Turbulence statistics of HI clouds entrained in the Milky Way's nuclear wind

TL;DR

This study presents the first observational analysis of turbulence in neutral hydrogen clouds entrained in the Milky Way's nuclear wind, revealing sub-to-trans-sonic, compressively-driven turbulence likely influenced by cloud-wind interactions.

Contribution

It introduces new measurements of turbulence parameters in HI clouds within the Galactic nuclear wind and develops a method to estimate HI temperature variations in molecular gas environments.

Findings

Both clouds exhibit sub-to-trans-sonic Mach numbers.

Turbulence is primarily compressively-driven ($b\,\sim1$).

Cloud-wind interactions may inject turbulence energy without star formation.

Abstract

The interstellar medium (ISM) is ubiquitously turbulent across many physically distinct environments within the Galaxy. Turbulence is key in controlling the structure and dynamics of the ISM, regulating star formation, and transporting metals within the Galaxy. We present the first observational measurements of turbulence in neutral hydrogen entrained in the hot nuclear wind of the Milky Way. Using recent MeerKAT observations of two extra-planar HI clouds above (gal. lat.$\,\sim7.0^{\circ}$) and below (gal. lat.$\,\sim-3.9^{\circ}$) the Galactic disc, we analyse centroid velocity and column density maps to estimate the velocity dispersion ($\sigma_{v,\mathrm{3D}}$), the turbulent sonic Mach number ($\mathcal{M}$), the volume density dispersion ($\sigma_{\rho/\rho_0}$), and the turbulence driving parameter ($b$). We also present a new prescription for estimating the spatial temperature variations of HI in the presence of related molecular gas. We measure these turbulence quantities on the global scale of each cloud, but also spatially map their variation across the plane-of-sky extent of each cloud by using a roving kernel method. We find that the two clouds share very similar characteristics of their internal turbulence, despite their varying latitudes. Both clouds are in the sub-to-trans-sonic Mach regime, and have primarily compressively-driven ($b\sim1$) turbulence. Given that there is no known active star-formation present in these clouds, this may be indicative of the way the cloud-wind interaction injects energy into the entrained atomic material on parsec scales.