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CANUCS/Technicolor Data Release 2: A Catalogue of Galaxy Structural Parameters in up to 29 HST+JWST bands and a Multi-Wavelength Exploration of the Galaxy Size-Mass Relation at $0.6 < z \leq 4$

This paper presents the second data release from the CANUCS/Technicolor surveys, providing structural parameters for approximately 41,000 galaxies across up to 29 HST and JWST bands, and utilizes a subset of 4,100 star-forming galaxies to reveal a wavelength-dependent size-mass relation with a critical crossover mass of $\sim 10^{9.5} M_\odot$ marking the transition between diffuse and compact morphologies.

Original authors: Maya Merchant, Lamiya A. Mowla, Georgios E. Magdis, Adam Muzzin, Chris J. Willott, Roberto Abraham, Yoshihisa Asada, Maruša Bradač, Gabriel B. Brammer, Guillaume Desprez, Kartheik G. Iyer, Nicholas S.

Published 2026-06-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Maya Merchant, Lamiya A. Mowla, Georgios E. Magdis, Adam Muzzin, Chris J. Willott, Roberto Abraham, Yoshihisa Asada, Maruša Bradač, Gabriel B. Brammer, Guillaume Desprez, Kartheik G. Iyer, Nicholas S. Martis, Gaël Noirot, Gregor Rihtaršič, Marcin Sawicki, Ghassan T. E. Sarrouh, Sunna Withers, Natalie Allen, Jacqueline Antwi-Danso, Westley Brown, Jon Judež, Danilo Marchesini, Rosa M. Mérida, Katherine Myers, Luke Robbins, Visal Sok

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, cosmic library. For decades, astronomers have been trying to read the "biographies" of galaxies to understand how they grow, change, and age. But there was a problem: they were mostly reading these biographies in black and white, or perhaps just one specific color of light. This made it hard to tell if a galaxy was a fluffy, young cloud of stars or a dense, compact ball of old stars.

This paper, written by Maya Merchant and a large team of astronomers, is like upgrading that library with a high-definition, full-color 3D scanner. Using the powerful James Webb Space Telescope (JWST), they took a massive snapshot of over 4,000 star-forming galaxies across a vast stretch of time (from 6 billion to 13 billion years ago). They didn't just look at them in one color; they looked at them in 19 different colors (filters), ranging from ultraviolet to near-infrared.

Here is what they discovered, explained simply:

1. The "Chameleon" Effect: Size Depends on Color

The biggest surprise is that a galaxy doesn't have just one fixed size. Its size changes depending on the "color" of light you use to measure it.

The Analogy: Think of a galaxy like a fruit salad.
- If you look at it under blue light (which highlights young, hot stars), you see the fresh, juicy berries on the outside. The salad looks big and spread out.
- If you look at it under red light (which highlights older, cooler stars), you see the dense, heavy fruit in the center. The salad looks smaller and more compact.
The Finding: For massive galaxies, the "blue" version looks much larger than the "red" version. This happens because young stars are forming in the outer edges (making the galaxy look big), while the older stars are packed tightly in the middle (making it look small).

2. The "Crossover" Point: The Magic Mass Number

The team found a specific "tipping point" in the mass of a galaxy, which they call the crossover mass.

The Analogy: Imagine a traffic light for galaxy growth.
- Below the light (Low Mass): Galaxies are like small, scattered campfires. Whether you look at them in blue or red light, they look roughly the same size. They are "diffuse" and don't have a strong center.
- Above the light (High Mass): Galaxies are like a bonfire with a dense core. Here, the color matters a lot. The blue light sees the sparks flying far out, but the red light sees the dense, hot center.
The Finding: This transition happens at a specific mass (about 3 billion times the mass of our Sun). Below this mass, galaxies are "outside-in" builders (forming stars in the middle first). Above this mass, they are "inside-out" builders (forming a dense core first, then growing a fluffy outer disk).

3. The Dusty Fog

Why do massive galaxies look so different in different colors? The paper suggests it's partly due to dust.

The Analogy: Imagine a foggy city.
- If you look at a city through a blue filter, the fog scatters the light, making the city look hazy and spread out over a large area.
- If you look through a red filter, the light cuts through the fog, revealing the actual, compact buildings in the center.
The Finding: Massive galaxies have a lot of dust concentrated in their centers. This dust blocks the blue light from the center, making the galaxy appear larger in blue light because you are only seeing the outer, unobscured edges. In red light, the dust is less of a problem, so you see the true, smaller size of the central core.

4. A New "Universal Ruler"

Because they measured galaxies in so many colors, the team created a new mathematical formula.

The Analogy: Before, if you wanted to know the size of a galaxy, you had to take a specific photo. Now, the team has built a universal translator.
The Finding: They created a formula that can predict a galaxy's size based on three things: how heavy it is, how old the universe is when you see it, and what "color" of light you are using. This means astronomers can now estimate the size of a galaxy even if they don't have a perfect photo of it in that specific color.

Summary

In short, this paper tells us that galaxies are not static objects with one fixed size. They are dynamic, multi-layered structures. To understand how they really grow, we have to look at them through many different "colors" of light. The study reveals a clear dividing line in the universe: small galaxies grow from the inside out, while massive galaxies build a dense core first and then grow a fluffy outer layer, all while hiding their true size behind a veil of cosmic dust.

Technical Summary: CANUCS/Technicolor Data Release 2 – A Catalogue of Galaxy Structural Parameters and Multi-Wavelength Size–Mass Relation

Problem Statement
Galaxy size is a fundamental tracer of galaxy evolution, mass assembly, and formation history. However, previous measurements of the galaxy size–mass relation have largely relied on single-band imaging, causing the probed rest-frame wavelengths to vary with redshift. To facilitate cross-wavelength comparisons, empirical color-gradient corrections are often applied to estimate sizes at a fixed rest-frame wavelength (typically 0.5 µm). This approach introduces systematic uncertainties because light is a biased tracer of stellar mass, dependent on stellar population age, metallicity, and dust attenuation geometry. Furthermore, simplifying assumptions regarding uniform stellar populations or dust models have obscured the true physical diversity of galaxy morphologies, particularly for star-forming galaxies which exhibit complex structures and diverse stellar populations. While Hubble Space Telescope (HST) studies provided extensive data in the rest-frame optical, they were limited in wavelength coverage beyond the rest-optical, forcing high-redshift ( $z > 3$ ) studies to rely on rest-frame UV measurements dominated by young stellar populations rather than the full underlying mass distribution.

Methodology
This study utilizes data from the Canadian NIRISS Unbiased Cluster (CANUCS) survey and the JWST in Technicolor program, covering five NIRCam flanking fields (MACS J1423, MACS J0417, Abell 370, MACS J0416, and MACS J1149). The dataset combines Cycle 1 and Cycle 2 JWST/NIRCam observations with ancillary HST imaging, providing coverage in up to 29 filters (19 used for morphology) spanning rest-frame wavelengths of $\sim 0.2$ – $3.2\,\mu$ m for galaxies at $0.6 < z \le 4$ .

Data Processing: Imaging data were processed using the grizli software package. Source detection was performed on a deep $\chi_{mean}$ detection image constructed from co-added JWST and HST images.
Morphological Fitting: Structural parameters were measured using GALFIT to fit single-component Sérsic profiles. The analysis focused on 19 JWST wide- and medium-band filters to maximize signal-to-noise and spatial resolution, excluding HST filters due to larger PSFs and the shortest-wavelength NIRCam filter (F070W) due to undersampling.
Sample Selection: From a parent sample of $\sim 41,000$ $\sim 41, 000$ galaxies, a robust multi-wavelength sample of $\sim 4,140$ $\sim 4, 140$ star-forming galaxies was selected. Selection criteria included:
- Redshift range: $0.6 < z \le 4$ .
- Stellar mass range: $8.5 < \log(M_*/M_\odot) \le 11.5$ (based on redshift-evolving mass completeness limits).
- Morphological quality: Sérsic index ( $0.1 < n < 10$ ) and axis ratio ( $0.05 < Q < 1$ ) must converge within bounds in at least 10 filters.
- Star-forming classification: Selected via rest-frame UVJ color criteria, excluding quiescent galaxies to isolate evolutionary pathways specific to star-forming systems.
Analysis Framework:
- Size–Mass Relations: Bayesian linear regression (LinMix) was used to fit size–mass relations at rest-frame 0.5 $\mu$ m (optical) and 1.5 $\mu$ m (NIR).
- Size–Wavelength Analysis: The dependence of galaxy size on rest-frame wavelength was analyzed by fixing stellar mass and redshift in turn, and then simultaneously introducing all three variables as free parameters in a weighted least-squares regression.

Key Contributions

Data Release: The paper presents the "CANUCS-Technicolor NCF Morphology Catalogue," providing structural parameters (effective radius, Sérsic index, axis ratio, position angle, magnitude) for $\sim 41,000$ galaxies across up to 29 JWST+HST filters.
Multi-Wavelength Parametrization: The authors derive a generalized parametrization of galaxy size as a continuous function of rest-frame wavelength ( $\lambda_{RF}$ ), stellar mass ( $M_*$ ), and photometric redshift ( $z_{phot}$ ) for $0.25\,\mu\text{m} < \lambda_{RF} < 2.0\,\mu\text{m}$ .
Identification of a Crossover Mass: The study identifies a characteristic stellar mass scale where the wavelength dependence of galaxy size transitions.

Key Results

Rest-Frame Size–Mass Relations:
- At rest-frame 0.5 $\mu$ m, the size–mass relation slope ( $\alpha$ ) remains relatively constant ( $\alpha \approx 0.23$ ) across redshifts $0.6 < z \le 4$ , with galaxy sizes decreasing as $(1+z)^{-0.71}$ .
- At rest-frame 1.5 $\mu$ m, the slope flattens moderately with increasing redshift, showing a $\sim 50\%$ decrease in $\alpha$ between $z \sim 0.8$ and $z \sim 1.7$ .
Size–Wavelength Dependence:
- A systematic gradient exists in the size–mass slope as a function of rest-frame wavelength. The NIR size–mass relation is consistently shallower than the rest-frame optical relation.
- This trend is strongly correlated with stellar mass ( $\rho = -0.47$ ) but shows negligible dependence on redshift ( $\rho = -0.01$ ).
- Crossover Mass: A characteristic crossover mass is identified at $\log(M_*/M_\odot) \approx 9.5 \pm 0.13$ . At this mass, size measurements across different filters converge, and the wavelength dependence of size is minimized.
- Above the Crossover Mass ( $\log M_* \gtrsim 9.5$ ): Galaxies are systematically smaller in the NIR than in the optical (by $\sim 10\%$ at $M_* \sim 10^{9.5}$ and $\sim 27\%$ at $M_* > 10^{10.5}$ ). This is interpreted as evidence of "inside-out" growth, where the central regions host older, more compact stellar populations and concentrated dust, while the outer disks are bluer and more extended.
- Below the Crossover Mass ( $\log M_* \lesssim 9.5$ ): Galaxies show little to no wavelength dependence, or are slightly larger in the NIR. This suggests a different structural configuration, potentially consistent with "outside-in" formation where star formation is centrally concentrated and the outskirts are dominated by older, diffuse populations.
Generalized Relation: The authors provide a functional form (Equation 9) to estimate effective radius based on mass, redshift, and wavelength, reproducing observed sizes with a residual scatter of $\sim 0.05$ dex.

Significance
The paper claims that rest-frame wavelength is a critical parameter in characterizing galaxy structure, and that multi-wavelength morphological measurements with JWST are essential for disentangling the effects of stellar population gradients, dust attenuation, and structural evolution. The identification of a stable crossover mass at $\log(M_*/M_\odot) \approx 9.5$ suggests a fundamental transition in galaxy formation physics, potentially marking the threshold where galaxies shift from centrally concentrated, outside-in star formation to extended, inside-out growth. This mass scale aligns with previous findings regarding the onset of significant bulge components, elevated dust attenuation, and the inflection point in the star-forming main sequence. The results provide quantitative constraints for hydrodynamical simulations of galaxy formation, specifically regarding the spatial distribution of stars and dust.

1. The "Chameleon" Effect: Size Depends on Color

2. The "Crossover" Point: The Magic Mass Number

3. The Dusty Fog

4. A New "Universal Ruler"

Summary

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