Studying the resolved stellar populations of the different structural components that build massive galaxies directly unveils their assembly history. We aim at characterizing the stellar population properties of a representative sample of bulges and pure spheroids in massive galaxies (M _⋆ > 10¹⁰ M _⊙) in the GOODS-N field. We take advantage of the spectral and spatial information provided by SHARDS and Hubble Space Telescope data to perform the multi-image spectrophotometric decoupling of the galaxy light. We derive the spectral energy distribution separately for bulges and disks in the redshift range 0.14 < z ≤ 1 with spectral resolution R ∼ 50. Analyzing these spectral energy distributions, we find evidence of a bimodal distribution of bulge formation redshifts. We find that 33% of them present old mass-weighted ages, implying a median formation redshift . They are relics of the early universe embedded in disk galaxies. A second wave, dominant in number, accounts for bulges formed at median redshift . The oldest (first-wave) bulges are more compact than the youngest. Virtually all pure spheroids (i.e., those without any disk) are coetaneous with the second-wave bulges, presenting a median redshift of formation . The two waves of bulge formation are distinguishable not only in terms of stellar ages but also in star formation mode. All first-wave bulges formed fast at z ∼ 6, with typical timescales around 200 Myr. A significant fraction of the second-wave bulges assembled more slowly, with star formation timescales as long as 1 Gyr. The results of this work suggest that the centers of massive disk-like galaxies actually harbor the oldest spheroids formed in the universe.

研究构建大型星系的不同结构组件的已解析恒星种群，可以直接揭示它们的组装历史。我们的目标是在GOODS-N 场中_表征大质量星系 ( M _⋆ > 10 ¹⁰ M _⊙ ) 中具有代表性的凸起和纯球体样本的恒星种群特性。我们利用 SHARDS 和哈勃太空望远镜数据提供的光谱和空间信息来执行星系光的多图像分光光度解耦。我们分别推导出红移范围 0.14 < z ≤ 1内的凸起和圆盘的光谱能量分布，光谱分辨率为R∼ 50. 分析这些光谱能量分布，我们发现了凸起地层红移双峰分布的证据。我们发现其中 33% 呈现出旧的质量加权年龄，这意味着中位地层红移。它们是嵌入盘状星系的早期宇宙的遗迹。数量上占主导地位的第二波解释了在中位红移处形成的凸起。最古老的（第一波）凸起比最年轻的更紧凑。几乎所有纯球体（即那些没有任何圆盘的球体）都与第二波凸起共生，呈现出形成的中值红移。两次凸起形成波不仅在恒星年龄方面而且在恒星形成模式方面都是可区分的。所有第一波凸起都在z 处快速形成∼ 6，典型的时间尺度约为 200 Myr。第二波凸起的很大一部分聚集得更慢，恒星形成的时间尺度长达 1 Gyr。这项工作的结果表明，巨大的盘状星系的中心实际上拥有宇宙中形成的最古老的球体。