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LoVoCCS. III. Third Generation Pipeline & The Hercules Supercluster

This paper presents the third-generation data reduction pipeline for the Local Volume Complete Cluster Survey (LoVoCCS) and validates it using multi-plane weak-lensing analysis of the Hercules Supercluster, revealing a total mass of approximately $8.9 \times 10^{14}~M_{\odot}$ and a dynamical bias in Abell 2147 due to its recent periapsis passage.

Original authors: Anthony M. Englert, Shenming Fu, Ian Dell'Antonio, Mohamed H. Abdullah, Shrouk Abdulshafy, Eddie Aljamal, William K. Black, Nicole Chidester, Doug Clowe, M. C. Cooper, Megan Donahue, Zacharias Escalan

Published 2026-07-21

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CC BY 4.0

Original authors: Anthony M. Englert, Shenming Fu, Ian Dell'Antonio, Mohamed H. Abdullah, Shrouk Abdulshafy, Eddie Aljamal, William K. Black, Nicole Chidester, Doug Clowe, M. C. Cooper, Megan Donahue, Zacharias Escalante, August Evrard, Eric Habjan, Soren Helhoski, Binyang Liu, Jacqueline McCleary, Hironao Miyatake, Mireia Montes, Priyamvada Natarajan, Jessica Nelson, Michelle Ntampaka, Elena Pierpaoli, Marc Postman, Rahul Shinde, Jubee Sohn, Fuyuko Tanaka, David Turner, Keiichi Umetsu, Yousuke Utsumi, Ray Wang, Gillian Wilson

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, invisible web made of dark matter, stretching across the cosmos like a spider's web spun in the dark. Galaxies don't just float randomly; they gather in clusters along the thick strands of this web, and sometimes, these clusters clump together to form even bigger structures called superclusters. To understand how this cosmic web was built and how it's changing, scientists need to weigh these massive structures. But here's the catch: you can't put a galaxy cluster on a bathroom scale. Instead, astronomers use a trick called "weak gravitational lensing." Think of it like looking at a distant streetlight through a wavy, distorted window. The glass (which is actually the gravity of the galaxy cluster) bends the light, making the streetlight look slightly squashed or stretched. By measuring exactly how much the light from thousands of background galaxies is squashed, scientists can calculate how heavy the invisible cluster in front of them must be. This is crucial because the weight of these structures tells us about the rules of the universe itself, helping us figure out how much dark matter exists and how the cosmos will evolve.

In this paper, the LoVoCCS team introduces a brand-new, super-advanced set of tools—a "third-generation pipeline"—to clean up and analyze their telescope images with incredible precision. They tested this new system on a massive, chaotic neighborhood in the sky called the Hercules Supercluster. This isn't just one cluster; it's a busy intersection where several galaxy clusters are crashing into each other, including the famous Abell 2147, 2151, and 2152. The team used their new pipeline to create a detailed "mass map" of this area, effectively weighing the entire supercluster. They found that the total mass of this cosmic complex is about $8.9^{+1.7}_{-1.4} \times 10^{14}$ times the mass of our Sun ( $M_\odot$ ).

However, the most exciting discovery comes from looking at Abell 2147. When the team weighed this specific cluster using their new lensing method, they got a different number than what was predicted by looking at how the galaxies inside were moving (a method called dynamical mass). The lensing method, which is considered more reliable because it measures gravity directly, showed the cluster was lighter than the movement of its stars suggested. The paper explains this mismatch by suggesting that Abell 2147 is in the middle of a violent crash. It appears to be about 0.2 to 0.4 billion years past the point where two clusters smashed into each other and bounced apart (periapsis). Because the cluster is still shaking and settling from this collision, the usual methods of weighing it by watching its stars move are getting confused and giving a falsely heavy reading. The new pipeline successfully caught this "dynamical bias," proving that the cluster is indeed out of equilibrium.

The paper also confirms that their new data processing tools are working beautifully. They managed to reach a depth of observation that rivals the upcoming Vera C. Rubin Observatory's first year of data, even though they are using older telescope equipment. They validated that their new way of correcting for "shear" (the stretching of galaxy shapes) is much better at filtering out errors than their old methods. While they found that some of their measurements of star positions and brightness aren't quite as perfect as the strictest future goals yet, they are close enough to be incredibly useful. The team concludes that their new pipeline is ready to weigh the rest of the nearby universe, providing a clearer picture of how these giant cosmic structures form and interact, long before the next generation of telescopes even turns on.

Technical Summary: LoVoCCS. III. Third Generation Pipeline & The Hercules Supercluster

Problem Statement
The Local Volume Complete Cluster Survey (LoVoCCS) aims to provide a volume-complete sample of nearby ( $0.03 < z < 0.12$ ), X-ray luminous galaxy clusters to serve as a precursor and validation dataset for the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). While previous reductions (Gen2) utilized LSST Science Pipelines (LSP) v19, significant updates to the LSP and new observational requirements necessitated a comprehensive overhaul of the data reduction pipeline. Furthermore, existing mass estimates for complex structures like the Hercules Supercluster often suffer from biases due to the dynamical state of merging clusters and the presence of massive background deflectors along the line of sight. The paper addresses the need for a "Third Generation" (Gen3) pipeline that corrects for specific systematics (such as the Brighter-Fatter Effect and Point Spread Function modeling) and implements advanced shear calibration to produce unbiased weak lensing mass estimates for superclusters.

Methodology
The authors present the Gen3 pipeline, built on LSP v26, which processes DECam observations for the LoVoCCS sample. Key methodological components include:

Calibration and Reference Data: The pipeline constructs master calibration frames (bias, flat, fringe) from raw data using the cp_pipe package, replacing NOIRLab's master frames. It incorporates the Brighter-Fatter Effect (BFE) correction using kernels derived from photon transfer curves, a step omitted in Gen2.
Sky Correction: To preserve low surface brightness features often removed by standard background models, the pipeline utilizes skycorr to generate sky-corrected coadds in addition to standard deep coadds.
Shear Calibration: The paper implements the metadetect algorithm for shear calibration. Unlike the previous HSC-Y1 calibration method, metadetect creates synthetic sheared realizations of the data itself to explicitly measure the response matrix ( $R$ ) and correct for PSF-induced biases and selection effects. This is performed on large $\sim 20' \times 20'$ patches to ensure PSF uniformity.
Multi-Plane Lensing: To analyze the Hercules Supercluster, the authors employ a multi-plane lens model. This accounts for multiple deflectors at different redshifts along the line of sight, calculating the magnification tensor recursively to avoid biases from background structures.
Mass Reconstruction: Mass maps are generated using the aperture mass statistic ( $M_{ap}$ ) with a Schirmer filter. Statistical significance is determined by generating $\sim 900$ noise maps via random phase rotation of galaxy shears to establish a peak-signal-to-noise distribution.
Validation: The pipeline is validated using the Hercules Supercluster field, comparing astrometry against Gaia DR3, photometry against Pan-STARRS/SDSS, and photometric redshifts against DESI DR1 spectroscopic data.

Key Contributions

Gen3 Pipeline Release: A fully documented, public pipeline (lovoccs_pipe) that integrates modern LSP features, including scarlet lite for multi-band deblending and piff for PSF modeling.
Systematic Corrections: Implementation of BFE correction and sky-frame subtraction to improve shape measurements and low surface brightness science.
Shear Calibration: Application of the metadetect algorithm to LoVoCCS data, providing a self-calibrated shear catalog that mitigates PSF modeling errors and selection biases.
Hercules Supercluster Analysis: A comprehensive weak lensing analysis of the Hercules Supercluster (covering $\sim 16$ deg $^2$ ), including Abell 2147, 2151, 2152, and Clumps A and B, utilizing multi-plane lensing to disentangle foreground and background structures.

Results

Pipeline Performance: The Gen3 pipeline achieves photometric scatter of $\sim 10-20$ mmag in $griz$ and astrometric scatter of $\lesssim 50$ mas. While these metrics fall short of LSST Y1 requirements (due to the empirical nature of joint calibration), they meet the specific goals for LoVoCCS science. Photometric redshifts show a scatter of $\sigma_z/(1+z) \sim 0.051$ with an outlier rate of $\sim 5.3\%$ .
Shear Calibration Validation: Null tests confirm that the metadetect catalog offers superior control over systematics compared to the HSM catalog, with PSF leakage ( $\zeta_{sys}$ ) reduced to $< 10^{-5}$ .
Hercules Mass Estimates:
- Abell 2147: The weak lensing mass ( $M_{200} \approx 3.2 \times 10^{14} M_\odot$ ) is consistent with X-ray estimates but significantly lower ( $>5\sigma$ ) than the dynamical mass ( $\sim 13.5 \times 10^{14} M_\odot$ ). This discrepancy supports the hypothesis that A2147 is in a post-periapsis merger state ( $\sim 0.2-0.4$ Gyr), where dynamical mass estimates are biased high.
- Abell 2151 & 2152: A2151 has a lensing mass of $2.7 \times 10^{14} M_\odot$ , consistent with dynamical and X-ray estimates, suggesting it is in early merger stages. A2152 is found to be a blend of a foreground cluster and a massive background cluster ( $z \sim 0.13$ ), requiring multi-plane modeling to separate their contributions.
- Clump A: Detected as a tri-modal structure with a lensing mass of $2.2 \times 10^{14} M_\odot$ , consistent with dynamical estimates, despite a lack of X-ray emission.
- Total Supercluster Mass: The total weak lensing mass of the Hercules Supercluster complex is estimated at $8.9^{+1.7}_{-1.4} \times 10^{14} M_\odot$ .

Significance
The paper demonstrates that the Gen3 pipeline successfully produces high-fidelity data products suitable for precision weak lensing science, serving as a critical testbed for LSST. The analysis of the Hercules Supercluster highlights the necessity of multi-plane lensing models and robust shear calibration to avoid biases in mass estimation, particularly in complex merging systems. The authors claim that LoVoCCS will remain the deepest and most homogeneous view of southern sky clusters until LSST Year 2 data products are released, providing a legacy dataset for studying cluster astrophysics, intracluster light, and supercluster dynamics. The specific finding regarding A2147's biased dynamical mass underscores the importance of weak lensing as a nearly unbiased mass estimator for clusters in non-equilibrium states.

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