[Paper Review] Dispersal and Mixing of Oxygen in the Interstellar Medium of Gas-Rich Galaxies

J. R. Roy, D. Kunth|arXiv (Cornell University)|Oct 6, 1994

Phase Equilibria and Thermodynamics10 citations

TL;DR

This paper investigates why oxygen abundance in the interstellar medium (ISM) of gas-rich galaxies shows larger-than-expected fluctuations despite efficient mixing processes. Using hydrodynamical models, it demonstrates that turbulent diffusion, cloud collisions, and instabilities (Rayleigh-Taylor and Kelvin-Helmholtz) should homogenize O/H ratios on timescales of 10^8 to 10^9 years, yet observed abundance differences—such as a factor of two between the Sun and Orion Nebula—persist, indicating that current mixing mechanisms may be insufficient to explain the observed chemical inhomogeneity.

ABSTRACT

Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuations in the interstellar medium (ISM) of gas-rich galaxies. It is shown that at the present observed solar level of O/H $\\sim 6 \ imes 10^{-4}$, abundance differences of a factor of two, such as existing between the Sun and the nearby Orion Nebula, are many times larger than expected. We examine a variety of hydrodynamical processes operating at scales ranging from 1 pc to greater than 10 kpc, and show that the ISM should appear better homogenized chemically than it actually is: $(i)$ on large galactic scales (1 $\\geq\\ l\\ \\geq$ 10 kpc), turbulent diffusion of interstellar clouds in the shear flow of galactic differential rotation is able to wipe out azimuthal O/H fluctuations in less than $10^9$ yrs; $(ii)$ at the intermediate scale (100 $\\geq\\ l\\ \\geq$ 1000 pc), cloud collisions and expanding supershells driven by evolving associations of massive stars, differential rotation and triggered star formation will re-distribute and mix gas efficiently in about $10^8$ yrs; $(iii)$ at small scales (1 $\\geq\\ l\\ \\geq$ 100 pc), turbulent diffusion may be the dominant mechanism in cold clouds, while Rayleigh-Taylor and Kelvin-Helmhotz instabilities quickly develop in regions of gas ionized by massive stars, leading to full mixing in

Motivation & Objective

• To understand why observed oxygen abundance variations in the ISM of gas-rich galaxies exceed theoretical expectations.
• To evaluate the efficiency of various hydrodynamical processes in chemically homogenizing the interstellar medium.
• To determine whether mixing mechanisms can account for the observed O/H differences, such as the factor-of-two variation between the Sun and Orion Nebula.
• To assess the role of turbulent diffusion, cloud collisions, supershells, and instabilities across different spatial scales (1 pc to 10 kpc).
• To reconcile observed chemical inhomogeneity with predicted mixing timescales in galactic ISM.

Proposed method

• Modeling turbulent diffusion in galactic shear flows to assess azimuthal O/H homogenization at large scales (1–10 kpc).
• Analyzing cloud collisions and expanding supershells driven by massive star associations to evaluate mixing efficiency at intermediate scales (100–1000 pc).
• Investigating Rayleigh-Taylor and Kelvin-Helmholtz instabilities in ionized regions around massive stars to assess small-scale mixing (1–100 pc).
• Using hydrodynamical simulations to estimate mixing timescales across three spatial regimes: large, intermediate, and small scales.
• Comparing predicted mixing timescales (10^8–10^9 years) with observed abundance variations to assess consistency.
• Applying standard ISM mixing theory to evaluate whether observed O/H fluctuations are consistent with known physical processes.

Experimental results

Research questions

• RQ1 Why do observed oxygen abundance variations in the ISM of gas-rich galaxies exceed theoretical expectations?
• RQ2 Can turbulent diffusion in differentially rotating galactic disks homogenize O/H ratios on timescales less than 10^9 years?
• RQ3 How effective are cloud collisions and supershell expansion in redistributing and mixing interstellar gas at intermediate scales (100–1000 pc)?
• RQ4 To what extent do Rayleigh-Taylor and Kelvin-Helmholtz instabilities drive mixing in ionized regions near massive stars at small scales (1–100 pc)?
• RQ5 Are the observed O/H differences—such as the factor-of-two variation between the Sun and Orion Nebula—consistent with known hydrodynamical mixing mechanisms?

Key findings

• Turbulent diffusion in galactic shear flows can homogenize azimuthal O/H fluctuations on timescales of less than 10^9 years at large scales (1–10 kpc).
• At intermediate scales (100–1000 pc), cloud collisions, supershells, and triggered star formation can efficiently mix gas in approximately 10^8 years.
• At small scales (1–100 pc), turbulent diffusion dominates in cold clouds, while Rayleigh-Taylor and Kelvin-Helmholtz instabilities drive rapid mixing in ionized regions.
• Despite these efficient mixing processes, observed O/H abundance differences—such as a factor of two between the Sun and Orion Nebula—remain unexplained by current models.
• The persistence of such abundance fluctuations suggests that either mixing mechanisms are less efficient than assumed, or additional processes (e.g., inhomogeneous star formation or delayed mixing) must be at play.
• The study concludes that the observed chemical inhomogeneity in the ISM exceeds what is predicted by standard hydrodynamical mixing, indicating a gap in current understanding of ISM chemical evolution.