Euclid: Early Release Observations -- Programme overview and pipeline for compact- and diffuse-emission photometry

Abstract: The Euclid ERO showcase Euclid's capabilities in advance of its main mission, targeting 17 astronomical objects, from galaxy clusters, nearby galaxies, globular clusters, to star-forming regions. A total of 24 hours observing time was allocated in the early months of operation, engaging the scientific community through an early public data release. We describe the development of the ERO pipeline to create visually compelling images while simultaneously meeting the scientific demands within months of launch, leveraging a pragmatic, data-driven development strategy. The pipeline's key requirements are to preserve the image quality and to provide flux calibration and photometry for compact and extended sources. The pipeline's five pillars are: removal of instrumental signatures; astrometric calibration; photometric calibration; image stacking; and the production of science-ready catalogues for both the VIS and NISP instruments. We report a PSF with a full width at half maximum of 0.16" in the optical and 0.49" in the three NIR bands. Our VIS mean absolute flux calibration is accurate to about 1%, and 10% for NISP due to a limited calibration set; both instruments have considerable colour terms. The median depth is 25.3 and 23.2 AB mag with a SNR of 10 for galaxies, and 27.1 and 24.5 AB mag at an SNR of 5 for point sources for VIS and NISP, respectively. Euclid's ability to observe diffuse emission is exceptional due to its extended PSF nearly matching a pure diffraction halo, the best ever achieved by a wide-field, high-resolution imaging telescope. Euclid offers unparalleled capabilities for exploring the LSB Universe across all scales, also opening a new observational window in the NIR. Median surface-brightness levels of 29.9 and 28.3 AB mag per square arcsec are achieved for VIS and NISP, respectively, for detecting a 10 arcsec x 10 arcsec extended feature at the 1 sigma level.

• The paper demonstrates Euclid ERO's advanced photometry pipeline optimized for both compact and diffuse emissions.
• It details a robust calibration and astrometric stacking process using state-of-the-art algorithms for millisecond precision and accurate PSF measurements.
• The study sets new benchmarks in sensitivity and imaging quality, offering pioneering insights for future cosmic structure and dark energy research.

Overview of the Euclid Early Release Observations Program

This scholarly article discusses the early phase operations of the Euclid mission, specifically focusing on the Euclid Early Release Observations (ERO) program. Euclid is a space mission initiated by the European Space Agency (ESA) under its Cosmic Vision program. The mission's primary goal is to explore dark energy through an extensive extragalactic survey, employing optical imaging alongside near-infrared (NIR) imaging and spectroscopy. This paper provides an in-depth evaluation of the ERO program, detailing its objectives, data processing pipeline, and the unique scientific capabilities demonstrated by Euclid's early observations.

Objectives and Methodology

The ERO program aims to illustrate Euclid's advanced capabilities by conducting scientific observations of 17 selected astronomical targets, ranging from galaxy clusters to star-forming regions. These preliminary observations were intended to engage the scientific community and the public by offering an early look at Euclid's potential before its main mission officially commenced. The resulting datasets were released promptly to contribute to early scientific research and public outreach.

Data Processing Pipeline

1. Detrending and Calibration: The pipeline meticulously removes instrumental artifacts and calibrates the data for astrometric and photometric accuracy. The approach borrowed from operations at the Canada-France-Hawaii Telescope (CFHT), was implemented to ensure optimal data quality.
2. Astrometry and Stacking: Using sophisticated algorithms, such as those found in the AstrOmatic suite, the pipeline achieves millisecond-precision astrometric calibration and efficient image stacking for both VIS and NISP instruments.
3. PSF and Photometric Assessment: The characterization of the Point Spread Function (PSF) is crucial for precise object detection and photometry, both in terms of extracting compact sources and examining extended emission.

Results and Performance

• Imaging Quality: The PSF was found to have a Full Width at Half Maximum (FWHM) of $\ang{0.16}$ in the optical band (IE) and $\ang{0.49}$ in the NIR bands (YE, JE, HE), highlighting Euclid's diffraction-limited performance.
• Sensitivity: The Euclid ERO demonstrated impressive point-source detection limits, reaching depths of 25.3 (IE) and 23.2 (NIR) AB magnitudes for galaxies at an SNR of 10, with equivalent capabilities in extended source detection.
• Surface Brightness Observations: Euclid's capacity to detect low-surface-brightness (LSB) features contrasts remarkably with existing imaging telescopes, offering unique opportunities for studies in diffuse emission regions, such as those caused by galactic cirrus.

Implications and Future Directions

The ERO program has established a foundation for Euclid's main mission, setting rigorous benchmarks for the examination of cosmic structures and the large-scale distribution of matter. By achieving unprecedented sensitivity and resolution, Euclid opens new avenues for understanding dark energy and cosmic acceleration.

The paper concludes by projecting future improvements in data processing and the potential inclusion of more automated photometric capabilities, which will enhance Euclid's utility in both high-redshift and LSB science. Moreover, the research insights gained from the ERO will pave the way for subsequent operational methodologies and advanced studies into foundational cosmological principles.

Overall, the Euclid ERO program showcases the mission's leading-edge scientific capabilities, underpinning its long-term objective to unravel the mysteries of the Universe's expansion dynamics.