Evidence for large scale compressible turbulence in the ism of CSWA13, a star-Forming Lensed Galaxy at z = 1.87 with outflowing wind

Recently, Keerthi Vasan G. et al. (2024) presented spatially resolved observations of a wind outflow in CSWA13, a gravitationally lensed Star-Forming galaxy at z = 1.87. The gravitational lensing allowed for a substantially improved spatial and kinematic resolution of the wind and of the nebular gas. In this paper we take advantage of the resolved data to test for the existence of turbulence and to study its nature. We obtained the autocorrelation functions of the two velocity fields, and derived the spatial structure functions of the residual nebular and wind velocities along the major axis of the galaxy. The structure functions, of both velocity fields, reveal the existence of an underlying k −2 power spectrum scaling. This scaling suggests the existence of supersonic compressible turbulence. The autocorre- lation functions exhibit correlations over scales comparable to the total extent of the velocity fields. Thus, the turbulence is a large scale one. The turbulent timescale corresponding to the largest scale is about 200 Myr, an order of magnitude larger than the estimated age of the wind and of the young stars. This implies that the turbulence in the ISM formed well before the wind and the young stars. Given the large spatial scale of the turbulence, it is plausible that the source of the turbulence is a large scale one e.g. a merger or tidal event that triggered the formation of molecular clouds, in the cores of which, the young stars formed. A steepening of the structure functions on the smaller scales provides an estimate for the effective depth along the line of sight of the turbulent layer. The latter turns out to be ∼ 2kpc