ODIN: Characterizing the Three-dimensional Structure of Two Protocluster Complexes at $z = 3.1$

TL;DR

This work develops and validates a probabilistic 3D reconstruction of the large-scale structure surrounding two prominent protocluster complexes at $z=3.1$ identified by ODIN. By merging photometric LAE distributions with targeted and wide-area spectroscopy, the authors map the 3D density field on ~50 cMpc scales, revealing irregular, filamentary structures with multiple density peaks and outskirts-rich LAB populations. Validation against the TNG300 simulation demonstrates that incorporating the full LAE sample substantially improves reconstruction accuracy over spectroscopic data alone, enabling robust descendant mass estimates within $\sim0.2$–$0.3$ dex. The results show COSMOS-z3.1-A as a proto-supercluster akin to Hyperion and illustrate how line-of-sight orientation shapes observed morphology, providing valuable guidance for optimizing future spectroscopic campaigns and deepening our understanding of the formation of ultra-massive structures in the early universe.

Abstract

We present a detailed study of the 3D morphology of two extended associations of multiple protoclusters at $z=3.1$. These protocluster 'complexes', designated COSMOS-z3.1-A and COSMOS-z3.1-C, are the most prominent overdensities of $z=3.1$ Ly$α$ emitters (LAEs) identified in the COSMOS field by the One-hundred-deg$^2$ DECam Imaging in Narrowbands (ODIN) survey. These protocluster complexes have been followed up with extensive spectroscopy from Keck, Gemini, and DESI. Using a probabilistic method that combines photometrically selected and spectroscopically confirmed LAEs, we reconstruct the 3D structure of these complexes on scales of $\approx$50 cMpc. We validate our reconstruction method using the IllustrisTNG300-1 cosmological hydrodynamical simulation and show that it consistently outperforms approaches relying solely on spectroscopic data. The resulting 3D maps reveal that both complexes are irregular and elongated along a single axis, emphasizing the impact of sightline on our perception of structure morphology. The complexes consist of multiple density peaks, ten in COSMOS-z3.1-A and four in COSMOS-z3.1-C. The former is confirmed to be a proto-supercluster, similar to {\it Hyperion} at $z=2.4$ but observed at an even earlier epoch. Multiple `tails' connected to the cores of the density peaks are seen, likely representing cosmic filaments feeding into these extremely overdense regions. The 3D reconstructions further provide strong evidence that Ly$α$ blobs preferentially reside in the outskirts of the highest density regions. Descendant mass estimates of the density peaks suggest that COSMOS-z3.1-A and COSMOS-z3.1-C will evolve to become ultra-massive structures by $z=0$, with total masses $\log(M/M_\odot) \gtrsim 15.3$, exceeding that of Coma.

Figures (15)

• Figure 1: Examples of the spectra of LAEs confirmed with Keck II/DEIMOS (top row) and Gemini/GMOS (bottom row). The black curve shows the $N501$ filter transmission function, while the peak of the Ly$\alpha$ line, used to infer the redshift, is indicated by a vertical red line. The 2D spectra (shown in the bottom of each panel) are slightly smoothed to better distinguish the presence of line emission. Red horizontal lines in the 2D spectra indicate the position of the source within the slit, slightly offset for clarity.
• Figure 2: As Figure \ref{['fig:spectra']}, but for LABs confirmed with DEIMOS. The Ly$\alpha$ line of LABs is, in general, wider than that of LAEs. The more luminous LABs show clear evidence of AGN origin, with a bright continuum and multiple observed emission lines (as shown in the bottom panel).
• Figure 3: Spectroscopically confirmed sources for COSMOS-z3.1-A (top) and COSMOS-z3.1-C (bottom), overlaid on the 2D LAE surface density map. White contours show the boundaries of the protocluster candidates identified from the 2D surface density map (i.e. as overdensities of photometrically selected LAEs). In the left-hand panels, all identified LAEs (dots) and LABs (stars) are shown, with blue symbols indicating objects confirmed via targeted DEIMOS/GMOS spectroscopy, green indicating those confirmed via wide-field DESI spectroscopy, and grey those which remain unconfirmed. In the right-hand panels, all confirmed LAEs and LABs are shown color-coded by redshift.
• Figure 4: Left: A 3D visualization of the smoothed priors showing the spatial distribution of galaxies, with two lines (green and blue) representing the line-of-sight at two randomly chosen $x$ and $y$ positions. Note that the third and fourth vertical lines from the left indicate the back face of the cubic volume under consideration. Right: Probability distributions along the line-of-sight for each corresponding position (green and blue lines), showing the prior used to assign redshifts to the LAEs with projected positions close to those $x$ and $y$ coordinates. The $N501$ transmission curve is shown as a grey dashed line.
• Figure 5: Comparison of the spatial distributions of LAEs and the underlying dark matter density in a 60x60x60 cMpc$^3$ region centered on the largest structure in TNG300 at $z~=~3$. Top: (a) Projected dark matter density (smoothed for visualization) over a 60x60x20 cMpc$^3$ slab. (b) Same dark matter projection, overlaid with spec-z mock LAE sample, color-coded by redshift. (c) Same dark matter projection, overlaid with all mock LAE sources in the sample (without redshift). Bottom: (d) The underlying dark matter distribution in 3D, serving as a theoretical reference. (e) The spec-z mock LAE sample in 3D, used as priors. (f) The full probabilistic reconstruction, which more closely follows the dark matter distribution, particularly in the central overdense regions. This demonstrates that probabilistic redshift assignment improves the recovery of LSS compared to relying solely on spec-z sources.