A "universal" supernova efficiency to drive the HI turbulence in nearby star-forming galaxies

summary

The gist

The gist The observed HI kinetic energy in dwarf galaxies requires only ≲ 3% of the SN energy, and that a "universal" efficiency of ≈ 2% explains the HI turbulence across our main-sequence sample.

In short

Researchers investigated whether a single 'universal' efficiency for supernova (SN) energy can drive cold gas turbulence in dwarf galaxies. They modeled HI kinetic energy and found that a universal efficiency of approximately 2% explains the observed turbulence across a sample of star-forming galaxies, suggesting SN feedback is sufficient to sustain this chaotic motion.

Key concepts

HI Turbulence: This refers to the chaotic, non-uniform motions within the neutral atomic gas (HI) in galaxies. It's measured by velocity dispersion and is thought to be caused by energy injection from supernovae, as it exceeds what thermal motions alone can explain.
SN Efficiency ($\eta$): This is a key parameter representing the fraction of supernova energy that successfully transfers into driving turbulence within the cold gas. The study tests if this efficiency stays consistent across different galaxies.
Two-Phase Scenario: The model considers the neutral gas in two states: cold (dense, star-forming) and warm (diffuse). The paper uses a mixed two-phase scenario to better reflect the actual gas distribution, finding a universal efficiency of about 1.2% for this case.

Terminology used across episodes

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Federated Learning — explained in 17 episodes
Retrieval-Augmented Generation (RAG) — explained in 16 episodes
Sample Complexity — explained in 16 episodes
Interpretability — explained in 15 episodes
Quantum Fisher Information (QFI) — explained in 15 episodes

This episode discusses

The paper

A "universal" supernova efficiency to drive the HI turbulence in nearby star-forming galaxies · Read on arXiv

Lauri Sassali, Cecilia Bacchini, Aku Venhola, Giuliano Iorio, Antonino Marasco

Space Physics and Astronomy Research Unit, University of Oulu · DARK, Niels Bohr Institute, University of Copenhagen · Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona · INAF – Padova Astronomical Observatory

The origin of cold gas turbulence in galaxies has long been debated. Supernovae (SNe), the dominant form of stellar feedback, were thought insufficient to sustain it against dissipation, especially in galactic environments with low star formation rates (SFRs). We investigate the origin of the neutral atomic gas (HI) turbulence in dwarf irregular galaxies, a challenging regime of low masses and SFRs. We build on a recent study of spirals showing that disc flaring prolongs the dissipation timescale, reducing the energy needed to sustain turbulence and making SN feedback viable. We measure the SN efficiency, that is the fraction of SN energy required to sustain HI turbulence, in 14 dwarfs, and test whether a "universal" efficiency holds across a sample representative of the star-forming main sequence. From the observed HI distributions and kinematics, we derive the radial profiles of the HI kinetic energy per unit area and model them as the sum of thermal energy and turbulent energy from SNe, constrained by the observed SFR surface density. We consider pure cold and warm neutral medium (WNM) phases as limiting cases, as well as a mixed two-phase scenario, obtaining the SN efficiency and WNM fraction via hierarchical Bayesian inference. We find that the observed HI kinetic energy in dwarfs requires only 3% of the SN energy, and that a "universal" efficiency of about 2% explains the HI turbulence across our main-sequence sample. Our findings suggest that the SN efficiency remains similar across galaxies with different properties, offering valuable constraint for feedback recipes in galaxy evolution models.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "A "universal" supernova efficiency to drive the HI turbulence in nearby star-forming galaxies".

Jocelyn: The gist The observed HI kinetic energy in dwarf galaxies requires only ≲ 3% of the SN energy,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: Building on what we discussed, this paper, "A "universal" supernova efficiency to drive the HI turbulence in nearby star-forming galaxies," focuses on dwarf irregular galaxies specifically, which are those challenging environments with low masses and low star formation rates. The authors investigate the origin of neutral atomic gas turbulence here.

Jocelyn: They look at the observed HI velocity dispersion in these dwarfs, finding it’s typically around six to twelve km/s, which often seems too high for just thermal motions alone. This excess is what they attribute to turbulence between fluid elements.

Subrahmanyan: The paper builds on prior work from spirals showing that disc flaring prolongs the time it takes for energy to dissipate, which makes supernova feedback a more viable way to sustain that turbulence in those lower star formation rate environments.

Vera: They measure the SN efficiency, which is defined as the fraction of supernova energy required specifically to sustain HI turbulence in these dwarf galaxies. They test if this efficiency is universal across a sample representative of the star-forming main sequence.

Jocelyn: From their observed HI distributions and kinematics, they derive radial profiles for the kinetic energy per unit area and model them as a sum of thermal energy and turbulent energy from supernovae, constrained by the observed surface density of star formation.

Subrahmanyan: They consider pure cold neutral medium phases and warm neutral medium phases as limiting cases, along with a mixed two-phase scenario to constrain the supernova efficiency and the warm neutral medium fraction using hierarchical Bayesian inference.

Conclusion: Vera: So to wrap up this paper, "A "universal" supernova efficiency to drive the HI turbulence in nearby star-forming galaxies," the core finding is that they found a single, universal supernova efficiency value for driving HI turbulence across many different dwarf galaxies.

Jocelyn: They found that when you look at the observed kinetic energy in these dwarf galaxies, it only requires about zero point zero two or two percent of the total supernova energy to keep things moving turbulently.

Subrahmanyan: That means this efficiency is not wildly dependent on whether the galaxy is a dwarf irregular or a spiral, which is pretty big for building models of galaxy evolution.

Vera: They show that this two percent efficiency holds up across their sample of star-forming main sequence galaxies, matching what they see in both dwarfs and spirals.

Jocelyn: It’s within about a factor two of the uncertainty they have, and it lines up well with what models predict for how supernova remnants evolve.

Subrahmanyan: This gives us a concrete benchmark for how much energy SNe transfer into the cold interstellar medium in those sub-grid models.

Vera: The paper suggests that supernovae alone are enough to drive this turbulence without needing extra mechanisms like gravitational instabilities to explain the motions in dwarf galaxies.

Jocelyn: They did mention that their method is a bit different from other ways people study this, and they haven't fully settled if SNe are truly sufficient on their own.

Subrahmanyan: That’s a fair point because this work provides a solid starting point for setting up the fraction of supernova energy that gets transferred into the cold ISM in those sub-grid models.

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