When do Disks Form in MW-mass galaxies?

Degree of rotational support, v_phi/sigma_Z, for young stars (age < 250 Myr) versus the lookback time of disk onset. Circles show isolated (m12) galaxies, and stars show galaxies in LG-like pairs. We examine 3 metrics to determine the time that each galaxy's disk started to settle, which yield similar but occasionally different times and ordering between galaxies. Top: disk onset time defined as age when v_phi/sigma_tot at formation permanently exceeded 1. Middle: disk onset time defined age as when v_phi/sigma_total as measured today permanently exceeded 1. Bottom:: disk onset time defined age as when v_phi/sigma_z as measured today permanently exceeded 1.8.

Overall, using kinematics at formation versus today or measuring different velocity components all lead to similar times of disk onset. All 3 metrics show that disks that started to settle earlier currently form stars on more rotationally supported orbits, which implies that the MW, which is unusually dynamically cold today, started to settle unusually early.