Angular Momentum Variation of the Milky Way Thick Disk: The Dependence of Chemical Abundance and Evidence of the Inside-out Formation Scenario

Abstract

We investigate the angular momentum of mono-abundance populations (MAPs) of the Milky Way thick disk by using a sample of 26,076 giant stars taken from APOGEE Data Release (DR) 17 and Gaia early DR3. The vertical and perpendicular angular momentum components, L Z and L P , of the MAPs in narrow bins have significant variations across the [α/M]–[M/H] plane. L Z and L P systematically change with [M/H] and [α/M] and can be alternatively quantified by the chemical gradients: d[M/H]/dL Z = 1.2 × 10−3 dex kpc−1 km−1 s, d[M/H]/dL P = −5.0 × 10−3 dec kpc−1 km−1 s, and d[α/M]/dL Z = −3.0 × 10−4 dex kpc−1 km−1 s, d[α/M]/dL P = 1.2 × 10−3 dec kpc−1 km−1 s. These correlations can also be explained as the chemical dependence of the spatial distribution shape of the MAPs. We also exhibit the corresponding age dependence of the angular momentum components. Under the assumption that the guiding radius (R g ) is proportional to L Z , this provides direct observational evidence of the inside-out structure formation scenario of the thick disk, with dR g /dAge = −1.9 kpc Gyr−1. The progressive changes in the disk thickness can be explained by the upside-down formation or/and the consequent kinematical heating.