Max Gronke

Astrophysical gases such as the interstellar-, circumgalactic- or intracluster-medium are commonly multiphase, which poses the question of the structure of these systems. While there are many known processes leading to fragmentation of cold gas embedded in a (turbulent) hot medium, in arXiv:2209.00732 , we focus on the reverse process: coagulation.

Coagulation is often seen in wind-tunnel and shearing layer simulations, where cold gas fragments spontaneously coalesce.

Perturbed, large ($ r > {\rm min}(c_{\rm s}t_{\rm cool})$) gas clouds can pulsate which leads to further cooling and pulsations (see the videos here). The associated mass transfer from the hot to the cold medium can be written as $ \dot m \sim A \rho_{\rm hot} v_{\rm mix}$ where $A\sim r^2$ is the surface area of the cloud, $\rho_{\rm hot}$ the density of the hot medium, and $v_{\rm mix}$ a characteristic velocity (with interesting physics in there, see arXiv:2008.12302 ).

Because of this growth, the hot medium at a distance $d$ has to move towards the cold cloud with velocity $v_{\rm hot}\sim v_{\rm mix} (r/d)^{\alpha}$ where $\alpha=2$ or $1$ in 3D or 2D, respectively.

Small cold gas droplets can become entrained in this hot gas bulk motion and, thus, coalesce with the big gas cloud.

This is an example of this process:

Or here, with a higher overdensity (of 50):

As stated above, in 3D $\alpha=2$ and thus the coagulation is much slower:

In an ensemble of clouds this implies they will also coagulate:

And because more clouds mean a greater mass growth this means faster coagulation:

In 3D, an ensemble of clouds will also merge - but slower:

Reality, is, however, turbulent which disperses the cold gas clouds, i.e., is competing with the coagulation effect.

However, if the turbulence is weak enough the coagulation can “win” and (somewhat) compensate fragmentation (here a Mach number of 0.1 versus 1 above):

In arXiv:2209.00732 , we develop a simple analytical model describing coagulation - and the threshold when it “wins”. Check out the paper! :)