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A unified model for the co-evolution of galaxies and their circumgalactic medium

How do turbulence and atomic cooling physics shape the circumgalactic medium and galaxy formation?

Published onDec 08, 2022
A unified model for the co-evolution of galaxies and their circumgalactic medium

Illustration of our new time-dependent, two-zone model connecting galaxy evolution to the circumgalactic medium through mass, metal, and energy flows (Figure 1 from Pandya et al. 2022). Credit: Lucy Reading-Ikkanda, Graphic Artist at the Simons Foundation

This figure from Pandya et al. (2022) illustrates a new time-dependent, two-zone model for connecting mass, metal, and energy flows between galaxies and their circumgalactic medium (CGM). Our approach involves setting up a system of coupled ordinary differential equations (ODEs) to predict the time evolution of eight state variables associated with each galaxy—CGM system: the total mass, metal mass, thermal energy, and turbulent kinetic energy of the CGM, and also the masses and metal masses of the interstellar medium (ISM) and long-lived stellar population of the central galaxy. The density and temperature structure of the CGM are assumed to follow power laws, and we envision that both cosmic accretion and supernova-driven winds stir large-scale turbulent eddies throughout the CGM. The turbulence accomplishes two things: (1) it provides pressure support for the CGM even when it has a very short cooling time, as in dwarfs and high-redshift galaxies, and (2) its dissipation leads to a heating term for the CGM that limits ISM accretion and star formation.

The right-hand side of our ODE system (see Equations 1-8 of the paper) has several individual terms that account for cosmic accretion, radiative cooling, turbulence dissipation, star formation, supernova-driven winds that heat and stir turbulence in the CGM, and large-scale outflows into the intergalactic medium when the CGM becomes overpressurized. Each of these terms has a physically-motivated functional form, and there are several free parameters that account for our ignorance of the relevant underlying physics. Our paper demonstrates that hydrodynamical simulations can constrain both the functional forms and values of our free parameters. A companion paper by Carr et al. (2022) shows that the stellar-to-halo-mass relation inferred from observations can also constrain the purely thermal limit of this model.

As an initial testbed, we use the FIRE-2 cosmological hydrodynamical “zoom-in” simulations (Hopkins et al. 2018) to calibrate our model parameters. Running our model on the merger trees of the individual FIRE-2 dark matter halos leads to predictions for mass assembly histories and baryon cycles that are in remarkable agreement with measurements of various bulk properties and gas flow rates directly from the simulation particle data.

The most striking prediction of our model is that the CGM should undergo a global “phase transition” from a cool, turbulence-supported phase at early times to a roughly virial-temperature, thermal pressure-supported phase at later times. Analyzing the equilibria of our ODE system reveals that the epoch of this “thermalization” is determined when CGM heating balances radiative cooling, with the former being sensitive to the specific energy of galactic winds and the largest eddy turnover time while the latter depends on the physics of atomic cooling and the ever-decreasing mean cosmic density (see the supplementary movies corresponding to Figure 14 of our paper). The CGM thermalizes at very early times for ultrafaint dwarfs and at very late times, if at all, for classical dwarfs, while Milky Way-mass halos undergo the CGM phase transition at z~0.5 with our assumed FIRE-2 parameters.

The novelty of our approach is in self-consistently tracking energy flows through the CGM and linking these directly to the galaxy formation process in a time-dependent way. It complements existing approaches for modeling the galaxy—CGM connection in two important ways: (1) it provides a framework for understanding hydrodynamical simulations whose growing complexity and computational cost demand simpler models like ours to help interpret, and (2) it predicts the time-dependent global conditions of the galaxy—CGM system, which can help inform instantaneous 1D physical models like steady-state cooling flow solutions and hydrostatic equilibrium and precipitation models that describe the detailed structure of the CGM at a single instant. Our successful initial application in reproducing the evolution of individual simulated galaxies and their CGM suggests that this approach will be a promising path to continue to build on.

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