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Search for XMP gas-rich dwarfs in Nearby Voids: results of BTA and SALT spectroscopy

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    In the framework of the modern cosmological paradigm, the great majority of galaxies have been formed during the first 1-2 Gyr after the Big Bang, which occured 13.7 Gyr ago. Initially, the galaxies were purely gas objects with the so-called 'primordial' element composition, comprised of H and He. Under gravitational force, gas was compressed and formed the first stars. Due to the thermonuclear reactions, the elements heavier than He (conditionally called 'metals') were synthesized in the stellar interiors. At the end of star life, the metals were deposited into the surrounding gas and mixed over the whole galaxy, thus increasing the Z(gas) - metallicity of the interstellar matter. The galaxy evolution is the combination of two related processes: transformation of gas into stars and enrichment of gas by the products of stellar evolution. Therefore, parameters 'gas mass-fraction' M(gas)/[M(gas)+M(stars)] and metallicity Z(gas) characterize the stages of galaxy evolution. Large disc galaxies are evolutionary advanced. They have a gas mass fraction of 1-10 percent and metallicity Z(gas) of about Zo = 0.02. Here Zo is the metallicity of solar matter. In smaller disc galaxies, gas fraction can be as high as tens percent while its metallicity is several times lower, down to the level of Zo/10. It should be noted, however, that galaxies with metallicities of Zo/50 - Zo/30 and gas fraction more than 90 percent are extremely rare in the modern Universe. It terms of their properties, they appear similar to galaxies in the early Universe, but at the same time, they are really 'close' to us, at distances of ten(s) Mpc, and thus can be studied in great detail. This creates a strong motivation for their active search.
    A prototype of such objects is IZw18 with Z(gas) = Zo/30 that was discovered 50 years ago. Since then, optical spectra of thousands of galaxies have been analyzed and estimates of their Z(gas) were obtained. However, only about 10 of them are found to be the 'eXtremely Metal-Poor' (XMP) galaxies with Z(gas) <= Zo/30.
    The majority of known XMP dwarfs, including IZw18, resides in voids. Therefore, working within the framework of our project of studying galaxies in the nearby voids (SAO Science news, 2018, https://www.sao.ru/Doc-k8/SciNews/2018/Pustilnik), we selected 60 candidate XMP dwarfs (out of 1,350 galaxies found in these voids) for for their spectral study. To date, using the large the large optical telescopes, BTA and SALT (South Africa), we have obtained the spectra of 46 of these candidates. Of them, ten galaxies turned out to be new XMP objects. With this addition, the number of such unusual galaxies known has grown to 19. Moreover, we found that the Z(gas) of 13 more void galaxies of Zo/28-Zo/23, is only a little higher than the adopted boundary Zo/30. Such galaxies are also very rare, so this new data will enable us to better study their group and individual properties. The model simulations by Tweed et al. (2018) predict the existence in the nearby Universe of rare galaxies, in which more than a half of stellar mass have formed within the last 1 Gyr. They are conditionally called 'Very Young Galaxies', VYGs. Of the known objects, IZw18 and its companion IZw18C can be confidently assigned to this group. The majority of newly found void XMP galaxies resemble the representatives of VYGs on their properties. Their further detailed studies will enable us to better understand their unusual properties and evolutionary status.
Figure 1. The images of two new XMP void objects with the lowest metallicity. Images are in the inverted colours, in order to better show the low surface brightness morphology, with the positions of the spectrograph long slit overlaid. Left panel: J0110-0000 = AGC411446 with Z = Zo/44. Right panel: J0256+0248 = AGC124629, with Z = Zo/53.
Figure 2. Optical spectra of the same two galaxies obtained at BTA (SAO) and SALT (South Africa), respectively. X axis - wavelength in Angstrems, Y - axis spectral flux density in relative units.

Pustilnik S.A., Egorova E.S., Kniazev A.Y., Perepelitsyna Y.A., Tepliakova A.L., Burenkov A.N., Oparin D.V. XMP gas-rich dwarfs in nearby voids: results of BTA spectroscopy, 2021, MNRAS, 501, 944-962
Pustilnik S.A., Kniazev A.Y., Perepelitsyna Y.A., Egorova E.S. XMP gas-rich dwarfs in nearby voids: results of SALT spectroscopy, 2020, MNRAS, 493, 830-846
Pustilnik S.A., Egorova E.S., Perepelitsyna Y.A., Kniazev A.Y. XMP gas-rich dwarfs in nearby voids: candidate selection, 2020, MNRAS, 492, 1078-1090

Contact person - Pustilnik S.A., DSc, Leading Researcher in the Extragalactic Astrophysics and Cosmology Laboratory