North-South Asymmetries In The Galactic Thin Disk Associated With The Vertical Phase Spiral

We select 1,052,469（754,635）thin disk stars from Gaia e DR3 and LAMOST DR7 in the range of Galactocentric radius（guiding center radius R＿g）from 8 to 11 kpc to study the asymmetries between the North and South of the disk mid-plane.More specifically we analyze the vertical velocity dispersion profiles (σ_vx（z）) in different bins of（R＿g）and[Fe/H].The most striking feature is the concavity in the profiles of σ_vx（z） located in both the North（z（?）0.6 kpc）and South（z（?）-0.5 kpc）of the disk at all radii and metallicity bins studied.The North-South difference（NS asymmetry）of vertical velocity dispersion profiles(Δσ_vx（|z|）)shows a shift between curves with differentand R＿g.A similar shift exists in the NS asymmetry profiles with different[Fe/H]ranges.The sample binned with R＿g more clearly displays the features in the velocity dispersion profiles.We confirm the coincidence in the NS asymmetries in the velocity dispersion,stellar number density and median metallicity profiles found previously,which is strong evidence of the connection between the NS asymmetry and the vertical phase spiral.The shift in the peaks ofΔprofiles and the variation in the spiral shape binned by metallicity indicates the variation of the vertical potential profiles and the radial metallicity gradient.We perform two test particle simulations perturbed differently to qualitatively reproduce the results in observations.Quantitative explanation of the NS asymmetry in the metallicity needs careful consideration on the spiral shape and the perturbation model,and is left to a future work.