Modeling Photoionized Turbulent Material in the Circumgalactic Medium II: Effect of Turbulence within a Stratified Medium

TL;DR

This study uses chemodynamical simulations to explore how turbulence in a stratified circumgalactic medium affects ionization states, reproducing observed ionic ratios and column densities in star-forming galaxies.

Contribution

It introduces a detailed simulation of turbulent, stratified CGM that matches observed ionic signatures, highlighting turbulence's role in ionization and cooling processes.

Findings

Turbulence with ~40 km/s velocity dispersion reproduces observed ionic ratios.

Cooling and turbulent mixing suppress NVI/OVI ratio from equilibrium.

Higher resolution simulations confirm the robustness of results.

Abstract

The circumgalactic medium (CGM) of nearby star-forming galaxies shows clear indications of OVI absorption accompanied by little to no detectable NV absorption. This unusual spectral signature, accompanied by highly non-uniform absorption from lower ionization state species, indicates that the CGM must be viewed as a dynamic, multiphase medium, such as occurs in the presence of turbulence. Motivated by previous isotropic turbulent simulations, we carry out chemodynamical simulations of stratified media in a Navarro-Frenk-White (NFW) gravitational potential with a total mass of $10^{12}$ solar masses and turbulence that decreases radially. The simulations assume a metallicity of 0.3 solar, a redshift zero metagalatic UV background, and they track ionizations, recombinations, and species-by-species radiative cooling using the MAIHEM package. We compare a suite of ionic column densities with the COS-Halos sample of low-redshift star-forming galaxies. Turbulence with an average one-dimensional velocity dispersion approximately 40 km/s, corresponding to an energy injection rate of approximately $10^{49}$ erg/yr, produces a CGM that matches many of the observed ionic column densities and ratios. In this simulation, the NVI to OVI ratio is suppressed from its equilibrium value due to a combination of radiative cooling and cooling from turbulent mixing. This level of turbulence is consistent with expectations from observations of better constrained, higher-mass systems, and could be sustained by energy input from supernovae, gas inflows, and dynamical friction from dark matter subhalos. We also conduct a higher resolution run which yields smaller-scale structures, but remains in agreement with observations.