![]() ), with the process possibly still ongoing as late as \(z\sim 5.5\). There is also some evidence for a late reionization scenario (e.g. ), although there exist exceptions such as in observations of UV-faint galaxies behind galaxy cluster lenses, massive galaxies, and possibly in cases of ionised bubbles around bright galaxies. These studies yield a drop in the fraction of Ly α emitting galaxies at z > 6 (e.g. A second approach has proven successful at probing the neutral fraction of the IGM at higher redshifts, namely searches for Ly α emission in spectroscopic observations of Lyman-break selected galaxies. Cosmic chemical evolution is very poorly constrained at high redshift, and even in the JWST era, metallicity estimates at z > 6 will rely on crude, or model dependent, emission line diagnostics for only a limited number of the brightest galaxies (typically, M UV 6.1 quasars (see, e.g., also Section 8 for supporting evidence from GRB afterglows). The growth of these early luminous sources is intimately connected to the reionization of the intergalactic medium (IGM) and build-up of heavy chemical elements. THESEUS will allow us to explore the faint end of the luminosity function of galaxies and the star formation rate density to high redshifts constrain the progress of re-ionisation beyond \(z\gtrsim 6\) study in detail early chemical enrichment from stellar explosions, including signatures of Population III stars and potentially characterize the dark energy equation of state at the highest redshifts.Ī major goal of contemporary astrophysics and cosmology is to achieve a detailed understanding of the formation of the first collapsed objects (Pop-III and early Pop-II stars, black holes and the primordial galaxies in which they were born) during the first billion years in the life of the Universe. The proposed THESEUS mission is designed to detect large samples of GRBs at z > 6 in the 2030s, at a time when supporting observations with major next generation facilities will be possible, thus enabling a range of transformative science. ![]() Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization era. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. ![]() At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known.
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