A lensed protocluster candidate at z = 7.66 identified in JWST observations of the galaxy cluster SMACS0723−7327

Abstract

Context. According to the current paradigm of galaxy formation, the first galaxies likely formed within large dark matter haloes. The fragmentation of these massive haloes led to the formation of galaxy protoclusters, which are usually composed of one to a few bright objects, surrounded by numerous fainter (and less massive) galaxies. These early structures could have played a major role in reionising the neutral hydrogen within the first billion years of the Universe, especially if their number density is significant. Aims. Taking advantage of the unprecedented sensitivity reached by the James Webb Space Telescope (JWST), galaxy protoclusters can now be identified and studied in increasing numbers beyond z ≥ 6. Characterising their contribution to the UV photon budget could supply new insights into the reionisation process. Methods. We analysed the first JWST dataset behind SMACS0723-7327 to search for protoclusters at z ≥ 6, combining the available spectroscopic and photometric data. We then compared our findings with semi-analytical models and simulations. Results. In addition to two bright galaxies (≤26.5 AB in F277W), separated by ∼11 and spectroscopically confirmed at z spec = 7.66, we identify six additional galaxies with similar colours within a θ ∼ 20 radius (corresponding to R∼ 60 − 90 kpc in the source plane). Using several methods, we estimate the mass of the dark matter halo of this protocluster as ∼3.3×10 11 M , accounting for magnification, consistent with various predictions. The physical properties of all protocluster members are also in excellent agreement with what has been previously found at lower redshifts: star formation main sequence and protocluster size. This detection adds to just a few protoclusters currently known in the first billion years of the universe. These z ≥ 7 galaxy protoclusters may play an important role in cosmic reionisation.

Early Results from GLASS-JWST. XIV. A Spectroscopically Confirmed Protocluster 650 Million Years after the Big Bang

“…Our estimated lower limit of the mass of A2744-z7p9OD is comparable to those of previously known protoclusters and protoscluster candidates using similar methods, including the recently reported candidate behind the SMACS0723 cluster (Laporte et al 2022), where two of the member candidates are spectroscopically confirmed to be z = 7.66. The real breakthrough of our observations, however, is the sheer number of spectroscopically confirmed redshift measurements, which allow us to establish secure membership to the protocluste and get a first estimate of the its velocity dispersion.…”

“…Second, we attempt to estimate the mass of the structure. Following previous work (e.g., Laporte et al 2022), we can estimate the halo mass of the individual components from the halo-mass-galaxy-luminosity relation. Using the relation derived by Mason et al (2022), we infer that the brightest member of the overdensity (ZD2, M UV = −20.1) lives in a M h ≈ (7 ± 2) × 10 10 M e halo.…”

Evidence for a Low Lyman Continuum Escape Fraction in Three Massive, Ultraviolet-bright Galaxies at z > 7

“…(e.g., Castellano et al 2022, Roberts-Borsani et al 2022. The origin of the ionizing photons that produce these ionized bubbles is still highly debated and could be either due to the intrinsic nature of the object (e.g., star formation or an active galactic nucleus) or the overdense environment near the most-massive galaxies formed at high redshift (Leonova et al 2021;Laporte et al 2022). Indeed previous research into these galaxies indicates that they reside within ionized bubbles, large enough that any blue peak escaping the host galaxy would be redshifted past the Lyα line center before leaving the ionized bubble and hence should be unaffected by IGM absorption.…”

COSMOS2020: Discovery of a Protocluster of Massive Quiescent Galaxies at z = 2.77

“…Exploring massive structures in the highredshift universe can unveil their origin. In recent decades, protoclusters, which are progenitors of local clusters with halo mass of ( )  > M M log 14 halo , have been found out to z ∼ 7-8 (e.g., Harikane et al 2019;Hu et al 2021;Laporte et al 2022). They have often been found through the overdensity of starforming galaxies, such as bright galaxies in rest-frame ultraviolet continuum (e.g., Steidel et al 1998;Overzier et al 2008;Toshikawa et al 2018), Lyα emission (e.g., Shimasaku et al 2003;Jiang et al 2018;Harikane et al 2019), Hα emission (e.g., Hayashi et al 2012;Darvish et al 2020), and infrared (e.g., Miller et al 2018;Oteo et al 2018).…”