Vertical scaleheights in a gravitationally coupled, three-component galactic disk

Abstract

The vertical scaleheight of the atomic hydrogen gas shows a remarkably flat distribution with the galactocentric radius in the inner Galaxy. This has been a long-standing puzzle (Oort 1962) because the gas scaleheight should increase with radius when treated as responding to the gravitational potential of the exponential stellar disk. We argue that the gravitational force of the molecular and atomic hydrogen gas should also be brought into the picture to explain this. We treat the stars, the HI and H ₂ gas as three gravitationally coupled components in the Galactic disk, and find the response of each component to the joint potential and thus obtain their vertical distribution in a self-consistent fashion. The effect of the joint potential is different for the three components because of their different velocity dispersions. We show that this approach cohesively and naturally explains the observed scaleheight distribution of all the three components, namely, the HI and H ₂ gas and the stars, in the region studied (2-12 kpc). This includes the constant scaleheight for the HI seen in the inner Galaxy. The effect of H ₂ dominates in the molecular ring region of 4-8.5 kpc, while that due to HI is dominant in the outer Galaxy.