EMPRESS. IX. Extremely Metal-Poor Galaxies are Very Gas-Rich Dispersion-Dominated Systems: Will JWST Witness Gaseous Turbulent High-z Primordial Galaxies?

Abstract

We present kinematics of 6 local extremely metal-poor galaxies (EMPGs) with low metallicities ($0.016-0.098Z_{\odot }$) and low stellar masses ($10^{4.7}-10^{7.6}{M}_{\odot }$). Taking deep medium-high resolution ($R\sim 7500$) integral-field spectra with 8.2-m Subaru, we resolve the small inner velocity gradients and dispersions of the EMPGs with H$\hat{I}\pm$ emission. Carefully masking out sub-structures originated by inflow and/or outflow, we fit 3-dimensional disk models to the observed H$\hat{I}\pm$ flux, velocity, and velocity-dispersion maps. All the EMPGs show rotational velocities ($v_{\mathrm{rot}}$) of 5--23 km s${}^{-1}$ smaller than the velocity dispersions ($\ddot{I}{ƒ}_0$) of 17--31 km s${}^{-1}$, indicating dispersion-dominated ($v_{\mathrm{rot}}/\ddot{I}{ƒ}_0=0.29-0.80<1$) systems affected by inflow and/or outflow. Except for two EMPGs with large uncertainties, we find that the EMPGs have very large gas-mass fractions of $f_{\mathrm{gas}}\simeq 0.9-1.0$. Comparing our results with other H$\hat{I}\pm$ kinematics studies, we find that $v_{\mathrm{rot}}/\ddot{I}{ƒ}_0$ decreases and $f_{\mathrm{gas}}$ increases with decreasing metallicity, decreasing stellar mass, and increasing specific star-formation rate. We also find that simulated high-$z$ ($z\sim 7$) forming galaxies have gas fractions and dynamics similar to the observed EMPGs. Our EMPG observations and the simulations suggest that primordial galaxies are gas-rich dispersion-dominated systems, which would be identified by the forthcoming James Webb Space Telescope (JWST) observations at $z\sim 7$.

ApJ in Press