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In the present Universe, the Inter-Galactic Medium (IGM) is fully ionized and maintained so from the integrated ultraviolet emission from stars and Active Galactic Nuclei (AGN). Some 380,000 years after the Big Bang the temperature of the Universe was low enough that the IGM was neutral. Exactly when and how the Universe has been re-ionised is still unknown and a debated topic in modern astrophysics. From the Planck 2018 results (Aghanim et al. 2020) we have indication that the IGM may have been already half-ionized by z~9, and the spectra of high redshift quasars suggest that the re-ionization process was concluded already around z~6 (e.g. Fan et al. 2006).

The identification of the main ionizing sources has been elusive until know due to their faintness. Thanks to its larger collecting area, the ELT will allow us to push the search of these sources to much fainter magnitudes and higher redshifts, compared to the currently achieved with 8-10 m telescopes. The overarching goal is to derive a precise characterization of the ionisation state of the IGM during the first Gyr of the life of the Universe, to construct the timeline and topology of reionisation, and to observe the formation and growth of the first galaxies.

Very deep optical-to-near-IR spectroscopy to probe the UV rest-frame emission of galaxies in the early Universe is crucial to study these questions. Some of the fundamental questions and hot topics are:

  • Measurements of Ly-alpha emission in galaxies (i.e., fraction of Ly-alpha emitters in the Lyman break galaxy population, evolution of the Ly-alpha luminosity function with redshift, Ly-alpha equivalent width measurements and information from the detailed Ly-alpha line profile, see Dijkstra et al. 2013, Mesinger et al. 2015, Mason et al. 2018) provide crucial information on the ionization state of the IGM;
  • Overdensities of Ly-alpha emitters at z~7-8 (Castellano et al. 2016, Tilvi et al. 2020) providing the first evidence for overlapping reionized bubbles in the IGM, i.e. a hint of topology. Also, the detailed Ly-alpha line profile of z>>6 galaxies can constrain the properties of reionized bubbles in the early Universe (Haiman 2002, Mason & Gronke 2020);
  • One of key quantities to establish if galaxies reionized the Universe, is the measurement of escape fraction of Lyman continuum photons, fesc, which relies on measurements of the Ly-alpha line profile (121.6 nm), the MgII doublet line (280.7,280.9 nm), and UV low ionization absorption lines (e.g. SiII 126.0 nm) (Izotov et al. 2018, Verhamme et al. 2017, Reddy et al. 2018, Gazagnes et al. 2018, Chisholm et al. 2018, 2020). In contrast, no reliable method has yet been found to determine fesc from rest-frame optical spectra (see e.g. Plat et al. 2019, Wang et al. 2020);
  • UV absorption lines and nebular emission lines also provide fundamental information on the ISM of high-z galaxies, such as the geometry and properties of the gas (outflows, the presence of channels allowing LyC photon escape, gas density, and others), chemical abundances (C, O, and Si, e.g);
  • Spectroscopy of distant galaxies with 8-10m class telescopes reveal UV emission lines that require moderately metal-poor gas and a harder radiation field than is seen in galaxies at lower redshift (cf. review by Stark 2016). Several z>6 galaxies show the presence of CIV 154.8nm emission, others HeII 164 nm, [OIII] 166.7 nm, [CIII] 190.9 nm with varying intensities. However, both nebular CIV and HeII emission, requiring higher energies, is challenging to produce and their origin is intensely debated (e.g. Schaerer et al. 2018, Saxena et al. 2019). Spatially resolved studies in, e.g., HeII in emission will be essential to constrain the ionizing sources;
  • Structure formation can also be investigated at the epoch of reionisation by looking for proto-clusters (Toshikowa et al. 2014; Overzier et al. 2016). Spatially-resolving the galaxies and AGN, and studying the circum-galactic and inter-galactic medium (CGM and IGM) within these proto-clusters provides us a unique opportunity to witness the early formation, growth, and co-evolution of these structures on a huge range of spatial scales.
Many of these questions cannot be addressed with the JWST, e.g. since they require a) measurements of very faint emission lines (in the rest-optical), which will only be accessible for relatively bright sources with the JWST (i.e. not for the bulk of the population, see Vanzella et al. 2014), b) UV absorption line observations with R>~2000, c) faint rest-UV emission lines (with EWrest~1-10A), d) R>~2000 Lya line profile measurements.

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