Galaxy Groups in Cosmological Sims: Central Galaxies & their surrounding CGM

Contrary to many stereotypes about massive galaxies, the observed systems are  diverse in their star formation rates, kinematic properties, and morphologies.  Studying how they evolve into and express such diverse characteristics is an important piece of the galaxy formation puzzle. Here, we focus on a subset of massive galaxies, the brightest group galaxies (BGGs).  We use a high-resolution cosmological suite of simulations based on the Romulus model, and compare simulated central galaxies in group-scale halos at 𝑧 = 0 to observed BGGs. Since most galaxy formation models are calibrated using measures that are strongly influenced by the properties and evolution of “normal”  Milky-Way like galaxies, this exercise is also an opportunity to test the limits of these models. The comparison encompasses the stellar mass-halo mass relation, various kinematic properties and scaling relations, morphologies, and the star formation rates.  We find Romulus BGGs that are early-type S0 and elliptical galaxies as well as late-type disk galaxies; we find Romulus BGGs that are fast-rotators as well as slow-rotators; and we observe galaxies transforming from late-type to early-type following strong dynamical interactions with satellites. In sum, we find that Romulus reproduces the full spectrum of diversity in the properties of the BGGs very well.  We also explore the properties of the CGM cocooning the BGGs.  We find that the CGM is multiphase and dynamic, with a wide spectrum of density perturbations. We investigate the origin of this multiphase structure and identify several cooling channels, including cooling flows and precipitation due to thermally unstable perturbations. We investigate and identify the drivers of these perturbations.  We emphasize that the Romulus simulations are only just scratching the surface in terms of probing the latter.  Moreover, we have also identified a problem:  We are finding a tendency towards lower than the observed fraction of quenched Romulus BGGs, with increasing halo mass.   The problem appears to be due to decreasing effectiveness of AGN feedback with increasing halo mass.  Examining some of the other galaxy formation models, we find that they too run into trouble on the same scale — but in an opposite way. I will conclude by discussing what we are to make of this.