标题： NGC 5128星系晕星的金属丰度分布：星系形成的意义 显示英文标题

标题： The Metallicity Distribution in the Halo Stars of NGC 5128: Implications for Galaxy Formation

摘要： 哈勃太空望远镜已被用于获取巨椭圆星系 NGC 5128 外晕中 10,000 颗红巨星的 WFPC2 (V,I) 光度。与银河系球状星团的标准 RGB 和模型等年龄线相比，表明这个外晕群体完全由老年恒星主导，具有极其宽广的金属丰度范围，从最贫金属的银河球状星团（[Fe/H] = -2）到接近太阳丰度。通过对标准线内插，我们推导出晕族的金属丰度分布函数 (MDF)，结果显示出明显的双组分结构：约 2/3 的恒星属于金属较丰富且尖锐的成分（[Fe/H] = -0.3, σ = 0.22 dex），而剩余的 1/3 属于更广泛的金属贫乏分布，逐渐下降至低金属丰度。MDF 可以通过一个简单的闭合盒子化学富集模型很好地匹配，其中第一组分以初始丰度 Z_0 = 0 开始，第二组分以 Z_0 = 0.18 Z_太阳 开始。与涉及吸积或合并重大贡献的其他模型相比，一种基本的“就地”形成图像，包含两个明显的恒星形成时期，能更好地拟合观测数据。 显示英文摘要

摘要： The Hubble Space Telescope has been used to obtain WFPC2 (V,I) photometry for a sample of 10,000 red giant stars in the outer halo of the giant elliptical NGC 5128. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globulars at [Fe/H] = -2 up to Solar abundance. By interpolation within the fiducial lines we derive the metallicity distribution function (MDF) for the halo, which turns out to have a distinct two-component structure: about 2/3 of the stars belong to a sharply peaked metal-richer component ([Fe/H] = -0.3, sigma = 0.22 dex), while the remaining one-third belong to a more extended metal-poor distribution sloping gradually down to low metallicity. The MDF can be remarkably well matched by a simple closed-box model of chemical enrichment, where the first component starts with an initial abundance Z_0 =0 and the second component with Z_0 = 0.18 Z_Sun. A basic "in situ" formation picture with two distinct epochs of star formation fits the observations better than other models involving major contributions from accretions or mergers.