Stationary models for the extraplanar gas in disc galaxies

The kinematics of the extraplanar neutral and ionized gas in disc galaxies show a systematic decline of the rotational velocity with height from the plane (vertical gradient). This feature is not expected for a barotropic gas, whilst it is well reproduced by baroclinic fluid homogeneous models. The problem with the latter is that they require gas temperatures (above 105 K) much higher than the temperatures of the cold and warm components of the extraplanar gas layer. In this paper, we attempt to overcome this problem by describing the extraplanar gas as a system of gas clouds obeying the Jeans equations. In particular, we consider models having the observed extraplanar gas distribution and gravitational potential of the disc galaxy NGC 891: for each model, we construct pseudo-data cubes and compare them with the H I data cube of NGC 891. In all cases, the rotational velocity gradients are in qualitative agreement with the observations, but the synthetic and the observed data cubes of NGC 891 show systematic differences that cannot be accommodated by any of the explored models. We conclude that the extraplanar gas in disc galaxies cannot be satisfactorily described by a stationary Jeans-like system of gas clouds.