Molecular scale height in NGC 7331

Abstract

Combined Poisson's-Boltzman equations of hydrostatic equilibrium were set up and solved numerically for different baryonic components to obtain the molecular scale height as defined by the Half Width at Half Maxima (HWHM) in the spiral galaxy NGC 7331. The scale height of the molecular gas was found to vary between $\sim 100-200$ pc depending on the radius and assumed velocity dispersion. The solutions of the hydrostatic equation and the observed rotation curve were used to produce a dynamical model and consequently a simulated column density map of the molecular disk. The modelled molecular disk found to match with the observed one reasonably well except at the outer disk regions. The molecular disk of NGC 7331 was projected to an inclination of 90$^o$ to estimate its observable edge-on thickness (HWHM), which was found to be $\sim 500$ pc. With an HWHM of $\sim 500$ pc, the observed edge-on extent of the molecular disk was seen to be $\sim$ 1 kpc from the mid-plane. This indicates that in a typical galaxy, hydrostatic equilibrium, in fact, can produce a few kilo-parsec thick observable molecular disk which was thought to be difficult to explain earlier.