Understanding the Galaxy-Halo Connection Using the Simba Simulation

Understanding the Galaxy-Halo Connection Using the Simba Simulation

Romeel Davé, PI

The Concordance Cosmological Model predicts the properties and distribution of dark matter halos with exceptional precision.  However, how these halos come to be populated with the diversity of galaxies that we observe remains unclear.  A curious recent observational result shows that at a given halo mass, quenched galaxies (i.e. those not forming stars currently) have lower stellar masses than star-forming galaxies.  Many models have attempted to explain this dichotomy, with mixed success.  In general, the difference is attributed to assembly bias, i.e. that halos hosting quenched galaxies assembled earlier and hence achieved a higher mass by today.  However, this does not pinpoint the physical origin of the dichotomy in star-forming properties; indeed, all models yield halo assembly bias yet can have qualitatively different predictions regarding the host galaxies!

In a paper published in Nature Astronomy (Cui et al., NatAs 2021, 5, 1069), Weiguang Cui and Romeel Davé showed that the Simba galaxy formation simulation quantitatively reproduces the observed trend.  This was a major success for Simba, which differs from other such simulations specifically by the mechanisms in which it quenches galaxies.  Taking advantage of the physical information provided by the simulation, we further explored the physics driving this dichotomy in galaxy properties.  We showed that Simba indeed produced halo assembly bias, but the key aspect that made it successful was the way that assembly bias interacted with Simba’s unique jet feedback model. 

In Simba, early halo assembly leads to increased cold gas accumulation at early epochs, resulting in longer sustained star formation and a bluer galaxy today.   Hence early-formed halos host star-forming galaxies.  Late-forming halos, meanwhile, accumulate less gas.  The key is that in Simba, galaxy quenching is driven by a low gas accretion rate onto the central black hole, which triggers radio jets and X-ray feedback that heat the surrounding gas.  Late-formed halos, having less gas, trigger jet and X-ray mode feedback earlier, shutting down the growth of stars and resulting in a galaxy with lower stellar mass by today.  Using Simba’s variant runs that switch off specific AGN feedback modes, we showed that without jet or X-ray feedback, we obtain the opposite trend to obsevrations, that at a given halo mass quenched galaxies are more massive!  Adding jets moved the results towards the observations, but only with both jet and X-ray feedback was Simba able to fully reproduce the observations.  This is a stunning success for Simba’s novel physics for black hole growth and feedback, and provides a springboard for using this observational relation to constrain models of black hole feedback.