[Paper Review] Galapagos-2/Galfitm/GAMA -- multi-wavelength measurement of galaxy structure: separating the properties of spheroid and disk components in modern surveys

Boris Häußler, Marina Vika+7 more|arXiv (Cornell University)|Apr 12, 2022

Galaxies: Formation, Evolution, Phenomena90 references43 citations

TL;DR

This paper introduces Galapagos-2 and Galfitm, a multi-wavelength fitting framework that improves galaxy structural decomposition by simultaneously fitting bulge and disk components across multiple bands. It demonstrates that multi-band fitting significantly enhances parameter accuracy—especially for SEDs, magnitudes, and component sizes—enabling reliable structural measurements down to fainter magnitudes compared to single-band methods.

ABSTRACT

We present the capabilities of Galapagos--2 and Galfitm in the context of fitting 2-component profiles to galaxies, on the way to providing complete multi-band, multi-component fitting of large samples of galaxies in future surveys. We release both the code and the fit results to 234,239 objects from the DR3 of the Gama survey, a sample significantly deeper than previous works. We use stringent tests on both simulated and real data, as well as comparison to public catalogues to evaluate the advantages of using multi-band over single-band data. We show that multi-band fitting using Galfitm provides significant advantages when trying to decompose galaxies into their individual constituents, as more data are being used, by effectively being able to use the colour information buried in the individual exposures to its advantage. Using simulated data, we find that multi-band fitting significantly reduces the deviations from real parameter values, allows component sizes and S\'ersic indices to be recovered more accurately, and, by design, constrains the band-to-band variations of these parameters to more physical values. On both simulated and real data, we confirm that the SEDs of the 2 main components can be recovered to fainter magnitudes compared to using single-band fitting, which tends to recover disks and bulges to - on average - have identical SEDs when the galaxies become too faint, instead of the different SEDs they truly have. By comparing our results to those provided by other fitting codes, we confirm that they agree in general, but measurement errors can be significantly reduced by using the multi-band tools developed by the MegaMorph project. We conclude that the multi-band fitting employed by Galapagos-2 and Galfitm significantly improves the accuracy of structural galaxy parameters and enables much larger samples to be be used in a scientific analysis.

Motivation & Objective

• To develop a scalable, multi-wavelength fitting framework for decomposing galaxy light profiles into bulge and disk components.
• To improve the accuracy of structural parameters (e.g., magnitude, size, Sérsic index) by leveraging color information across multiple bands.
• To enable robust component SED recovery at faint magnitudes, where single-band fitting fails to distinguish bulge and disk SEDs.
• To release a comprehensive catalog of 234,239 multi-band, two-component fits from the GAMA DR3 survey for public use.
• To validate the method using simulated and real data, comparing results to existing codes and demonstrating reduced measurement errors.

Proposed method

• Galapagos-2 automates multi-band profile fitting using Galfitm, applying 2-component (bulge/disk) models across multiple photometric bands.
• The method uses multi-band data to constrain band-to-band variations in structural parameters, enforcing physical consistency across wavelengths.
• It employs a joint fitting approach that simultaneously fits light profiles in all bands, using color information to break degeneracies in component parameters.
• The framework includes dust correction models (e.g., from Pastrav et al.) and supports rest-frame SED estimation via interpolation of Chebyshev polynomials.
• It is built on the Galfit codebase with extensions for multi-band fitting and automated source processing.
• The pipeline is tested on simulated galaxies and real GAMA data, with comparisons to single-band fitting and other codes (e.g., Gim2d, ProFit) to validate performance.

Experimental results

Research questions

• RQ1 Can multi-band fitting significantly improve the accuracy of bulge and disk structural parameters compared to single-band fitting?
• RQ2 To what extent can multi-band fitting recover distinct SEDs for bulge and disk components at faint magnitudes?
• RQ3 How do the recovered parameters (e.g., magnitude, size, Sérsic index) compare to truth values in simulated galaxies with known input parameters?
• RQ4 What are the limitations of two-component fitting in extreme B/T regimes (e.g., B/T < 0.2 or B/T > 0.8)?
• RQ5 How does the performance of Galapagos-2 compare to existing fitting codes in terms of measurement error and consistency?

Key findings

• Multi-band fitting reduces parameter deviations from true values in simulations, improving recovery of component sizes and Sérsic indices.
• The method enables reliable SED recovery for bulge and disk components down to fainter magnitudes than single-band fitting, which tends to produce identical SEDs for both components in faint galaxies.
• Component magnitudes and colors are measured with significantly reduced errors compared to single-band fitting, especially in the presence of noise and resolution limits.
• In extreme B/T regimes (B/T < 0.05 or B/T > 0.9), two-component models are less reliable, and single-Sérsic fits often provide a better match to the data.
• The code achieves good agreement with other fitting codes (e.g., Gim2d), but measurement errors are systematically lower when using the multi-band approach.
• The Sérsic index from a single-Sérsic fit is a weak indicator of B/T ratio, limiting its use as a proxy for bulge prominence.