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Dissecting the Perseus-Pisces supercluster observed with CFHT-MegaCam: Exploring late-type galaxy shape alignments within the local cosmic web

Using deep CFHT-MegaCam imaging of the Perseus-Pisces supercluster, this study reveals distinct intrinsic alignment mechanisms where late-type galaxies in filaments are shaped by tidal torquing while early-type galaxies near cluster centers are influenced by tidal stretching, providing crucial local constraints for modeling cosmic shear contamination in future large-scale surveys.

Original authors: M. Mondelin, S. Codis, J. -C. Cuillandre, R. Paviot, T. de Boer

Published 2026-05-25

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CC BY 4.0

Original authors: M. Mondelin, S. Codis, J. -C. Cuillandre, R. Paviot, T. de Boer

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 not as a random scattering of stars, but as a giant, invisible spiderweb made of gravity. This "cosmic web" is made of long, thin strands called filaments, with massive knots where galaxies cluster together.

This paper is like a high-resolution detective story about how galaxies get their shapes and orientations while living in this web. Specifically, the authors looked at a huge neighborhood of galaxies called the Perseus-Pisces Supercluster. They used a powerful telescope (CFHT-MegaCam) to take incredibly deep photos, seeing not just the bright centers of galaxies, but their faint, outer edges—like seeing the faint ripples around a stone dropped in a pond.

Here is what they found, explained simply:

1. The Two Main Types of Galaxies

The researchers split the galaxies into two groups, like sorting people into two different sports teams:

Early-types (The "Round" Team): These are elliptical galaxies. They look like fluffy, round balls or footballs. They usually live in the crowded "cities" of the universe (dense galaxy clusters).
Late-types (The "Flat" Team): These are spiral galaxies (like our Milky Way). They look like flat, spinning pancakes or frisbees. They are more common in the "suburbs" and along the strands of the cosmic web.

2. The Big Discovery: They Are All "Dancing" Together

The main question was: Do these galaxies just point in random directions, or do they align with the cosmic web?

The answer is yes, they align, but the two teams dance to different tunes:

The Round Team (Early-types) follows the "City Center":
Think of these galaxies as people in a crowded room. They tend to stretch out and point their long axes directly toward the center of the crowd (the galaxy cluster). The authors found that these galaxies are being "stretched" by the heavy gravity of the cluster, like dough being pulled by a baker. They align radially, pointing inward.
The Flat Team (Late-types) follows the "Highway":
These galaxies are like cars driving on a highway. They don't point toward a single center; instead, they line up along the long strands of the cosmic web (the filaments). The authors found a very strong signal here: the spiral galaxies are spinning and stretching in a way that matches the direction of the cosmic "highways."
- Crucial Detail: The authors found that the spiral galaxies are actually the ones driving the strongest alignment signals in the universe, even though they are just one type of galaxy. They are the "super-aligners."

3. Looking at the "Outer Edges"

One of the clever things this paper did was look at galaxies at different distances from their center.

Imagine a galaxy as an onion. The authors looked at the inner layers (bright core), the middle layers, and the very outer, faint skin (the "low surface brightness" regions).
The Surprise: Even though the outer skin of a galaxy is very faint and loosely held together, it still points in the same direction as the bright core.
The Analogy: It's like a kite. The tail (the outer edge) is loose and flapping, but it still points in the same general direction as the handle (the core) because the wind (the cosmic tidal field) is blowing on the whole thing. The alignment signal didn't disappear in the outer edges; it stayed strong.

4. Why This Matters (According to the Paper)

The authors explain that understanding this "dance" is critical for future telescopes (like Euclid and LSST) that are trying to map the invisible "dark matter" in the universe.

The Problem: When we look at distant galaxies, their shapes are distorted by gravity (lensing), which helps us find dark matter. But, galaxies are also distorted by the cosmic web itself (intrinsic alignment).
The Solution: If we don't account for the fact that spiral galaxies naturally line up with the cosmic web, we might mistake that natural alignment for dark matter. This paper proves that we need to treat spiral galaxies very carefully when doing these calculations because they are the ones creating the strongest "noise" in the signal.

Summary

The paper is a detailed map of how galaxies in a specific neighborhood of the universe are oriented. It reveals that:

Round galaxies point toward the crowded centers of galaxy clusters.
Flat, spiral galaxies line up along the long strands of the cosmic web.
Spiral galaxies are the most important players in this alignment game.
This alignment happens all the way out to the faint, outer edges of the galaxies, proving that the cosmic web's influence reaches far beyond the bright centers.

They did this by taking super-deep photos and using 3D data to see exactly where every galaxy is, ensuring they weren't just looking at a flat picture but a real, three-dimensional structure.

Technical Summary: Dissecting the Perseus-Pisces Supercluster Observed with CFHT-MegaCam

Problem Statement
Intrinsic alignments (IA) of galaxy shapes constitute a critical systematic for weak gravitational lensing surveys, as they can mimic or dilute the cosmic shear signal, thereby biasing cosmological parameter estimates. While previous studies have largely relied on statistical samples from large photometric surveys (e.g., SDSS, CFHTLS, UNIONS) to characterize IA amplitudes as a function of redshift and luminosity, there remains a need to understand the physical mechanisms driving these alignments in well-characterized local environments. Specifically, the interplay between galaxy morphology, stellar mass, and the large-scale cosmic web (filaments, clusters, voids) requires high-resolution investigation to distinguish between tidal stretching (dominant in dense environments) and tidal torquing (dominant in filaments). Furthermore, the behavior of galaxy shapes in the low-surface-brightness (LSB) outer regions—where stars are less gravitationally bound and potentially more susceptible to external tidal fields—has not been extensively probed in the context of intrinsic alignments.

Methodology
This study analyzes the Perseus–Pisces Supercluster (PPSC) at $z < 0.03$ , leveraging a combination of deep imaging and spectroscopic data.

Data: The authors utilize deep $r$ -band imaging from the Canada–France–Hawaii Telescope (CFHT) MegaCam, covering 367 deg $^2$ (primarily from the UNIONS survey), reaching a surface brightness limit of 28 mag arcsec $^{-2}$ . This is combined with the spectroscopic redshift catalog from a companion study (Paper I) to establish precise 3D positions.
Sample: The analysis focuses on 2004 galaxies with $\log M_*/M_\odot > 8.6$ . Galaxies are stratified by morphological type: Early Types (ET; Ellipticals and S0) and Late Types (LT; Spirals and Irregulars). Approximately 10% of galaxies showing strong tidal interactions were excluded to avoid local contamination.
Shape Measurements: Galaxy shapes are measured at three isophotal radii: $R_{23}$ , $R_{25}$ , and $R_{27}$ (corresponding to surface brightness thresholds of 23, 25, and 27 mag arcsec $^{-2}$ ). Ellipticity ( $e$ ) and position angle ( $\theta_{PA}$ ) are decomposed into Cartesian components ( $e_1, e_2$ ).
Statistical Analysis: The study computes two-point correlation functions in comoving coordinates:
- Shape–Shape ( $\xi_+$ ): Measures the coherence of galaxy orientations at a given separation (probing intrinsic-intrinsic alignment).
- Position–Shape ( $\xi_{g+}$ ): Measures the radial or tangential alignment of surrounding galaxies relative to a reference object (probing tidal shear effects).
Definitions: "Strongly Correlated Galaxies" (SCGs) are identified as systems whose cumulative contribution to the correlation signal in the first three comoving separation bins exceeds the mean by one standard deviation.
Environmental Context: Alignments are analyzed relative to the cosmic web structure (filament spines and cluster/group centers) reconstructed in Paper I using DisPerSE.

Key Results

Detection of Signals: Positive intrinsic alignment signals are detected for both early- and late-type galaxies, extending to $\sim 1$ Mpc/ $h$ in comoving space. The shape–shape correlation ( $\xi_+$ ) is substantially stronger than the position–shape correlation ( $\xi_{g+}$ ), with a signal-to-noise ratio of $\sim 10$ at $r \lesssim 0.1$ Mpc/ $h$ for $\xi_+$ .
Morphological Dependence:
- Late-Type Dominance: Late-type galaxies strongly dominate the shape–shape signal. They constitute 86% of the SCGs in $\xi_+$ , despite representing only 69% of the total sample. These galaxies exhibit systematically higher ellipticities ( $e \approx 0.55–0.65$ ) compared to the full sample, consistent with edge-on disk configurations aligned with the tidal field.
- Early-Type Behavior: Early-type galaxies show no significant over-representation in the shape–shape SCG population. However, they are preferentially found in position–shape correlations ( $\xi_{g+}$ ) near cluster and group centers, where their orientations are radially aligned toward the halo center.
Environmental Segregation:
- SCGs are systematically closer to filament spines and cluster/group centers than the general population.
- Late-type SCGs are found preferentially in filamentary regions, supporting a mechanism of coherent tidal torquing during accretion.
- Early-type SCGs concentrate near cluster/group centers, supporting a mechanism of tidal stretching in dense environments.
Radial Dependence: Correlation profiles show minimal variation across the three isophotal radii ( $R_{23}, R_{25}, R_{27}$ ), despite 10–20% of galaxies showing ellipticity variations between radii. This suggests that local morphological perturbations do not significantly alter the statistical alignment signal at the population level, though outer isophotes show a slight tendency for increased large-scale amplitude.
Direct Alignment: Direct measurements of the angle between galaxy major axes and cosmic web directions generally show isotropy. A marginal signal of radial alignment for ellipticals toward cluster centers and a tentative perpendicular alignment for spirals in specific filaments (Region B) were observed, but these are limited by sample size and projection effects.

Significance and Claims
The paper claims to provide a high-resolution, local-universe calibration of intrinsic alignment systematics, which is essential for interpreting upcoming large-scale surveys like Euclid, DESI, and LSST.

Mechanism Differentiation: The results offer observational evidence distinguishing between two alignment mechanisms: tidal stretching in dense cluster environments (dominating early-types) and tidal torquing in filaments (dominating late-types).
Role of Late-Types: The study highlights that blue, spiral late-type galaxies are a major contributor to intrinsic alignment signals in the low-redshift Universe, challenging the historical assumption that they contribute negligibly. This necessitates careful modeling of these populations in cosmic shear analyses.
Methodological Value: The combination of deep LSB imaging, precise 3D structure reconstruction, and multi-radius shape measurements demonstrates the utility of local superclusters as laboratories for testing alignment physics. The authors note that while direct geometric alignment measurements were limited by the volume of a single supercluster, the pair-based correlation statistics proved to be more sensitive probes in this regime.
Future Calibration: The findings underscore the need for future surveys to account for morphology and environment when modeling IA systematics. The authors suggest that future observations with Euclid's high image quality could extend these shape measurements to even fainter isophotes ( $R_{29}$ and beyond), further probing the coupling between outer stellar halos and the cosmic web.

1. The Two Main Types of Galaxies

2. The Big Discovery: They Are All "Dancing" Together

3. Looking at the "Outer Edges"

4. Why This Matters (According to the Paper)

Summary

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