Euclid: Early Release Observations – Deep anatomy of nearby galaxies⋆

Euclid is poised to make significant advances in the study of nearby galaxies in the Local Universe. Here we present a first look at six galaxies observed for the Nearby Galaxy Showcase as part of the Euclid Early Release Observations acquired between August and November, 2023. These targets, three dwarf galaxies (Holmberg II, IC 10, and NGC6822) and three spirals (IC 342, NGC2403, and NGC6744), range in distance from about 0.5 Mpc to 8.8 Mpc. We first assess the surface brightness depths in the stacked Euclid images, and confirm previous estimates in 100 arcsec2 regions for VIS of 1σ limits of 30.5 mag arcsec−2, but find deeper than previous estimates for NISP with 1σ = 29.2–29.4 mag arcsec−2. By combining Euclid HE, YE, and IE into RGB images, we illustrate the large field-of-view (FoV) covered by a single Reference Observing Sequence, together with exquisite detail on scales of < 1–4 parsecs in these nearby galaxies. Analysis of radial surface brightness and color profiles demonstrates that the photometric calibration of Euclid is consistent with what is expected for galaxy colors according to stellar synthesis models. We perform standard source selection techniques for stellar photometry, and find approximately 1.3 million stars across the six galaxy fields. After subtracting foreground stars and background galaxies, and applying a color and magnitude selection, we extract stellar populations of different ages for the six galaxies. The resolved stellar photometry obtained with Euclid 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 two galaxies individually for surrounding systems of dwarf galaxy satellites and globular cluster populations. Our analysis of the ensemble of dwarf satellites around NGC6744 recovers all the previously known dwarf satellites within the Euclid FoV, and also reveals a new system, EDwC1, a nucleated dwarf spheroidal at the end of a spiral arm. Our new census of the globular clusters around NGC2403 yields nine new star-cluster candidates, eight of which with colors indicative of evolved stellar populations. In summary, our first investigation of the six Showcase galaxies demonstrates that Euclid is a powerful probe of stellar structure and stellar populations in nearby galaxies, and will provide vastly improved statistics on dwarf satellite systems and extragalactic globular clusters in the local Universe, among many other exciting results. Key words. Galaxies: dwarf – Galaxies: irregular – Galaxies: spiral – Galaxies: starburst – Galaxies: stellar content

欧几里得（Euclid）空间望远镜有望在本地宇宙近邻星系的研究中取得重大突破。本文首次呈现了近邻星系展示计划（Nearby Galaxy Showcase）的观测结果，该计划作为欧几里得早期释放观测（Euclid Early Release Observations）的组成部分，于2023年8月至11月间完成观测数据采集，共包含6个星系的观测资料。本次观测的目标天体包括3个矮星系（Holmberg II、IC 10与NGC6822）以及3个旋涡星系（IC 342、NGC2403与NGC6744），其距离跨度约为0.5兆秒差距（Mpc）至8.8 Mpc。我们首先对叠加后的欧几里得图像开展面亮度极限评估，确认可见光波段相机（VIS）在100平方角秒区域内的1σ极限为30.5星等每平方角秒（mag arcsec⁻²），同时发现近红外光谱与光度计（NISP）的面亮度极限较此前预估更深，1σ范围为29.2~29.4 mag arcsec⁻²。通过将欧几里得的HE、YE与IE波段数据整合为RGB合成图像，我们展示了单条参考观测序列所覆盖的宽视场（FoV），以及这些近邻星系中小于1至4秒差距尺度上的精细结构细节。对径向面亮度与颜色轮廓的分析表明，欧几里得的测光校准结果与基于恒星合成模型得到的星系颜色预期完全一致。我们采用标准恒星星源选择技术开展已分辨恒星星测光分析，在6个星系的视场内共识别出约130万颗恒星。在扣除前景星与背景星系，并应用颜色与星等筛选流程后，我们提取出了这6个星系中不同年龄阶段的恒星族群。借助欧几里得获取的已分辨恒星星测光数据，我们可以通过区分年轻恒星、渐近巨星支（AGB）以及红巨星支（RGB）恒星族群的分布特征，约束这些星系的恒星形成历史。我们还分别针对两个星系开展了矮星系卫星系统与球状星团族群的专项研究。对NGC6744周围矮卫星系统的分析，不仅找回了欧几里得视场内所有此前已知的矮卫星星系，还发现了一个新的天体系统EDwC1——一个位于旋臂末端的有核矮球状星系。对NGC2403周围球状星团的全新普查共得到9个新的星团候选体，其中8个的颜色特征表明其属于演化后的恒星族群。综上，我们对6个展示星系的首次研究证实，欧几里得是研究近邻星系恒星结构与恒星族群的强大探测工具，并且将在本地宇宙矮卫星系统统计、河外球状星团研究等诸多令人振奋的研究方向上提供大幅提升的观测数据与统计结果。关键词：星系：矮星系；星系：不规则星系；星系：旋涡星系；星系：星暴星系；星系：恒星成分