The Origin of Steep Vertical Stellar Distribution in the Galactic Disk

Abstract

Over the past two decades, observations have revealed that the vertical density distribution of stars in galaxies near the midplane is substantially steeper than the sech(2) function that is expected from an isothermal approximation. However, the physical origin for this has not been explained so far. Here we show that such steep profiles result naturally, even within the isothermal regime, on taking into account the gravitational force due to the gas. Because of its low velocity dispersion, the gas is concentrated closer to the Galactic midplane than the stars, and hence it strongly affects the vertical stellar distribution, even though its contribution to the total surface density is small. We apply a three-component Galactic disk model consisting of gravitationally coupled stars and H I and H-2 gas embedded in the dark matter halo, and calculate the vertical density distribution of stars for the Galaxy. The resulting vertical density distribution of stars is shown to be steeper than the sech(2) function, lying between the sech and an exponential function, in good agreement with observations of galaxies. We also show that a multicomponent stellar disk consisting of coupled dwarfs and two populations of giants does not explain the observed steep stellar profiles.