Leo I is considered one of the youngest dwarf spheroidals (dSph) in the Local Group. Its isolation, extended star formation history (SFH), and recent perigalacticon passage (~1 Gyr ago) make Leo~I one of the most interesting nearby stellar systems. Here, we analyse deep photometric Hubble Space Telescope data via colour-magnitude diagram fitting techniques to study its global and radially-resolved SFH. We find global star formation enhancements in Leo I ~13, 5.5, 2.0, and 1.0 Gyr ago, after which it was substantially quenched. Within the context of previous works focused on Leo I, we interpret the most ancient and the youngest ones as being linked to an early formation (surviving reionisation) and the latest perigalacticon passage (transition from dIrr to dSph), respectively. We clearly identify the presence of very metal poor stars ([Fe/H]~-2) ageing ~5-6 and ~13 Gyr old. We speculate with the possibility that this metal-poor population in Leo I is related to the merging with a low mass system (possibly an ultra-faint dwarf). This event would have triggered star formation (peak of star formation ~5.5 Gyr ago) and accumulated old, metal poor stars from the accreted system in LeoI. Some of the stars born during this event would also form from accreted gas of low-metallicity (giving rise to the 5-6 Gyr low-metallicity tail). Given the intensity and extension of the 2.0 Gyr burst, we hypothesise that this enhancement could also have an external origin. Despite the quenching of star formation around 1 Gyr ago (most probably induced by ram pressure stripping with the Milky Way halo at pericentre), we report the existence of stars as young as 300-500 Myr. We also distinguish two clear spatial regions: the inner ~190 pc presents an homogeneous stellar content (size of the gaseous star forming disc in LeoI from ~4.5 to 1 Gyr ago), whereas the outer regions display a clear positive age gradient.

中文翻译：

---

剖析狮子座 I 的恒星含量：一颗陷入过渡的不规则矮星

狮子座 I 被认为是本星系群中最年轻的矮椭球体 (dSph) 之一。它的隔离、延长的恒星形成历史 (SFH) 和最近的近半轴通过（约 1 Gyr 前）使 Leo~I 成为附近最有趣的恒星系统之一。在这里，我们通过色等图拟合技术分析深测光哈勃太空望远镜数据，以研究其全局和径向分辨的 SFH。我们发现 Leo I 的全球恒星形成增强大约在 13、5.5、2.0 和 1.0 Gyr 之前，之后它被基本淬灭。在之前专注于狮子座 I 的作品的背景下，我们将最古老和最年轻的解释为分别与早期形成（幸存的再电离）和最新的近星系通道（从 dIrr 到 dSph 的过渡）有关。我们清楚地识别出非常缺乏金属的恒星 ([Fe/H]~-2) 老化~5-6 和~13 Gyr 的存在。我们推测，狮子座 I 中缺乏金属的群体可能与低质量系统（可能是超微弱矮星）的合并有关。这一事件将触发恒星形成（恒星形成的峰值~5.5 Gyr 前）并从狮子座的吸积系统中积累旧的、贫金属的恒星。在这次事件中诞生的一些恒星也会由低金属度的吸积气体形成（产生 5-6 Gyr 的低金属度尾）。鉴于 2.0 Gyr 爆发的强度和延伸，我们假设这种增强也可能有外部来源。尽管大约 1 Gyr 前恒星形成已经停止（最有可能是由撞击压力引起的，银河系晕在中心附近），我们报告了年龄在 300-500 Myr 之间的恒星的存在。我们还区分了两个清晰的空间区域：内部~190 pc 呈现出均匀的恒星含量（LeoI 中气态恒星形成盘的大小从~4.5 到 1 Gyr 前），而外部区域显示出明显的正年龄梯度。