The Photometric Analysis of the Environment Around Two Dusty Star-Forming Galaxies at $z\sim 2$

Abstract

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 $z_{\text{spec}}=2.3049$ and HELMS-55 at $z_{\text{spec}}=2.2834$). Using Gemini South Flamingos-2 (F2) $K_s$-band imaging together with ancillary Subaru Hyper Suprime-Cam and Hubble Space Telescope multi-band photometry, we obtain photometric redshifts, $z_{\text{phot}}$, as well as star formation rates and stellar mass estimates for companion galaxies of the DSFGs. At least $5\pm 2$ and $15\pm 3$ companion galaxies exist with consistent $z_{\text{phot}}$ ($dz\le 0.2$) within a projected separation of 5.5 cMpc of SDP.17b and HELMS-55, respectively. These correspond to galaxy overdensities of $\delta =0.1\pm 0.2$ and $\delta =1.0\pm 0.3$, with significances of $(0.2\pm 0.4)\sigma$ and $(2.2\pm 0.6)\sigma$, respectively. On the $M_{{\mathrm{H}}_2}$-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.

23 pages (16 main text, 7 Appendix), 8 figures, 7 tables. Accepted to ApJ

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