The circumgalactic medium (CGM) is the gaseous component of dark matter-dominated haloes, surrounding all galaxies. It plays a major role in feeding the embedded galaxies, providing fuel for future that formation. Galactic outflows driven by, for example, supernovae and supermassive black holes, heat and enrich the CGM. This, in turn, affect the accretion of gas onto galaxies. Studying the CGM thus allows us to better understand galaxy growth and feedback.
However, standard cosmological simulations are designed to have adaptive resolution, which increases with density. Where the density is high, inside galaxies, the resolution is high, but where the density is lower, in the CGM, the resolution also decreases. We previously designed simulations that uniformly increased the resolution in the CGM, finding various properties were well converged, but finding much more cool gas far out in the halo. This method has unfortunately been pushed to its limits due to computational and memory constraints.
Therefore, we designed a hybrid method: Most of the CGM has the same uniform resolution, but the cool, dense CGM is assigned an eight times better mass resolution (corresponding to two times better spatial resolution). We first ran various simulations to test different methods how to set the threshold between the two resolution levels and settled on the one that performed best. We then used this method to run the final, highest resolution simulation with a resolution of about 500 parsec (1500 lightyears) in the hot CGM and 250 pc (750 lightyears) in the cool CGM of a Milky Way-mass galaxy. This helps us to understand the effect of resolution in the multiphase CGM, the origin of cool gas in the hot halo, and the impact on gas accreting onto the central galaxy. What the simulation looks like can be seen in the image, which shows the density in an infinitesimally thin slice through the simulation. The Milky Way-like galaxy is shown edge-on in the centre. Strong outflows push out material to large distances beyond 200 kiloparsec (600,000 lightyears) above and below the galaxy disc. Small, dense clouds and filaments can be seen around the disc and far out into the halo.
Our work primarily focused on the CGM of Milky Way-mass galaxies, but we did not know how our results would generalize across the galaxy population. Massive galaxies and their CGM can behave quite differently from Milky Way-like systems. The amount of gas in the interstellar medium and the star formation rate are comparatively low and feedback is dominated by active galactic nuclei (powered by supermassive black holes). The virial temperature of the halo is much higher and thus the cooling rate of the hot gas is much lower. Despite this, cool gas has been found around massive galaxies in observations. To understand whether or not this agrees with simulations, we need to quantify the effect of numerical resolution on the predicted cool gas observables.
We therefore built a sample of 4 haloes, 2 of which are three times more massive than the halo of the Milky Way and 2 of which are ten times more massive and can be classed as galaxy groups. We simulated these at standard resolution, in which the size of resolution elements decreases with decreasing density, and with a uniform resolution of 1 kpc. This increases the number of resolution elements in the CGM by 2 orders of magnitude. We find that the covering fraction of cool gas traced by neutral hydrogen is increased, though by a smaller amount than in our previous Milky Way-mass simulations. Most of it appears to have been stripped from satellite galaxies, but we are investigating what fraction of the cool gas is stripped and what fraction is cooling out of the CGM. We will also use these simulations to study turbulence and shocks in galaxy haloes with the full cosmological context at unprecedented resolution.
