Speaker
Description
Despite major advances in cosmological simulations, key discrepancies remain between models and observations of high-redshift protocluster environments. Simulations suggest that protoclusters contribute significantly to the stellar mass budget of the Universe, yet they often fail to reproduce the elevated star formation rates and the abundance of massive galaxies observed in protocluster cores at $z > 2$. The reliability of dusty star-forming galaxies (DSFGs) as tracers of these early structures also remains debated, due to their rarity and short duty cycles. To test the signposting ability of DSFGs, we study the environments of two luminous, gravitationally lensed DSFGs: SDP.17b ($z = 2.305$) and HELMS-55 ($z = 2.283$), using deep $K_s$-band Gemini South Flamingos-2 imaging and multiwavelength ancillary photometry. Based on photometric redshifts, we identify at least 6 and 15 companion galaxies within 5.5 cMpc, corresponding to galaxy overdensities of $\delta = 0.3 \pm 0.2$ ($0.6 \pm 0.4\sigma$) and $\delta = 1.0 \pm 0.3$ ($2.2 \pm 0.6\sigma$), respectively. Lower-limit halo mass estimates suggest the SDP.17b field is likely to evolve into a present-day galaxy group, while the HELMS-55 field is consistent with the progenitor of a Fornax-like cluster. Notably, 50% of the most likely companion galaxies in HELMS-55 appear quiescent, suggesting early environmental quenching. These findings illustrate both the potential and the limitations of DSFGs as signposts of structure formation and highlight the importance of deep near-infrared imaging for probing the early growth of large-scale structure. The unexpectedly high fraction of quiescent galaxies in the HELMS-55 field underscores the need for simulations to better capture early environmental quenching in dense protocluster cores.