GHOSTS: the Nature of Galactic Stellar Halos from Resolved Stellar Populations

近年来，我们开始意识到星系的外围包含有关于星系形成过程的有价值的信息。在分级星系的形成中，星系的恒星晕和厚盘被认为是小卫星积累的结果，主要是在早期的组装阶段。因此，现今晕的大小、金属度和子结构的数量与密度起伏的原始功率谱的小尺度特性和小暗物质晕中恒星形成的抑制等问题直接相关。为了利用这一星系形成过程的化石记录，我启动了对附近18个大质量盘状星系的已分解恒星群的鬼影成像调查。在这里，我申请资金，用地面大双筒望远镜成像扩大这个200多个HST轨道的计划，并分析合并的数据集。我们将使用明亮的示踪剂RGB星来测量光晕表面的亮度分布，以达到极低的等效表面亮度水平，最高可达~60kpc。这些古老的RGB恒星的颜色也是极好的金属丰度指标，进一步限制了形成历史。年轻的主序、He燃烧和AGB星制约着外盘区的演化。晕子结构的数量将与最新的等级星系形成模型进行比较。

In recent years we have started to appreciate that the outskirts of galaxies contain valuable information about the formation process of galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are thought to be the result of accretion of minor satellites, predominantly in the earlier assembly phases. The size, metallicity, and amount of substructure in current day halos are therefore directly related to issues like the small scale properties of the primordial power spectrum of density fluctuations and the suppression of star formation in small dark matter halos.To exploit this fossil record of the galaxy formation process I have initiated the GHOSTS imaging survey of the resolved stellar populations of 18 nearby, massive disk galaxies. Here I request funding to expand this program of 200+ HST orbits with ground-based Large Binocular Telescope imaging and analyse the combined data sets. We will use bright tracer RGB stars to measure halo surface brightness profiles to extremely low equivalent surface brightness levels out to ~ 60 kpc. The colors of these old RGB stars are also excellent metallicity indicators putting further constraints on formation history. Younger Main Sequence, He burning and AGB stars constrain evolution of outer disk regions. The amount of halo substructure will be compared against the latest hierarchical galaxy formation models.