Research Byte

Published in the RSAA Lunations
Vol1 Issue33 1–31 October 2022

Integrated field units (IFUs) provide a unique window to explore heavy element (metal) enrichment in the interstellar medium (ISM) of local galaxies. Compared with the well-studied metallicity gradient in the past decades, a next-generation metric in the IFU era is the two-point correlation, a two-dimension-version of the auto-correlation function of a zero-averaged metallicity residual map. It has the advantages of making full use of the spatial information now available and exploring the poorly understood gas diffusion inside galaxies. A recent study (Krumholz & Ting 2018) shows that the two-point correlation can be described by an injection-diffusion model which requires only two free parameters. Correlation length is the latter. It has a physical meaning of a characteristic diffusive length of ISM mixing, and in theory  correlates with gas velocity dispersion and equilibrium time (Li et al. 2021). 

With the knowledge that star formation rate (SFR) can also be a function of gas velocity dispersion (Krumholz et al. 2018), the figure shows the correlation lengths vs. global SFRs for the AMUSING++ galaxy observations compilation, skipping gas velocity dispersion. The comparison between the observed correlation lengths and the model-predicted ones reveals that the measured correlation length is roughly in good agreement with the combination of the two models, but not as large as we expected before at low-SFR. One plausible hypothesis is that at high SFR, supernovae distribute most of the energy into driving the surrounding turbulence. At low SFR, however, most of them are dwarf galaxies and due to their significant gas outflows, the metals have very short time (several hundred million years) to mix into ISM and are removed before they can be diffused.

Zefeng Li