Accretion-Driven Turbulence in the Circumgalactic Medium

TL;DR

This paper investigates how gas accretion drives turbulence in the circumgalactic medium (CGM), revealing that accretion amplifies turbulence to virial velocities, with implications for CGM energetics and observable signatures.

Contribution

It provides an analytic and simulation-based study showing accretion as the primary driver of turbulence in the CGM across certain halo masses and redshifts, establishing a turbulence-dominated regime.

Findings

Accretion amplifies turbulence velocities from ~10 km/s to ~100 km/s near the virial radius.

Inner CGM is dominated by turbulence with properties similar to isothermal supersonic turbulence.

Turbulence dissipation rate regulates accretion rate in halos with mass 10^{10}-10^{12} M_sun.

Abstract

Simulations suggest that turbulence is ubiquitous in the circumgalactic medium (CGM), though the source and properties of CGM turbulence is uncertain. Using analytic considerations and hydrodynamic simulations we study how CGM turbulence is driven by gas accretion, thus providing a baseline for additional turbulence driving processes such as galaxy feedback. We demonstrate that in halos with mass $\sim 10^{10}-10^{12} M_{\odot}$ at $0 < z < 2$, accretion amplifies mild turbulent velocities near the virial radius of $\sigma_t(R_{\rm vir}) \sim 10 \, {\rm km \, s^{-1}}$ to virial velocities at inner CGM radii, $\sigma_t(0.1 R_{\rm vir}) \approx v_{\rm vir} \sim 100 \, {\rm km \, s^{-1}}$. Rapid cooling at these inner radii further implies that thermal pressure support is small, and the gas is dominated by the cool and warm ($\sim 10^4-10^5 \, {\rm K}$) phases. Inner CGM energetics in these halos is thus dominated by turbulence, with gas density distributions and velocity structure functions similar to those seen in simulations of isothermal supersonic turbulence, rather than those seen in subsonically turbulent stratified media such as the ICM. The accretion rate in these systems is regulated by the turbulence dissipation rate, in contrast with being regulated by the cooling rate as in more massive halos. We argue that galaxy feedback is unlikely to qualitatively change our conclusions unless it significantly depletes the CGM or continuously injects high specific energy material ($\gg v^2_{\rm vir}$). Such `turbulence-dominated' CGM can be identified in observations via the predicted wide lognormal ionization distributions and large velocity dispersions in UV absorption spectra.