Exploring the Milky Way Circumgalactic Medium in a Cosmological Context with a Semianalytic Model

Abstract

We combine the Santa Cruz semianalytic model (SAM) for galaxy formation and evolution with the circumgalactic medium (CGM) model presented in Faerman et al. to explore the CGM properties of L * galaxies. We use the SAM to generate a sample of galaxies with halo masses similar to the Milky Way (MW) halo, M vir ≈ 1012 M ⊙, and find that the CGM mass and mean metallicity in the sample are correlated. We use the CGM masses and metallicities of the SAM galaxies as inputs for the FSM20 model and vary the amount of nonthermal support. The density profiles in our models can be approximated by power-law functions with slopes in the range of 0.75 < a n < 1.25, with higher nonthermal pressure resulting in flatter distributions. We explore how the gas pressure, dispersion measure, O VI–O VIII column densities, and cooling rates behave with the gas distribution and total mass. We show that for CGM masses below ∼3 × 1010 M ⊙ photoionization has a significant effect on the column densities of O VI and O VIII. The combination of different MW CGM observations favors models with similar fractions in thermal pressure, magnetic fields/cosmic rays, and turbulent support and with M CGM ∼ (3–10) × 1010 M ⊙. The MW O VI column requires t cool/t dyn ∼ 4, independent of the gas distribution. The AGN jet-driven heating rates in the SAM are enough to offset the CGM cooling, although exact balance is not required in star-forming galaxies. We provide predictions for the column densities of additional metal ions—N V, Ne VIII, and Mg X.

Constraints on Cosmic Rays in the Milky Way Circumgalactic Medium from O viii Observations

“…Our constraints allow larger values of η for most of the cases in comparison to the isentropic model by Faerman et al (2020Faerman et al ( , 2022. In their recent work (Faerman et al 2022), they have considered three cases for their model: (1) with only thermal pressure, i.e., α = (P nth /P th ) + 1.0 = ((P CR + P B )/ P th ) + 1.0 = 1.1; (2) with the standard case as considered in Faerman et al (2020), i.e., α = 2.1; and (3) with significant nonthermal pressure where α = 2.9. Note that the abovementioned values of α are the values at the outer boundary and the profiles of α will follow a declining pattern as radius decreases (see the blue curve in the right panel of Figure 2 in Faerman et al 2020).…”

“…This diminished value of thermal pressure would also significantly change the abundance of high ionization species and thereby jeopardize the interpretation of their column densities. In a recent work by Faerman et al (2022), they modified their previous isentropic model (Faerman et al 2020) with significant nonthermal pressure (α = (P nth /P th ) + 1.0 = 2.9) and found that the gas temperature in the central region for the nonthermal model becomes lower than the value at which O VIII collisional ionization equilibrium temperature peaks, which in turn results in a lower O VIII column density. At low CGM masses, photoionization compensated for this effect by the formation of O VIII at larger radii, but for large gas masses and large mean densities (as in the MW), the photoionization effect is negligible and the total O VIII column density is low.…”

Can Cooling and Heating Functions Be Modeled with Homogeneous Radiation Fields?

“…Radiation fields can ionize and change the energy levels of ions, thus affecting which atomic transitions can emit or absorb photons. Previous works indicate that the presence of ionizing radiation fields can significantly modify the cooling and heating functions of gas (e.g., Wiersma et al 2009;Gnedin & Hollon 2012;Faerman et al 2022;Romero et al 2021).…”

Figuring Out Gas & Galaxies In Enzo (FOGGIE) VI: The Circumgalactic Medium of $L^*$ Galaxies is Supported in an Emergent, Non-Hydrostatic Equilibrium

“…However, recent simulation work that investigates the degree of hydrostatic equilibrium obtained by the CGM of Milky Way mass galaxies finds that there is a significant amount of nonthermal pressure support, such that a purely hydrostatic equilibrium model is not a good description of the halo gas (Oppenheimer 2018;Lochhaas et al 2020). Faerman et al (2022) applied the analytic model of Faerman et al (2020) to a semi-analytic model for galaxy formation and also found that non-thermal support was neces-sary to fit the observations. These recent works suggest that perhaps the hydrostatic hot halo model needs to be modified in order to describe the complexities of the observed and simulated CGM.…”