Cosmic accretion through massive haloes

Cosmic accretion through massive haloes

During the hierarchical gravitational collapse of primordial dark matter overdensities, the baryonic content of haloes grows by accreting gas and stars from the intergalactic medium (IGM). This gas is what then feeds the formation of a galaxy at the halo’s centre. The mechanism by which this feeding occurs is not fully understood, as it involves a complex interplay between cooling, shock heating, and feedback from a variety of physical processes in the galaxy itself. All of these processes act in concert in the circumgalactic medium (CGM), which thus plays host to the baryon cycle that regulates the process of galaxy formation.

Representation of the AREPO Voronoi shown for the ‘base refinement’ (left) and ‘shock refinement’ (right) runs.

In the `classical’ analytic model of accretion onto massive dark matter haloes gas shock heats as it infalls, forming a hot atmosphere. Recently however, a significant amount of observational evidence points to a more complex, multi-phase CGM with a substantial cold component across a wide variety of galaxy masses.

In massive haloes, cold gas is thought to bypass accretion shocks and penetrate deep into haloes via filaments that deliver cold gas through the CGM. Numerically this is difficult to model, as the CGM is primarily made up of low-density, multi-phase gas that cosmological simulations do not focus resolution on. Accretion shocks themselves are also numerically broadened in simulations which may affect gas shock heating and energy dissipation at the shocks. These effects can potentially lead to inaccurately modelled properties of CGM gas.

From top to bottom: Maps of average  Mach number, temperature, metallicity and turbulent velocity in the halo. The left column shows the ‘base refinement’ run and the right column shows the ‘shock refinement’ run. The black dashed line indicates the virial radius.

Within this project we introduce a novel computational technique that automatically and on-the-fly boosts numerical resolution around shocks during the simulation. This allows us to better resolve the accretion shocks at the boundary of the CGM and IGM in massive haloes. This is the region that determines if primordial gas filaments can penetrate into the hot halo, and how turbulent motions are generated in the wake of curved shocks.

To do this, we resimulate a massive, high-redshift galaxy cluster progenitor with the successful physical model employed in the FABLE suite of simulations using the moving-mesh code AREPO. Figure 1 shows how our ‘shock refinement’ scheme boosts resolution significantly in and around cosmic filaments and at the accretion shocks, compared with AREPO‘s ‘base refinement’ scheme which is commonly adopted in the literature.

Figure 2 demonstrates clearly the physical effect of our novel method in maps of  Mach number, temperature, metallicity and turbulent velocity. The top panels show that with the ‘shock refinement’ scheme active we see sharper and better defined accretion shocks around the halo. The second row illustrates how the amount of cold gas penetrating into the hot halo is increased with ‘shock refinement’, leading to a much more multi-phase CGM more in line with observations. These cold gas filaments are shown to also have a low metallicity in the third row, indicating how our ‘shock refinement’ scheme allows primordial gas to more easily penetrate deep within the virial radius of the halo. Finally we see that higher numerical resolution and better defined curved accretion shocks lead to a significant boost in turbulent velocities in the halo, which is important when comparing models to the next-generation of X-ray observations from eROSITA and Athena.

3D surface showing accretion shocks from two different orientations, coloured by Mach number. Cosmic filaments are shown in grey.

Having a well-resolved accretion shock also allows us to create a 3D shock surface, as shown in Figure 3. Here we colour the surface with the Mach number, indicating that a significant amount of the surface area has no associated accretion shock at all. To explain this we also plot the positions of inflowing gas filaments in grey, whose points of intersection with the halo correspond very well with the unshocked regions of the shock surface. This visually demonstrates the filamentary feeding of galaxy growth, and how even in massive dark matter haloes with a hot gaseous atmosphere, cold, primordial gas can be delivered directly to the central galaxy.

Improved physical models and a better understanding of the CGM and the feeding of galaxies are important for a wide variety of current and upcoming observations, including those by MUSE and the Keck Cosmic Web Imager and enhanced Sunyaev-Zel’dovich observations from SPT and ACTpol.

The simulations for this project were carried out on COSMA7 in Durham and Peta4 in Cambridge as part of the DiRAC project dp012.