Paper detail

Turbulence driving in a star-forming Milky-Way-type galaxy

The life-cycle, structure, and dynamics of the interstellar medium (ISM) is regulated by turbulence. Complex physical processes, including supernova (SN) explosions, shear, and gravitational collapse, drive and maintain turbulence, but it is still an open question what turbulence driving mode is primarily excited by these different mechanisms. The turbulence driving parameter, b, can be used to quantify the ratio of solenoidal to compressive modes in the acceleration field that drives the turbulence. Compressive driving is characterised by b ~ 1, while purely solenoidal driving gives b ~ 0.3. To quantify the turbulence in the galactic ISM, we investigate the time evolution of b, as well as the turbulent Mach number, and plasma beta (thermal-to-magnetic pressure ratio), and its correlation with star formation in the magnetised warm neutral medium (WNM) of the TIGRESS shearing-box simulations of a kpc-sized patch of a Milky-Way-like galaxy, over a 100 Myr time period (~ half an orbital time). In this simulation the turbulence is driven by a combination of shear, gravitational collapse, and star formation feedback in the form of radiation and SNe. We find that the turbulence driving parameter fluctuates in time between b ~ 0.4 and b ~ 1. We find a time-dependent correlation of b with star formation activity, such that high star formation rates follow about one turbulent turnover time (~ 10 Myr) after phases of highly compressive driving (b > 0.5). About 20 Myr after the peak in star formation, type-B SN feedback drives up the WNM fraction and turbulent Mach numbers, and reduces plasma beta and the driving to b ~ 0.4-0.5.