Is the Milky Way A Typical Disk Galaxy? Probably Not!

Comparing stellar velocity dispersion versus age in the MW and nearby galaxies

Top: The line-of-sight (1D) velocity dispersion of stars, σ_1D, versus age, as observed in the MW, M31 (Dorman 2015), M33 (Quirk 2022), and 16 nearby galaxies from the PHANGS survey (Pessa 2023). For the MW, the teal line and shaded region show the mean and full scatter across the observational works in Figure MW obs for σ_1D = σ_3D / √3. The light green line and shaded regions show the median, 68% scatter, and full range across the PHANGS sample.
Bottom: Same, but scaled to each galaxy’s rotational velocity, v_φ,0, for young stars today, to provide a dimensionless metric of dynamical coldness that is more comparable across these different-mass galaxies. σ_1D and σ_1D / v_φ,0 increase with age for all galaxies and at all ages, though at differing rates with age. M31 and M33 have broadly similar σ_1D / v_φ,0, which also agrees with (lies within the 68% scatter) of PHANGS. However, among this total sample of 19 galaxies, the MW is an outlier, being dynamically colder in terms of σ_1D / v_φ,0 than all but one galaxy from PHANGS.

For more on the MW’s kinematics, see McCluskey et al. (2025), which discusses the possibility that the Galaxy’s unusually cold dispersions reflect rapid early assembly, disc settling, and a quiet late-time merger history.