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:

SILCC with cosmic rays

Cosmic rays (CRs) are an important energy component in the interstellar medium. Being high-energy particles their coupling to the gas is not via direct particle-particle interactions as in the case for thermal gas because their cross section is very small. Instead CRs couple via the magnetic field. By gyrating around magnetic field lines and scattering off of magnetic irregularities, CRs transfer energy and momentum to the magntic field. Together with the approximation of ideal MHD, which freezes the magnetic field lines to the gas, CRs can effectively transfer energy and momentum to the gas and accelerate it. The interaction with the magnetic field allows them to move relative to the gas and effectively diffuse or stream through the ISM. By this mechanism the CR energy can be deposited far away from their production sites. The small effective cross sections also result in low cooling efficiencies for CRs with a momentum of a few GeV/c, which carry most of the CR energy in the ISM. As a result, CRs can prodive a long-lived energy reservoir with a vertical stratification in the galactic disk that can lauch galactic outflows.
We included CRs as a relativistic fluid into the SILCC setup in the diffusion-advection approximation. We investigate the impact of CRs in driving outflows in two papers:

• Girichidis et al. 2016, ApJL 816, 19
• Girichidis et al. 2018a, MNRAS, 479, 3042

We find that CRs are able to drive stable warm and smooth outflows with mass loading factors (ratio of outflow to star formation rate) of order unity. The simulation data (after acceptance of the papers) is publicly available at the SILCC data website, hosted at the Max-Planck Computing and Data Facility in Garching (MPCDF). An expample video for CR-driven outflows is shown in here:

or as mov: mov (7MB)

Movies from Girichidis et al. 2018a (column density)

large vertical extent

upper part

lower part

Movies from Girichidis et al. 2016 (density, density, CR energy)

Movies from Girichidis et al. 2016 (density, density)

Magnetic fields in the interstellar medium

Magnetic fields are everywhere in the interstellar medium and are locally strong enough to modify the dynamics. Diffuse warm gas in the ISM is very likely to be supported by magnetic pressure. In dense, star forming regions the magnetic fields can be very strong such that the gas flow is channeld by the field structure. Using three-dimensional magneto-hydrodynamical simulations we study the impact of magnetic fields on the ISM, in particular the transition from diffuse warm atomic to dense colde molecular gas. In addition, we investigate the oriantation of the magnetic field compared to the orientation of the gas structures and the gas flows.

I published two papers related to the magnetic fields in the ISM (the first is linked the second will be there soon):

• Girichidis et al. (2018) MNRAS 480, 3511G

The following video shows the total column density of a fraction of the galactic disc egde-on (top) and face-on (bottom) for three different magnetic fields; no field (left-hand panels), a 3μG field (middle panel), and a 6μG field (right-hand panel).

other video formats

There are two main differences between the simulations. The first is that the runs including magnetic fields have smaller density contrasts. The dense gas is embedded in larger cloud complexes rather than in locally isolated clouds. The second is that dense gas forms later because the additional magnetic pressure supports the gas against fast compression and the resulting faster cooling. This can be nicely seen in the formation of molecular gas here:

and some other video formats