Galaxy Formation and Reionization: Key Unknowns and Expected Breakthroughs by the James Webb Space Telescope

Abstract

The scheduled launch of James Webb Space Telescope (JWST ) in late 2021 marks a new start for studies of galaxy formation at high redshift (z 6) during the era of Cosmic Reionization. JWST can capture sensitive, high-resolution images and multi-object spectroscopy in the infrared that will transform our view of galaxy formation during the first billion years of cosmic history. This review summarizes our current knowledge of the role of galaxies in reionizing intergalactic hydrogen ahead of JWST , achieved through observations with Hubble Space Telescope and ground-based facilities including Keck, the Very Large Telescope, Subaru, and the Atacama Large Millimeter/Submillimeter Array. We identify outstanding questions in the field that JWST can address during its mission lifetime, including with the planned JWST Cycle 1 programs. These findings include:• Surveys with JWST have sufficient sensitivity and area to complete the census of galaxy formation at the current redshift frontier (z ∼ 8-10). • Rest-frame optical spectroscopy with JWST of galaxies will newly enable measures of star formation rate, metallicity, and ionization at z ∼ 8 − 9, allowing for the astrophysics of early galaxies to be constrained. • The presence of evolved stellar populations at z ∼ 8-10 can be definitively tested by JWST , which would provide evidence of star formation out to z ∼ 15.

EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-Poor Galaxies: Uncovering the Origins of Strong HeII and the Impact on Cosmic Reionization

“…Moreover, the contributions of the high energy (>54.4 eV) photons would be further reduced because the photoionization crosssection has the photon energy dependency of approximately E −3 (Osterbrock & Ferland 2006). Thus, we conclude that the hard ionizing radiation does not affect the standard picture of cosmic reionization (Robertson 2021).…”

“…We evaluate the contribution of X-ray photons from the young galaxies to cosmic reionization, assuming that the young galaxies have the same ionizing spectrum shapes as the spectrum of I Zw 18. We adjust the ionizing photon escape fraction of 20% to match the standard picture of cosmic reionization (e.g., Ouchi et al 2009;Robertson et 2013). To do so, we consider the model spectrum that linearly combines the incident ionizing spectrum and the net transmitted spectrum with their luminosities multiplied by 0.2 and 0.8, respectively.…”

ALMA as a Redshift Machine: Using [CII] to Efficiently Confirm Galaxies in the Epoch of Reionization

“…Though 100s of UV-bright galaxy candidates at redshifts z > 6.5 (e.g., McLure et al 2013;Bouwens et al 2015;Finkelstein et al 2015;Ishigaki et al 2018;Bouwens et al 2021a;Harikane et al 2021) are known, characterization of their physical properties has been difficult. Deriving these properties from optical and near-IR photometry is complicated by uncertainties in the redshifts (e.g., Bouwens et al 2021b;Robertson 2021), dust extinction (e.g., Fudamoto et al 2020;Schouws et al 2021;Bowler et al 2021), and rest-frame optical nebular emission line properties (e.g., Smit et al 2015;Endsley et al 2021;Stefanon et al 2021) of z > 6 galaxies.…”

“…Fortunately, with ALMA, it is possible to make great progress on the characterization of galaxies at z > 6 (e.g., Hodge & da Cunha 2020;Robertson 2021). The [CII]158µm line is especially interesting as it is the strongest cooling line of warm gas (T < 10 4 K) in galaxies.…”

CLEAR: Boosted Ly$α$ Transmission of the Intergalactic Medium in UV bright Galaxies

“…Reionization marks the last major phase transition of the universe, when the first stars and galaxies ionized the intergalactic medium (IGM) neutral hydrogen (H I) gas. Galaxies in the early universe are inherently coupled with the process of reionization, as the galaxies were likely the primary sources of ionizing photons (e.g., Robertson et al 2015;Mc-Quinn 2016;Finkelstein et al 2019a;Dayal & Ferrara 2018) while the contribution from active galactic nucleus (AGN) activity could also play an important (though subdominant) role at z 6 (e.g., Matsuoka et al 2018;Kulkarni et al 2019;Robertson 2021). Thus, observations of this epoch also provide key information for studying the dominant source of the ionizing photons: galaxies in the early universe.…”