2025ApJ...995...28B

Studying the environments of dusty star-forming galaxies (DSFGs) provides insight into whether these luminous systems are reliable signposts of large-scale overdensities. Evidence suggests that individual DSFGs can trace overdense environments, although this association may not be universal. To test this, we investigate the environments surrounding two luminous, gravitationally lensed DSFGs (SDP.17b at z_spec = 2.3049 and HELMS-55 at z_spec = 2.2834). Using Gemini South Flamingos-2 (F2) K_s-band imaging together with ancillary Subaru Hyper Suprime-Cam and Hubble Space Telescope multiband photometry, we obtain photometric redshifts, z_phot, as well as star formation rates and stellar mass estimates for companion galaxies of the DSFGs. At least 5 ± 2 and 15 ± 3 companion galaxies exist with consistent z_phot (dz ≤ 0.2) within a projected separation of 5.5 cMpc of SDP.17b and HELMS-55, respectively. These correspond to galaxy overdensities of δ = 0.1 ± 0.2 and δ = 1.0 ± 0.3, with significances of (0.2 ± 0.4)σ and (2.2 ± 0.6)σ, respectively. On the ${M}_{{{\rm{H}}}_{2}}$-overdensity-significance plane, HELMS-55 may follow the positive correlation between the gas mass and the overdensity significance, while SDP.17b lies well above the relation despite its large gas reservoir, making it a potential outlier. Based on this study of two DSFGs, our photometric analysis suggests that DSFGs can trace the outskirts of protoclusters or associated large-scale structures. However, our small sample prevents firm conclusions about their ability to pinpoint dense cluster cores. Future multi-object spectroscopic observations are required to confirm the membership and star formation properties of the companion galaxies.