ELTs and the study of individual massive stars typical of "primordial" galaxies
A.Y. Kniazev¹, S.A. Pustilnik^2
1 South African Astronomical Observatory, Cape Town, South Africa
² Special Astrophysical Observatory RAS, Nizhnij Arkhyz, 369167, Russia
We discuss the opportunities to use the future ELTs to study in detail the properties and the evolution peculiarities of individual massive stars with the metal content typical of galaxies in the Universe during the first 0.5-1 Gyr, corresponding to the earliest known newly formed galaxies at redshifts of z = 6-10. This is possible in principle due to the existence in the local Universe of starbursting galaxies with metallicities of ~ 1/30 of the solar value. The nearest such galaxies are DDO 68 at ~ 6.5 Mpc with 12+log(O/H) = 7.21 and I Zw 18 at 15 Mpc with 12+log(O/H) = 7.17. For the youngest star clusters (with ages of T < 4-5 Myr) in these most metal-poor galaxies, stars with masses up to 40-60 M_\odot should be present on both the Main Sequence and the later evolution stages, including the WR stage. They are expected to have apparent magnitudes as bright as V=21^m for DDO 68 and V=23^m for I Zw 18. Their good S/N-ratio spectroscopy with telescopes like OWL or JWST, allowing near-milli-arcsecond angular resolution, will provide unique information to check the most up-to-date models of massive star evolution in the very low metallicity regime, and thus establish a firm basis for modelling the effects of star formation in `primordial' galaxies for the epoch of mass galaxy formation. Such observations will also provide an independent channel for probing the primordial Helium abundance.

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