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

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 approximate to 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) similar to 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 lcl following bNT mu l(cl)(0.3)over three decades in spatial scale from l(cl) approximate to 1 pc to l(cl) approximate to 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 epsilon approximate to 0.003 cm(2) s(-3) and a dissipation timescale of less than or similar to 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 epsilon 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.