Gas-phase Metallicity as a Diagnostic of the Drivers of Star-formation on Different Scales

Enci Wang, Simon J. Lilly

Published 2020-09-03Version 1

This work mainly includes two parts: the theoretical predictions and observational results on the correlation of SFR and gas-phase metallicity $Z$. We first predict the correlation between SFR, cold gas mass and $Z$ in the gas-regulator frame that the instantaneous SFR is regulated by the interplay between inflow, outflow and star formation. The mean SFR is determined by mean inflow rate and mass-loading factor $\lambda$, while the mean $Z$ is determined by the metallicity of inflow gas $Z_{\rm 0}$ and the effective yield ($y_{\rm eff}\equiv y/(1+\lambda)$). The SFR and $Z$ relative to their mean values, denoted as $\Delta$SFR and $\Delta Z$, are found to be negatively (or positively) correlated when driving the gas-regulator with time-varying inflow rate (or SFE). We then study the correlation of $\Delta$sSFR and $\Delta Z$ (defined in a similar way as in the model) from the observation, at both $\sim$100 pc scale and galactic scale based on two 2d-spectroscopic surveys of different spatial resolutions, MAD and MaNGA. We find that $\Delta$sSFR and $\Delta Z$ are found to be, negatively correlated at galactic or sub-galactic scale across galaxy population, and positively correlated at $\sim$100 pc scale within galaxies. This strongly supports the conclusion that the star formation is primarily driven by the time-varying inflow at galactic scale, and driven by the time-varying SFE at $\sim$100 pc scale. Furthermore, at sub-galactic scale, the variation of $\Delta$sSFR and $\Delta Z$ as a function of gas depletion time are well matched with the model predications of time-varying inflow rate, strongly strengthening our conclusion. We build a comprehensive frame to understand the correlation between SFR, cold gas mass and metallicity, as well as the variability of them, which potentially uncovers the relevant physical processes of star formation at different scales.