Jamie Sullivan wrote:
The standard model of cosmology (ΛCDM
) tells us the matter content of the universe with exquisite precision. Matter can be broken up into two chunks: normal, visible, matter including galaxies, stars, and gas that we know and love in the night sky, and the other much larger piece – invisible dark matter. The visible matter is referred to as “baryonic” (as in, made of baryons
), and comes along with all the rich physics of fluids, thermodynamics, and radiation. In contrast, dark matter (at least, the cold
kind) is extremely simple. Dark matter only interacts gravitationally, and this simple fact allows for very accurate simulation of dark matter dynamics using Newtonian gravity N-body
Dark-matter-only N-body simulations are excellent for describing the formation of dense dark matter halos
, where galaxy formation takes place, but do not tell us anything about galaxy formation or its effects, where normal baryonic matter plays a large role. To accurately capture the full physics at play, hydrodynamic cosmological simulations
are required to include gas, stars, and radiation (see Figure 1
). However, even these “full-hydro” simulations are fundamentally limited by the smallest length scales that can be resolved in the simulation (often of order 1 kpc – for reference, the closest star to the solar system is roughly 1 pc away). As a result, they cannot capture the full details of small-scale effects that drive gas outflows (or “feedback
“) via supernovae explosions
or Active Galactic Nuclei (AGN
). The modern solution to this is to use so-called sub-grid models, which approximately capture the effect of the small-scale physics that cannot be resolved.
Today’s paper is concerned with the effect of baryons – and particularly feedback – on the distribution of dark matter. You may have heard that we live in the age of precision cosmology
, and amazingly, cosmologists now need to worry about percent-level effects in their models of the dark matter density distribution at relatively small length scales. To reach this level of accuracy, models and simulations need to account for the impact of baryons on the dark matter distribution, which can be gravitationally disturbed by something like an AGN jet. Such disturbances are not accounted for in the dark-matter-only simulations widely used in cosmological analyses, usually because hydro simulations are astronomically more computationally costly than dark-matter-only ones. This difference in cost is so enormous that using only hydro simulations in cosmological analyses would be impossible. Faced with this dilemma, what do we do? ...