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

TL;DR

This study investigates whether dusty star-forming galaxies (DSFGs) at $z \sim 2$ reliably trace protocluster environments by analyzing two gravitationally lensed DSFGs, SDP.17b and HELMS-55, with wide-field $K_s$-band imaging plus optical and NIR data. Using photometric redshifts from EAZY-PY, a COSMOS field control, Monte Carlo companion counts, and CIGALE SED fitting, the authors quantify environment overdensities and derive halo-mass implications. They find HELMS-55 resides in a protocluster-like overdensity while SDP.17b lies in a field-like environment, illustrating that DSFGs inhabit a range of environments and that large, spectroscopically-confirmed samples are needed to establish a general trend. The work highlights the value of wide-field, rest-frame-optical/NIR surveys for mapping DSFG environments and guiding future protocluster searches.

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 $5\pm2$ and $15\pm3$ companion galaxies exist with consistent $z_\text{phot}$ ($dz \leq 0.2$) within a projected separation of 5.5 cMpc of SDP.17b and HELMS-55, respectively. These correspond to galaxy overdensities of $δ= 0.1 \pm 0.2$ and $δ =1.0 \pm 0.3$, with significances of $(0.2 \pm 0.4)σ$ and $(2.2 \pm 0.6) σ$, 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.

Figures (8)

• Figure 1: A subset of the SPOTLESS sample of Herschel DSFGs with available line FWHM measurements from the literature. Redshifts are determined via targeted, mostly interferometric, submillimeter CO line observations. The $y$-axis shows the FWHM of the line, serving as a proxy for the galaxy’s internal kinematics. Red markers indicate galaxies at $2.1 \lesssim z \lesssim 2.3$, where prominent spectral features fall within the $K$-band for MOS follow-up. Green and yellow markers highlight the two DSFGs studied in this paper: SDP.17b and HELMS-55, respectively. Blue markers denote the rest of the SPOTLESS sample. This plot is zoomed in to redshifts $1.5 \lesssim z \lesssim 4$ and FWHM $\leq 1450$ km s$^{-1}$ to highlight the bulk population of the sample.
• Figure 2: Summary of the photometric data for each target environment, including companion galaxies associated with the central DSFGs. Gemini South F2 $K_s$-band footprints are shown as RGB images: $K_s$ (red), $i$ (green), and $g$ (blue). Gaps in the F2 circular FOV arise from masking the OIWFS probe shadows during observations. The HSC optical grizy footprint (grayscale) extends beyond the F2 coverage, and HST F110W/F160W footprints are shown as white $\sim 2.05' \times 2.27'$ rectangles. Companion galaxies are marked with cyan circles, while the central DSFG is highlighted in green for SDP.17b and yellow for HELMS-55. Example companion galaxies are displayed in $5" \times 5"$ RGB cutouts, with their object ID indicated. Central DSFGs are shown in larger $10" \times 10"$ cutouts. SDP.17a is circled in magenta in SDP.17b's cutout.
• Figure 3: Example EAZY-PY SED fits of maximum likelihood companion galaxies. The left and right panels correspond to a companion galaxy from the SDP.17b and HELMS-55 target environments, respectively. Each panel shows the SED plot on the left and the probability distribution function of the photometric redshift, $p(z)$, with respect to the redshift on the right. The $z_\text{spec}$ value of the central DSFG is shown in red in the $p(z)$ plots, and the maximum likelihood $z_\text{phot}$ estimate, $z_\text{ml}$, is used to compute the best-fit SEDs, which are plotted in blue, with the $1\sigma$ error shaded around them. Template fluxes are shown as blue circles, while observed fluxes and their error bars are shown in black (SNR $> 0$) and gray (SNR $\leq 0$). Open markers at the bottom of the SED panels mark bands without flux measurements (e.g., HST F110W and F160W). For each source, cutouts measuring $10"$ by $10"$ are shown in the available filters, with $2.5"$ in diameter apertures overlaid, and filter names shown in yellow (e.g., grizy, $K_s$).
• Figure 4: Overdensity significances as a function of redshift for galaxy cluster and protocluster candidates photometrically identified in the literature. The overdensity significances of the SDP.17b and HELMS-55 environments are plotted in comparison as green and yellow triangles, respectively. Symbols represent different studies: gray circles for Castignani_2014, light-blue squares for Castignani_2014_radio, blue pluses for Castignani_2019, and purple diamonds for Calvi_2023. However, these papers do not report error bars on their overdensity significance. Spectroscopically confirmed protoclusters are marked with magenta crosses. The median value of the overdensity significance, assuming the number of companion galaxies in a given realization is equal to the number of field galaxies drawn from the COSMOS control sample ($N_{\text{target}, i} = N_{\text{COSMOS}, i}$), is shown as gray and black crosses at the $K_s$-band magnitude cuts of the SDP.17b and HELMS-55 environments, respectively.
• Figure 5: Molecular gas masses ($M_{\mathrm{H}_2}$) of our target DSFGs compared with the Calvi_2023 DSFGs sample, assuming $\alpha_\text{CO} =4.0\, M_\odot\,(K\,{\rm km\,s^{-1}\,pc^2})^{-1}$. SDP.17b is shown in green and HELMS-55 in yellow. For HELMS-55, we show estimates assuming different lensing magnifications ($\mu=4$ and $\mu=10$), since no lensing model currently exists in the literature. The shaded blue region marks the threshold below which overdensity significances are considered protocluster candidates. For the Calvi_2023 DSFGs, average overdensity significances were adopted; their colors correspond to their spectroscopic redshifts as indicated by the color bar, and marker shapes distinguish the individual DSFGs listed in the figure, where DSFGs near each other were grouped.