2021MNRAS.507..944P

We present the second part of results of the ongoing project aimed at searching for and studying eXtremely Metal-Poor (XMP) - adopted as those with Z_gas <= Z_☉/30, or with 12+log (O/H) <= 7.21 dex - very gas-rich blue dwarfs in voids. They were first identified in the course of the 'unbiased' study of the galaxy population in the nearby Lynx-Cancer void. These very rare and unusual galaxies seem to be the best proxies of so-called Very Young Galaxies (VYGs) defined recently in model simulations by Tweed et al. To date, for 16 pre-selected void XMP candidates, using the Big Telescope Alt-azimuth (BTA), the SAO 6-m telescope, we have obtained spectra suitable for the determination of O/H. For majority of the observed galaxies, the principal line [O III] λ4363, used for the direct classical T_e method of O/H determination, is undetected. Therefore, to estimate O/H, we use a new 'strong-lines' method by Izotov et al. This appears to be the most accurate empirical O/H estimator for the range of 12+log (O/H) <= 7.4-7.5. For objects with higher O/H, we use the semi-empirical method by Izotov & Thuan with our modification accounting for variance of the excitation parameter O₃₂. Six of those 16 candidates are found, with confidence, to be XMP dwarfs. In addition, eight studied galaxies are less metal-poor, with 12+log (O/H) = 7.24-7.33, and these can also fall into the category of VYG candidates. Taking into account our recently published work and the previously known (nine prototype galaxies) XMP gas-rich void objects, the new findings increase the number of this type of galaxy known to date to a total of 19.