Non-isothermal vertical distribution functions for the Milky Way

Abstract

The vertical structure of our Galaxy has commonly been assumed to follow a pseudo-isothermal distribution. However, there is no \textit{a priori} reason to expect this form to arise from scattering by giant molecular clouds (GMCs), since GMCs are confined to a narrow layer around the Galactic midplane and therefore do not randomise stars uniformly in vertical phase space. We first present a simple statistical argument to derive a new vertical distribution function (DF), in the limiting case of a razor-thin distribution of GMCs, which admits an extra factor of ${J_z}^{-α}$, where $α>0$ and varies for different potentials. In light of this, we revisit the diffusion coefficients for vertical heating from the mechanics of two-body scattering, in a more realistic, extended distribution of GMCs. The resulting drift and diffusion coefficients are strongly non-linear in $v_z$, particularly at low relative velocities, where the effective range of weak, local encounters can vanish. We determine stationary solutions, and also numerically solve the orbit-averaged Fokker--Planck equation with time-dependent coefficients. We find, in both cases, a vertical action distribution that deviates from that of the pseudo-isothermal case, and propose a new, family of vertical DFs that generalise the pseudo-isothermal form. We find the two primary agents of non-isothermality are: (i) a GMC distribution confined near the midplane, and (ii) the fact that the Coulomb logarithm, $Λ$ is strongly velocity-dependent at small velocities, and cannot be treated as a constant, contrary to the commonly adopted assumption.