Vertical scaleheights of stars and gas in the galaxy

Abstract

The vertical scaleheight of atomic hydrogen gas is observed to be nearly constant with radius in the inner Galaxy « 8.5 kpc). This has been a long-standing puzzle (Oort 1962) because the gas scaleheight should increase exponentially with radius as a response to the decreasing gravitational potential of the stellar disk alone. We treat the stars, H I and H2 as three gravitationally coupled components in the Galactic disk and find their scaleheights as a function of galactocentric radius. This approach not only explains the near-constancy of H I scaleheight but also reproduces the observed scaleheight variation for H2 and stars in the Galaxy.