Euclid: Early Release Observations -- Deep anatomy of nearby galaxies

Abstract: Euclid is poised to make significant advances in the study of nearby galaxies in the local Universe. Here we present a first look at 6 galaxies observed for the Nearby Galaxy Showcase as part of the Euclid Early Release Observations acquired between August and November, 2023. These targets, 3 dwarf galaxies (HolmbergII, IC10, NGC6822) and 3 spirals (IC342, NGC2403, NGC6744), range in distance from about 0.5 Mpc to 8.8 Mpc. Our assessment of the surface brightness depths in the stacked Euclid images confirms previous estimates in 100 arcsec^2 regions of 1sigma=30.5 mag/arcsec^2 for VIS, but slightly deeper than previous estimates for NISP with 1sigma=29.2-29.4 mag/arcsec^2. By combining Euclid HE, YE, and IE into RGB images, we illustrate the large field-of-view covered by a single Reference Observing Sequence, together with exquisite detail on parsec scales in these nearby galaxies. Radial surface brightness and color profiles demonstrate galaxy colors in agreement with stellar population synthesis models. Standard stellar photometry selection techniques find approximately 1.3 million stars across the 6 galaxy fields. Euclid's resolved stellar photometry allows us to constrain the star-formation histories of these galaxies, by disentangling the distributions of young stars, as well as asymptotic giant branch and red giant branch stellar populations. We finally examine 2 galaxies individually for surrounding satellite systems. Our analysis of the ensemble of dwarf satellites around NGC6744 reveals a new galaxy, EDwC1, a nucleated dwarf spheroidal at the end of a spiral arm. Our new census of the globular clusters around NGC2403 yields 9 new star-cluster candidates, 8 of which with colors indicative of evolved stellar populations. In summary, our investigation of the 6 Showcase galaxies demonstrates that Euclid is a powerful probe of the anatomy of nearby galaxies [abridged].

• The paper demonstrates Euclid's capability to capture high-resolution details, unveiling intricate structural and stellar properties in nearby galaxies.
• It employs VIS and NISP instruments to achieve surface brightness depths up to 30.5 AB magnitudes, enhancing analysis of low-brightness features.
• The study identifies satellite systems and globular clusters, supporting hierarchical galaxy formation models and advancing insights into galaxy evolution.

Overview of the Euclid Consortium Study on the Deep Anatomy of Nearby Galaxies

The paper "Euclid: Early Release Observations -- Deep anatomy of nearby galaxies" provides a comprehensive analysis of observations from the Euclid mission, focusing on elucidating the structural and stellar characteristics of six galaxies within the Local Universe. This study utilizes early release observations to demonstrate the capabilities of the Euclid spacecraft, particularly in capturing the intricate details of both dwarf and spiral galaxies at varying distances.

Key Observations

1. Sample Selection and Characteristics: The selected galaxies, comprising three dwarfs (Holmberg II, IC 10, and NGC 6822) and three spirals (IC 342, NGC 2403, and NGC 6744), span distances from approximately 0.5 Mpc to 8.8 Mpc. This diverse selection enables the examination of galaxies with differing morphologies and mass.
2. Instrumentation and Methodology: The Euclid satellite utilized both the Visible Imaging Channel (VIS) and the Near Infrared Spectrometer and Photometer (NISP) to capture high-resolution images in the visible and infrared spectra, enabling the study of features at scales down to 1–4 parsecs within these galaxies.
3. Surface Brightness and Photometry: The study confirms that Euclid can achieve surface brightness depths reaching 30.5 AB magnitudes in VIS, exceeding previous predictions for NISP, enabling detailed analysis of low surface brightness features across wide fields.
4. Structural and Stellar Analysis: Through Euclid’s data, detailed RGB images were constructed, illustrating the spatial distribution of stellar populations and interstellar material. Analysis of the surface brightness profiles and color gradients revealed insights into disk structures and stellar population age gradients, aligning with hierarchical galaxy formation models.
5. Identification of Satellite Systems and Star Clusters: The study effectively identified dwarf galaxy satellites around NGC 6744 and numerous globular cluster candidates in NGC 2403, emphasizing Euclid’s potential in augmenting the census of such extragalactic populations.

Implications and Future Prospects

This study underscores Euclid's dual capability for mapping large areas while resolving fine details, reinforcing its role in advancing both our understanding of local galaxy formation and broader cosmological structures. The enhanced sensitivity to faint features will facilitate studies on topics such as:

• Hierarchical Galaxy Formation: By resolving low surface brightness features and faint satellite systems, Euclid can test models of galaxy assembly and merger histories.
• Extragalactic Globular Clusters: The ability to identify and study extragalactic globular clusters will deepen our understanding of star cluster formation and its role in galaxy evolution.
• Stellar Populations and Feedback: High-resolution imaging of resolved stellar populations promises to refine models of star formation feedback mechanisms and their influence on galactic morphology.

The observational power of Euclid, combined with its wide-field capabilities, represents a significant leap forward in astronomical surveys, promising revolutionary insights into the nature of dark matter and dark energy, thanks to its contributions to the study of cosmic structures. The forthcoming Euclid Wide and Deep surveys are expected to further these investigations substantially, expanding on the findings from this early release study.