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The final frontier in constructing a coherent picture of cosmic history is to understand cosmic reionization: a period of the Universe when it transitioned from a predominently neutral intergalactic medium to an ionized one. Neutral hydrogen first formed ~370,000 years after the Big Bang, once the Universe expanded and cooled sufficiently. The recombination process released what is currently observed as the cosmic microwave background (CMB) but, devoid of any sources of light, the Universe entered a period of darkness known as the "Dark Ages". This period lasted until the formation of the first metal-free stars, which in turn formed the first galaxies and cosmic structures that bathed the Universe in intense ultraviolet light capable of ionizing their surroundings, thereby beginning the (re)ionization process. The formation of these first structures is believed to have occured within the first 200-400 Myr of the Universe and, as the structures developed and expanded, so did their ionized spherical bubbles. Over time, the bubbles grew sufficiently to overlap and, at a redshift of z~5.8, the reionizing process was completed. Understanding the link between the first galaxies and the Epoch of Reionization (EoR) is of fundamental importance, not only because the first generation of galaxies are the likely culprits for the reionizing process, but also because they were the seeds that gave rise to the diverse populations of galaxies and AGN that we observe today.

As such, detecting and characterising the first galaxies is a major consitituent towards our understanding of the Universe, and affirming their role in reionization requires constraints on:

  • the number of ionizing UV photons produces by the young stars

  • the escape fraction of the ionizing photons from the galaxy to the CGM

  • the number density of galaxies as a function of luminosity

  • the start and end of reionization


These are the main themes that drive galaxy evolution studies within the EoR. However, the challenges of finding and characterising these objects are significant: observations of high-z galaxies are limited by the objects' small and compact sizes, extremely low luminosities and the sensitivities of our instruments. An additional difficulty is the intervening hydrogen along the line of sight, which attenuates the rest-frame UV continuum and lines, make redshift identification difficult.


Several ways exist to partially get around these: gravitational lensing has yielded many faint galaxy candidates that otherwise would not be visible, whilst techniques such as the Lyman dropout or HST/Spitzer photometric colours have produced impressive catalogs of high-z candiates from z~4 to z~10. The Lyman-α (Lyα) line, typically used for redshift identifications because of it's brightness in star-forming galaxies has also recently provided the high-z community with some of the most distant galaxies known, although the line gets scattered by the intervening hydrogen and is therefore difficult to detect. Using state of the art telescopes such as Keck, the VLT and ALMA, the recent detections of other lines in the interstellar medium of high-z galaxies has also allowed astronomers to constrain their natures in unprecedented detail. However, large statistical samples (with secure redshifts), remain a luxury for EoR studies. Despite the difficulties, new facilities such as the James Webb Space Telescope, Thirty Metre Telescope, Extremely Large Telescope and the Square Kilometre Array, will open a new window onto galaxy evolution studies in the EoR and push the frontier of the observable universe in unprecedented fashion. One such example is the approved JWST ERS program, "Through the Looking GLASS" (PI: Treu), which aims to observe the Frontier Fields cluster, Abell 2744 (see the image below), with the NIRISS and NIRSpec instruments and NIRCam observations in parallel, providing some of the deepest NIR spectroscopic and imaging observations ever taken.

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