The onset of large-scale turbulence in the interstellar medium of spiral galaxies

Abstract

Turbulence is ubiquitous in the interstellar medium (ISM) of the Milky Way and other spiral galaxies. The energy source for this turbulence has been much debated with many possible origins proposed. The universality of turbulence, its reported largescale driving, and that it occurs also in starless molecular clouds, challenges models invoking any stellar source. A more general process is needed to explain the observations. In this work we study the role of galactic spiral arms. This is accomplished by means of three-dimensional hydrodynamical simulations which follow the dynamical evolution of interstellar diffuse clouds (∼ 100cm −3 ) interacting with the gravitational potential field of the spiral pattern. We find that the tidal effects of the arm's potential on the cloud result in internal vorticity, fragmentation and hydrodynamical instabilities. The triggered turbulence result in large-scale driving, on sizes of the ISM inhomogeneities, i.e. as large as ∼ 100pc, and efficiencies in converting potential energy into turbulence in the range ∼ 10 to 25 % per arm crossing. This efficiency is much higher than those found in previous models. The statistics of the turbulence in our simulations are strikingly similar to the observed power spectrum and Larson scaling relations of molecular clouds and the general ISM. The dependency found from different models indicate that the ISM turbulence is mainly related to local spiral arm properties, such as its mass density and width. This correlation seems in agreement with recent high angular resolution observations of spiral galaxies, e.g. M51 and M33.

Photoionization feedback in turbulent molecular clouds

“…These processes would maintain some level of turbulence even in ionized regions. Another possible driver of turbulence has been shown to be the interaction of the molecular clouds with the potential of the spiral arms of our galaxy (Falceta-Gonçalves et al 2015). Contrarily to previous drivers mentioned, the resulting turbulence does not depend on local properties such as star formation rates.…”

A diversity of starburst-triggering mechanisms in interacting galaxies and their signatures in CO emission

“…Sub-kpc scale simulations of the ISM are commonly used to study the inner physics of molecular clouds, down to the individual star level (see a review in Hennebelle & Falgarone 2012). These simulations do not include the galactic scales from which part of the turbulence is generated ( 0.1 -1 kpc, see Agertz et al 2009;Bournaud et al 2010;Renaud et al 2013;Falceta-Gonçalves et al 2015;Grisdale et al 2017), and they must resort to manually forced turbulence. Such works indicate that the turbulent ISM yields a log-normal density PDF (see e.g.…”

Interrelations Between Astrochemistry and Galactic Dynamics

“…The OB stars that we observe in the arms of external galaxies are believed to be the tip of the iceberg of clusters of stars that are not visible to us (Wright, 2020). Falceta-Gonçalves et al (2015) presented a detailed hydrodynamical simulation of interstellar clouds penetrating in the grooveshaped arms, due to the relative velocity of the galactic material with respect to the arms, showing the generation of turbulence and conditions favorable to star formation. Interestingly, at the end of their paper, Falceta-Gonçalves et al…”