EMPRESS. XII. Statistics on the Dynamics and Gas Mass Fraction of Extremely Metal-poor Galaxies

Abstract

We  present the demography of the dynamics and gas mass fraction of 33 extremely metal-poor galaxies (EMPGs) with metallicities of 0.015–0.195 Z ⊙ and low stellar masses of 10 4 –10 8 M ⊙ in the local universe. We conduct deep optical integral field spectroscopy (IFS) for the low-mass EMPGs with the medium-high resolution ( R = 7500) grism of the 8 m Subaru FOCAS IFU instrument by the EMPRESS 3D survey, and investigate the H α emission of the EMPGs. Exploiting the resolution high enough for the low-mass galaxies, we derive gas dynamics with the H α lines by the fitting of three-dimensional disk models. We obtain an average maximum rotation velocity ( v rot ) of 15 ± 3 km s −1 and an average intrinsic velocity dispersion ( σ 0 ) of 27 ± 10 km s −1 for 15 spatially resolved EMPGs out of 33 EMPGs, and find that all 15 EMPGs have v rot / σ 0 < 1 suggesting dispersion-dominated systems. There is a clear decreasing trend of v rot / σ 0 with the decreasing stellar mass and metallicity. We derive the gas mass fraction ( f gas ) for all 33 EMPGs, and find no clear dependence on stellar mass and metallicity. These v rot / σ 0 and f gas trends should be compared with young high- z galaxies observed by the forthcoming JWST IFS programs to understand the physical origins of the EMPGs in the local universe.