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 104–108 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 Hα emission of the EMPGs. Exploiting the resolution high enough for the low-mass galaxies, we derive gas dynamics with the Hα 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.