Resolving the C/IGM in Cosmological Simulations

The IGM contains most of the baryons in the Universe and sets the boundary conditions for gas flows into galactic halos. However, resolving it in cosmological simulations is notoriously difficult, since resolution elements typically have fixed mass yielding poor spatial resolution in low density regions.

My collaborators and I in the HITS-Yale Program in Astrophysics, in a paper led by Freeke van den Voort, have pioneered a novel method to greatly enhance the resolution in the CGM, without affecting the resolution in the galaxy and without prohibitive computational cost.  In a pilot study, we used this method to simulate a Milky-Way mass halo to z = 0, revealing much more small scale structure and increased HI in the CGM, in better agreement with observations. 

 

However, the IGM lies outside the “zoom-in” region of these simulations and is still poorly resolved. I have tackled this issue by running the highest resolution cosmological simulation to date of a large patch of the IGM, zooming-in on a > Mpc scale cosmic filament connecting two massive halos at z~2 using the moving-mesh code AREPO. This simulation reveals the growth of large-scale structure and the cosmic web in exquisite detail. In a paper published in ApJL, I demonstrated how the growth of large-scale structure generates strong shocks in the cosmic web, leading to its “shattering” via thermal instabilities which are unresolved in standard simulations. This results in a multiphase medium of ~kpc scale cold clouds embedded in a hot sheet, large fluctuations in the HI column density not associated with underlying fluctuations in the dark matter, and metal-free
Lyman Limit Systems resembling observations.

Several press releases associated with this work can be found at the links below

* Finding a Cosmic Fog Within Shattered Intergalactic 'Pancakes'

* Most Detailed and Beautiful Cosmic Fog Simulation

* Eurekalert

The multiphase IGM. Shown are temperature maps of a cosmological zoom-in simulation of the IGM surrounding two massive halos, which lie in a cosmic sheet shown edge-on and face-on in the left and right panels respectively. The black box marks the region shown in the figure below. The growth of large-scale structure triggers a shock, visible in the edge-on view. Most halos, marked with circles, lie along cold filaments, while the sheet in between is multiphase with hot and cold regions. Adapted from Mandelker et al. 2019b

cosmo_refinement.png

Enhanced resolution in the C/IGM. The left-two panels show the gas density surrounding the region marked in  the figure above, in a slice through the sheet mid-plane, in simulations with enhanced (left) and standard (right) IGM resolution. Adapted from Mandelker et al. 2019b. The right-two panels show the gas density in a slice through simulations of a MW halo at z = 0, with enhanced (left) and standard (right) CGM resolution. Adapted from van de Voort et al. 2019. In both cases, enhanced resolution leads to a multiphase medium with significantly more dense and neutral gas