Windtunnels

There is strong observational evidence that the gas flowing out of galaxies contains not only hot dilute gas but also cold molecular gas. This fact poses the question of whether the molecular can form within the outflo, i.e. above the galacitc disc, or whether it forms in the disc and is then transported to large altitudes above the disc via the hot gas or hot winds coming from star forming regions. We inverstigate this problem with idealised simulations, in which we expose a warm, dense cloud to a hot, low density wind. We follow how the wind can accelerate, destroy or compress the cloud. There are a number of paramters involed and we mainly focus on the density of the cloud and the magnetic field configuration in the wind. The movie below shows the comparison between three different wind configurations, namely a non-magnetic wind, a magnetic field that is orianted parallel to the direction of the wind and a field perpendicular to the wind

We find that magnetic fields perpendicular to the wind velocity are able to best accelerate the cloud, keep the cloud compact and thereby allow the formation of a coherent dense molecular phase. We find that it might be possible to form molecular gas inside the outflows of galaxies.

A second important fact is that self-gravity plays an important role, even if the initial conditions do not suggest that self-gravity matters. The continous compression of gas via the wind combined with radiative cooling of the gas gradually increases the density, decreases the Jeans mass and thus forms regions that are dominated by gravitational effects.

Here are more videos showing the different cloud densities and the chemical composition: