# Euclid preparation XXVI: The Euclid Morphology Challenge. Towards structural parameters for billions of galaxies

**Authors:** S. Davini, N. Mauri, L. Patrizii, G. Sirri, L. Wang, Y. Wang, A. A. Nucita, O. Ilbert, M. Meneghetti,  Euclid Collaboration, G. Desprez, S. Paltani, J. Coupon, M. Brescia, S. Cavuoti, S. Fotopoulou, W. G. Hartley, M. Castellano, F. Dubath, E. Merlin  
**arXiv:** [2209.12907](https://arxiv.org/abs/2209.12907)  
**Date:** 2022-09-26  
**Summary page:** https://www.summarizepaper.com/en/paper/2209.12907v1/

## Key points

- The Euclid imaging surveys will cover an unprecedented area of 15,000 square degrees with high spatial resolution and become a reference for studies of galaxy morphology.
- The Euclid Morphology Challenge was conducted to understand the capabilities of measuring morphologies from Euclid-detected galaxies and to help implement measurements in the pipeline.
- Five state-of-the-art surface-brightness-fitting codes were evaluated on a sample of about 1.5 million simulated galaxies resembling reduced observations with the Euclid VIS and NIR instruments.
- All methods tended to achieve reliable structural measurements down to an apparent magnitude of about 23 in one component and 21 in two components, but results were typically degraded by a factor of three when tested on non-analytic profiles due to systematics.
- Different behavior at the faint end is found to be explained by the set of adopted priors for various structural parameters.
- Bulge-disk decompositions are more challenging to perform than single-Sérsic simulations.
- Reliable structural measurements can be achieved down to an apparent magnitude of about 23 in one component and 21 in two components using state-of-the art surface brightness fitting codes such as DeepLeGATo, Galapagos -2, Morfometryka, Profit, and SourceXtractor++.
- The official Data Releases from Euclid are expected to deliver robust structural parameters for at least 400 million galaxies in the Euclid Wide Survey by the end of its mission.

## Summary

The Euclid imaging surveys are set to become a reference for studies of galaxy morphology by delivering imaging over an unprecedented area of 15,000 square degrees with high spatial resolution. To understand the capabilities of measuring morphologies from Euclid-detected galaxies and to help implement measurements in the pipeline, the Euclid Morphology Challenge was conducted. This challenge is presented in two papers, with one focusing on photometry analysis and the other assessing the accuracy of parametric galaxy morphology measurements in imaging predicted from within the Euclid Wide Survey. In this paper, five state-of-the-art surface-brightness-fitting codes (DeepLeGATo, Galapagos-2, Morfometryka, Profit, and SourceXtractor++) were evaluated on a sample of about 1.5 million simulated galaxies resembling reduced observations with the Euclid VIS and NIR instruments. The simulations included analytic S\'ersic profiles with one and two components as well as more realistic galaxies generated with neural networks. Despite some code-specific differences, all methods tended to achieve reliable structural measurements down to an apparent magnitude of about 23 in one component and 21 in two components. This corresponds to a signal-to-noise ratio of approximately 1 and 5 respectively. When tested on non-analytic profiles, however, results were typically degraded by a factor of three due to systematics. A key factor explaining different behavior at the faint end is found to be the set of adopted priors for various structural parameters. Looking ahead to Fig.6's summary plot reveals that all codes slightly overestimate galaxy sizes but there are no major differences between them regarding bias values. For intermediate magnitudes (IE =17-23), Galapagos-2 and SourceXtractor++ perform similarly while DeepLeGATo and Morfometryka have a higher positive bias. Single-Sérsic simulations yield the best results, with the lowest residuals, always remaining below 4σ of the noise level. Bulge-disk decompositions are more challenging to perform, as evident from the residuals of subtraction. In conclusion, this paper's evaluation shows that reliable structural measurements can be achieved down to an apparent magnitude of about 23 in one component and 21 in two components using state-of-the art surface brightness fitting codes such as DeepLeGATo , Galapagos - 2 , Morfometryka , Profit , and SourceXtractor ++ . The official Data Releases from Euclid are expected to deliver robust structural parameters for at least 400 million galaxies in the Euclid Wide Survey by the end of its mission .