From giant clumps to clouds -- III. The connection between star formation and turbulence in the ISM

Abstract

Supersonic gas turbulence is a ubiquitous property of the interstellar medium. The level of turbulence, quantified by the gas velocity dispersion (𝜎 g ), is observed to increase with the star formation rate (SFR) rate of a galaxy, but it is yet not established whether this trend is driven by stellar feedback or gravitational instabilities. In this work we carry out hydrodynamical simulations of entire disc galaxies, with different gas fractions, to understand the origins of the SFR-𝜎 g relation. We show that disc galaxies reach the same levels of turbulence regardless of the presence of stellar feedback processes, and argue that this is an outcome of the way disc galaxies regulate their gravitational stability. The simulations match the SFR-𝜎 g relation up to SFRs of the order of tens of M yr −1 and 𝜎 g ∼ 50 km s −1 in neutral hydrogen and molecular gas, but fail to reach the very large values (> 100 km s −1 ) reported in the literature for rapidly star forming galaxies. We demonstrate that such high values of 𝜎 g can be explained by 1) insufficient beam smearing corrections in observations, and 2) stellar feedback being coupled to the ionised gas phase traced by recombination lines. Given that the observed SFR-𝜎 g relation is composed of highly heterogeneous data, with 𝜎 g at high SFRs almost exclusively being derived from H𝛼 observations of high redshift galaxies with complex morphologies, we caution against analytical models that attempt explain the SFR-𝜎 g relation without accounting for these effects.

Turbulence in Milky Way Star-Forming Regions Traced by Young Stars and Gas

“…The kinematic coupling between these phases is not well understood (for a review of the multi-phase ISM model, see Cox 2005). More specifically, it is suggested that each phase possesses varying levels of turbulence, where ionized gas has been observed to have higher velocity dispersion than molecular gas in both high-and low-redshift galaxies (Girard et al 2021;Ejdetjärn et al 2021). In our study, we observe a similar trend comparing the VSF of Hα and other tracers of turbulence.…”