The vertical distribution and kinematics of H I and mass models of the galactic disk

We present full modeling of tangent point emission of H I as seen in the 21 cm transition in the inner Galaxy. This is done in order to measure the scale height and velocity dispersion of H I as a function of Galactic radius. The scale height and the velocity dispersion are used to consider the vertical equilibrium of the gas and constraints on the forces acting to keep it in such an equilibrium. Besides the scale height and velocity dispersion of H I, the rotation curve and the z-centroid of the atomic layer are also derived. The model used takes into account emission from a large path length along the line of sight, corresponding to an interval (ΔR) of typically 1 kpc or smaller in galactic radius; and is parameterized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion. These parameters are assumed to be constant over the interval ΔR. This model is then fitted to the 21 cm surveys of Weaver & Williams, Bania & Lockman, and Kerr et al. to determine the best-fit parameters. The terminal velocity values are found to be in good agreement with previous measurements. The velocity dispersion is constant with radius at the theoretically expected value of 9-10 km s^-1. The Gaussian scale height of the H I layer increases with Galactocentric radius. The centroid of the layer deviates significantly from z = 0. Apart from small local variations, the four quantities measured: terminal velocity V_T, velocity dispersion σ_v, scale height σ_z and the centroid z₀ show similar behavior in the first and the fourth quadrants. On balancing the turbulent pressure support of the layer against the disk gravitational potential, we confirm that additional support is needed for the H I layer. The radial profile of the reduced midplane mass density is an exponential with a scale length of 3.3 ± 0.3 kpc. This picture is consistent with a constant mass-to-light ratio of the disk, and extra support for the H I layer. This extra support (not provided by the turbulent pressure) should be constant with radius in the inner Galaxy.