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The vertical scaleheight of the atomic hydrogen gas shows a remarkably flatdistribution with the galactocentric radius in the inner Galaxy. This has beena long-standing puzzle (Oort 1962) because the gas scaleheight should increasewith radius when treated as responding to the gravitational potential of theexponential stellar disk. We argue that the gravitational force of themolecular and atomic hydrogen gas should also be brought into the picture toexplain this. We treat the stars, the HI and H_2 gas as three gravitationallycoupled components in the Galactic disk, and find the response of eachcomponent to the joint potential and thus obtain their vertical distribution ina self-consistent fashion. The effect of the joint potential is different forthe three components because of their different velocity dispersions. We showthat this approach cohesively and naturally explains the observed scaleheightdistribution of all the three components, namely, the HI and H_2 gas and thestars, in the region studied (2-12 kpc). This includes the constant scaleheightfor the HI seen in the inner Galaxy. The effect of H_2 dominates in themolecular ring region of 4-8.5 kpc, while that due to HI is dominant in theouter Galaxy.Comment: 15 pages, 6 .EPS figures, 2 tables, Astron. & Astrophys, Vol 394,  89-9

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