Stationary models for the extra-planar gas in disc galaxies

TL;DR

This study investigates whether stationary Jeans models can explain the vertical rotational velocity gradients of extra-planar gas in disc galaxies, finding that such models are insufficient to fully reproduce observations.

Contribution

It introduces a Jeans-equation-based modeling approach for extra-planar gas and demonstrates its limitations in matching observed galaxy data.

Findings

Jeans models qualitatively reproduce velocity gradients

Synthetic data cubes differ systematically from observations

Stationary Jeans models are inadequate for describing extra-planar gas

Abstract

The kinematics of the extra-planar neutral and ionised gas in disc galaxies shows 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 $10^5$ K) much higher than the temperatures of the cold and warm components of the extra-planar gas layer. In this paper, we attempt to overcome this problem by describing the extra-planar gas as a system of gas clouds obeying the Jeans equations. In particular, we consider models having the observed extra-planar gas distribution and gravitational potential of the disc galaxy NGC 891: for each model we construct pseudo-data cubes and we compare them with the HI 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 extra-planar gas in disc galaxies cannot be satisfactorily described by a stationary Jeans-like system of gas clouds.