Bar Fraction Evolution Anomaly

The fraction of disk galaxies hosting a central bar increases from the present toward moderate redshifts (z~0.2–0.5) and then drops steeply above z~0.8, reaching very low values at z>1 despite abundant disk galaxies at these epochs. Standard ΛCDM simulations predict that bar formation should be efficient whenever a disk is gravitationally self-consistent and cold, implying significant bar fractions at all redshifts with disks. The observed low bar fraction at high redshift implies that early disks were either too dynamically hot, too gas-rich, or too geometrically thick to support bar instabilities — but the ΛCDM framework does not straightforwardly predict these conditions in conjunction with the abundance of massive disks observed at z>1. Successive Collision Theory provides a natural explanation through the high angular momentum and high gas fraction of early galaxies. Early post-collision disk galaxies inherited very high specific angular momentum, which produced thick, turbulent, gas-rich disk configurations with high Toomre Q values — precisely the conditions that suppress bar instability.

Bar formation requires a thin, kinematically cold, stellar-dominated disk that is susceptible to gravitational instabilities on the scale of the bar pattern speed. The high gas fractions and turbulent velocity dispersions of early SCT disk galaxies — set by the post-collision angular momentum distribution and the high star formation rates driven by abundant cold gas — maintain Q values above the bar instability threshold throughout the early universe. As the tensor mesh dissipates and gas fractions decline through ongoing star formation and feedback, disks become progressively thinner and kinematically colder, eventually falling below the bar instability threshold. The bar fraction therefore rises naturally from high redshift to moderate redshift as disks evolve from the turbulent post-collision state to the cold, thin configuration that supports bar formation. The observed bar fraction evolution is a direct temporal record of disk kinematic cooling driven by gas consumption and angular momentum redistribution.