The Cosmic Ultraviolet Baryon Survey: Empirical Characterization of Turbulence in the Cool Circumgalactic Medium

Abstract

This paper reports the first measurement of the relationship between turbulent velocity and cloud size in the diffuse circumgalactic medium (CGM) in typical galaxy halos at redshift z ≈ 0.4–1. Through spectrally resolved absorption profiles of a suite of ionic transitions paired with careful ionization analyses of individual components, cool clumps of size as small as l cl ∼ 1 pc and density lower than n H = 10−3 cm−3 are identified in galaxy halos. In addition, comparing the line widths between different elements for kinematically matched components provides robust empirical constraints on the thermal temperature T and the nonthermal motions b NT, independent of the ionization models. On average, b NT is found to increase with l cl following b NT ∝ l cl 0.3 over three decades in spatial scale from l cl ≈ 1 pc to l cl ≈ 1 kpc. Attributing the observed b NT to turbulent motions internal to the clumps, the best-fit b NT–l cl relation shows that the turbulence is consistent with Kolmogorov at <1 kpc with a roughly constant energy transfer rate per unit mass of ϵ ≈ 0.003 cm2 s−3 and a dissipation timescale of ≲100 Myr. No significant difference is found between massive quiescent and star-forming halos in the sample on scales less than 1 kpc. While the inferred ϵ is comparable to what is found in C iv absorbers at high redshift, it is considerably smaller than observed in star-forming gas or in extended line-emitting nebulae around distant quasars. A brief discussion of possible sources to drive the observed turbulence in the cool CGM is presented.

An Ensemble Study of Turbulence in Extended QSO Nebulae at z ≈ 0.5–1

“…This estimated range for ò with our sample is comparable to the measurements for Hα filaments in core regions of nearby cool-core clusters (Li et al 2020;Ganguly et al 2023) and molecular clouds in nearby H II regions (e.g., Hennebelle & Falgarone 2012). Much lower estimates of ò ≈ 10 −7 -10 −3 cm 2 s −3 were obtained for CGM cool clumps probed through absorption line spectroscopy (Rauch et al 2001;Chen et al 2023a), and a Milky Way high-velocity cloud (HVC; Marchal et al 2021).…”

“…σ pos is quantified as the standard deviation of the lineof-sight velocity from spaxels included in the VSF measurements (see discussion in Section 2.3 and the velocity maps in Figures 9-16), and 〈σ los 〉 is the mean line width (obtained through a single-component Gaussian fit) for the same set of spaxels. We show results for both [O II] λλ3727,3729 and [O III]λ5008 emission as they can differ in σ pos and 〈σ los 〉 due to the different Chen et al (2023a). The turbulent heating rates in the QSO nebulae, the cool-core cluster ICM, and the star-forming molecular clouds are on average ∼1000 times higher than that in Complex C and cool gas clumps probed in absorption.…”

“…It can be seen that the turbulent heating rates in the QSO nebulae, the cool-core cluster ICM, and the star-forming molecular clouds are on average ∼1000 times higher than that in the Milky Way HVC and cool gas clumps probed in absorption. Given that both Complex C and cool absorption clumps are expected to be in relatively quiescent, undisturbed environments (Chen et al 2023a), a possible explanation for this difference is that feedback due to star formation and AGN activities can significantly elevate the turbulent energy in the gaseous halos. However, caveats remain in this interpretation.…”

“…Until recently, observing turbulence in circumgalactic/intergalactic gas has had to rely on two approaches employing high-resolution absorption line spectra of background QSOs. One approach is to observe line widths of ions with different masses and isolate the turbulent contribution to the velocity profile along the line of sight (e.g., Rauch et al 1996;Rudie et al 2019;Qu et al 2022;Chen et al 2023a). Alternatively, if multiple lines of sight (e.g., to gravitationally lensed QSO images) are available, turbulence can be measured as a function of transverse separation between the lines of sight to form the structure functions for the line-of-sight velocities (Rauch et al 2001).…”

“…Marchal et al (2021) measured Q turb for a bright concentration location in the HVC Complex C at scales ≈6-28 pc, as shown by the green shaded region. The gray points show the results for CGM cool clumps at scales ≈10 pc-1 kpc based on absorption line measurements presented inChen et al (2023a). The turbulent heating rates in the QSO nebulae, the cool-core cluster ICM, and the star-forming molecular clouds are on average ∼1000 times higher than that in Complex C and cool gas clumps probed in absorption.…”