Constraining Circumgalactic Turbulence with QSO Absorption-line Measurements

Abstract

Our knowledge of the circumgalactic medium (CGM) is mostly based on quasar absorption-line measurements. These have uncovered a multiphase medium that is likely highly turbulent, but constraints of this turbulence are limited to measurements of the non-thermal width of absorption-line components ($b_{turb}$) and the line-of-sight velocity dispersion between components ($\sigma_{LOS}$). Here we analyze a suite of CGM simulations to determine how well these indirect measures are related to the underlying CGM. Our simulations track the non-equilibrium evolution of all commonly observed ions, and consist of two main types: small-scale simulations of regions of homogeneous CGM turbulence and global simulations of inhomogeneous turbulence throughout a galactic halo. From each simulation, we generate mock spectra of Si II, Si IV, C IV, and O VI, which allow us to directly compare $b_{turb}$ and $\sigma_{LOS}$ to the true line-of-sight turbulence ($\sigma_{1D}$). In the small-scale simulations, $b_{turb}$ is only weakly correlated with $\sigma_{1D}$, likely because it measures random motions within individual warm CGM clouds, which do not sample the overall random motions. Meanwhile, $\sigma_{LOS}$ and $\sigma_{1D}$ are strongly correlated, with $\sigma_{1D}\approx\sigma_{LOS}+10$ km s$^{-1}$ in the densest regions we simulated, though, the strength of this correlation depended weakly on the gas phase being probed. Our large-scale simulations also indicate that $b_{turb}$ and $\sigma_{1D}$ are largely uncorrelated, and that $\sigma_{1D}\approx\sigma_{LOS}+10$ kms$^{-1}$ on average, although it varies along individual sightlines. Moreover, the $\sigma_\mathrm{LOS}$ distributions from our global simulations are similar to recent observations, suggesting that this quantity may provide useful constraints on circumgalactic turbulence regardless of the axis probed.