XMP gas-rich dwarfs in nearby voids: results of BTA spectroscopy

TL;DR

This study identifies and analyzes extremely metal-poor, gas-rich dwarf galaxies in nearby voids, using spectroscopy and new empirical methods to determine their metallicity, revealing a rare class of very young galaxies.

Contribution

It presents the discovery and spectroscopic analysis of 19 XMP gas-rich dwarf galaxies in voids, employing a new empirical O/H estimation method suitable for very low metallicities.

Findings

Six galaxies confirmed as confident XMP dwarfs.

Eight galaxies are less metal-poor but still potential VYGs.

Total of 19 such galaxies identified in the study.

Abstract

We present the second part of results of the on-going project of searching for and studying eXtremely Metal-Poor (XMP, adopted as those with Z(gas) <~ Zo/30, or with 12+log(O/H) <~ 7.21~dex) very gas-rich blue dwarfs in voids.They were first identified in course of the 'unbiased' study of 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 preselected void XMP candidates, we obtained with the SAO 6-m telescope (BTA) spectra suitable for determination of O/H. For majority of the observed galaxies, the principal line [OIII]4363 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 the most accurate empirical O/H estimator for the range of 12+log(O/H) < 7.4-7.5. For higher O/H objects, we use the semi-empirical method by Izotov and Thuan with our modification accounting for variance of the excitation parameter O32. Six of those 16 candidates are found to be the confident XMP dwarfs. In addition, eight studied galaxies are somewhat less metal-poor, with 12+log(O/H) = 7.24-7.33. They also can fall into the category of VYG candidates. With account of the recently published by us and previously known (9 prototype galaxies) XMP gas-rich void objects, the new findings increase the number of this type galaxies to the total of 19.