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

Abstract

We present demography of the dynamics and gas-mass fraction of 33 extremely metal-poor galaxies (EMPGs) with metallicities of $0.015-0.195Z_{\odot }$ and low stellar masses of $10^4-10^8{M}_{\odot }$ 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 8m-Subaru FOCAS IFU instrument by the EMPRESS 3D survey, and investigate H$\alpha$ emission of the EMPGs. Exploiting the resolution high enough for the low-mass galaxies, we derive gas dynamics with the H$\alpha$ lines by the fitting of 3-dimensional disk models. We obtain an average maximum rotation velocity ($v_{\mathrm{rot}}$) of $15\pm 3\mathrm{km}{\mathrm{s}}^{-1}$ and an average intrinsic velocity dispersion (${\sigma }_0$) of $27\pm 10\mathrm{km}{\mathrm{s}}^{-1}$ for 15 spatially resolved EMPGs out of the 33 EMPGs, and find that all of the 15 EMPGs have $v_{\mathrm{rot}}/{\sigma }_0<1$ suggesting dispersion dominated systems. There is a clear decreasing trend of $v_{\mathrm{rot}}/{\sigma }_0$ with the decreasing stellar mass and metallicity. We derive the gas mass fraction ($f_{\mathrm{gas}}$) for all of the 33 EMPGs, and find no clear dependence on stellar mass and metallicity. These $v_{\mathrm{rot}}/{\sigma }_0$ and $f_{\mathrm{gas}}$ 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.

18 pages, 9 figures, accepted for publication in ApJ

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