Stellar Kinematics across the Formation Histories of MW-like Galaxies

Median velocities and velocity dispersions for stars versus age, within cylindrical R = 6 - 10 kpc and |Z| < 3 kpc at z=0. Age bins have a width of 250 Myr.The lines show the average and the shaded regions show the standard deviation across 11 MW-mass galaxies, with velocities measured at z=0 (solid) and at the time of formation (dashed, including only in-situ stars). The shaded vertical bars show when our galaxies transitioned from the Pre-Disk to the Early-Disk Era (~8 Gyr ago) and the Early-Disk to the Late-Disk Era (~4 Gyr ago), on average.

Top row:Median velocity versus age. For most of the Pre-Disk Era, stars formed with little net rotation, but v_phi,form rapidly increased in the Early-Disk Era when the disk initially formed, and it increased at a lower rate in the Late-Disk Era when the disk further settled. Stars that formed in the Pre-Disk Era have v_phi,now > v_phi,form from dynamical torquing after formation, while stars that formed in the Late-Disk Era now have slightly smaller v_phi, now than at formation from post-formation dynamical heating.

v_R and v_Z fluctuated at formation in the Pre- and Early-Disk Eras, because of frequent mergers, rapid accretion, bursty star formation, and gaseous outflows, but today they are 0 across all ages.v_tot,form steadily increased at all times, but v_tot,now is nearly independent of age, because all stars have been dynamically heated to orbit near the virial velocity.

Bottom row: Velocity dispersion versus age. All 3 components and the total show the same trends with age. sigma_form increased throughout the Pre-Disk Era as the galaxy grew, peaking ~8 Gyr ago, but decreased throughout the Early-Disk Era and remained constant throughout the Late-Disk Era. By contrast, sigma_now monotonically increases with stellar age because of the dynamical heating of stars after their formation.

Thus, the kinematics of stars today do not simply reflect their kinematics at formation.