Evidence for supernova feedback sustaining gas turbulence in nearby star-forming galaxies

Abstract

It is well known that gas in galaxy discs is highly turbulent, but there is much debate on which mechanism can energetically maintain this turbulence. Among the possible candidates, supernova (SN) explosions are likely the primary drivers but doubts remain on whether they can be sufficient in regions of moderate star formation activity, in particular in the outer parts of discs. Thus, a number of alternative mechanisms have been proposed. In this paper, we measure the SN efficiency η, namely the fraction of the total SN energy needed to sustain turbulence in galaxies, and verify that SNe can indeed be the sole driving mechanism. The key novelty of our approach is that we take into account the increased turbulence dissipation timescale associated to the flaring in outer regions of gaseous discs. We analyse the distribution and kinematics of HI and CO in 10 nearby star-forming galaxies to obtain the radial profiles of the kinetic energy per unit area, for both the atomic gas and the molecular gas. We use a theoretical model to reproduce the observed energy with the sum of turbulent energy from SNe, as inferred from the observed star formation rate (SFR) surface density, and the gas thermal energy. For the atomic gas, we explore two extreme cases in which the atomic gas is made either of cold neutral medium or warm neutral medium, and the more realistic scenario with a mixture of the two phases. We find that the observed kinetic energy is remarkably well reproduced by our model across the whole extent of the galactic discs, assuming η constant with the galactocentric radius. Taking into account the uncertainties on the SFR surface density and on the atomic gas phase, we obtain that the median SN efficiencies for our sample of galaxies are η atom = 0.015 +0.018 −0.008 for the atomic gas and η mol = 0.003 +0.006 −0.002 for the molecular gas. We conclude that SNe alone can sustain gas turbulence in nearby galaxies with only few percent of their energy and that there is essentially no need for any further source of energy.

A Search for correlations between turbulence and star formation in LITTLE THINGS dwarf irregular galaxies

“…The curves in the KED plot show fitted relationships between the KED generated by supernovae and the Σ SFR for the indicated efficiencies of converting SN energy into turbulence, and for galaxy scale heights of 850 pc and 540 pc. These theoretical KEDs come from equation 3.7 in Bacchini et al (2020), which is…”

“…where η is the efficiency of energy conversion from supernova to turbulence, f cc = 1.3 × 10 −2 M −1 is the number of core-collapse supernovae per solar mass of stars, E SN = 10 51 erg is the supernova energy, H is the disk thickness and v turb is the turbulent gas velocity dispersion (the ratio of these latter two quantities gives the turbulent dissipation time). Bacchini et al (2020) compare the radial profiles of turbulent energies in 10 nearby galaxies with the SFRs and derive an average efficiency of 1.5 1.8 0.8 % if all of the turbulence comes from star formation. Because the required efficiency is relatively low, they concluded that supernovae related to star formation can drive most of the interstellar turbulence.…”

“…The total dispersions used to evaluate η include thermal and turbulent motions, which were distinguished in several limiting cases by Bacchini et al (2020) to get the desired v turb . If we assume Mach∼ 1 turbulence in the general H i ISM, then v turb = v disp /2 0.5 and the derived values of η decrease by the factor 0.7, preserving the ratio η/v used to match the KED.…”

“…If we assume Mach∼ 1 turbulence in the general H i ISM, then v turb = v disp /2 0.5 and the derived values of η decrease by the factor 0.7, preserving the ratio η/v used to match the KED. Alternatively, we could use thermal dispersions of 4.9 km s −1 and 6.1 km s −1 modeled for NGC 4736 and NGC 2403 respectively by Bacchini et al (2020) to estimate that v turb /v disp ∼ 0.8, given that v disp ∼ 10 km s −1 here. Then our derived η should decrease by ∼ 0.8.…”

“…On the other hand, Übler et al (2019) interpreted the increase in the ionized gas velocity dispersion with SFR density for high redshift galaxies as the result of gravitational instabilities, following the theory in Krumholz et al (2018). Bacchini et al (2020) consider radial profiles of turbulent speeds and SFRs in local spiral galaxies and account for all of the gas turbulence using supernovae from young massive stars. They get more effective turbulence driving than other studies because they include the radial increase in disk thickness, which decreases the dissipation rate.…”