Re-visiting the relations: Galactic thin disc age-velocity dispersion relation

TL;DR

This study re-examines the age-velocity dispersion relation in the Galactic thin disc, revealing that vertical heating does not saturate early and that a power-law model is not necessary, challenging previous assumptions.

Contribution

The paper demonstrates that the in-plane velocity distributions are distorted by local structure, and that the age-sigma(W) relation can be constrained without assuming a power law, showing disc heating may saturate.

Findings

Vertical disc heating does not saturate early.

A power law is not required to fit the data.

No local signature of the stellar warp detected.

Abstract

The velocity dispersion of stars in the solar neighbourhood thin disc increases with time after star formation. Nordstrom et al. (2004) is the most recent observational attempt to constrain the age-velocity dispersion relation. They fitted the age-velocity dispersion relations of each Galactic cardinal direction space velocity component, U (towards the Galactic centre), V (in the direction of Galactic rotation) and W (towards the North Galactic Pole), with power laws and interpreted these as evidence for continuous heating of the disc in all directions throughout its lifetime. We re-visit these relations with their data and use Famaey et al. (2005) to show that structure in the local velocity distribution function distorts the in-plane (U and V) velocity distributions away from Gaussian so that a dispersion is not an adequate parametrization of their functions. The age-sigma(W) relation can however be constrained because the sample is well phase-mixed vertically. We do not find any local signature of the stellar warp in the Galactic disc. Vertical disc heating does not saturate at an early stage. Our new result is that a power law is not required by the data: disc heating models that saturate after ~ 4.5 Gyr are equally consistent with observations.