy bars
• Milky Way formation
• Milky Way Galaxy physics
• Milky Way evolution
• Milky Way dynamics