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arXiv 1107.0911astro-ph.GAastro-ph.CO
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Subaru/Suprime-Cam巡天的NGC55恒星晕的结构和星族

Structure and Population of the NGC55 Stellar Halo from a Subaru/Suprime-Cam Survey

Mikito Tanaka, Masashi Chiba, Yutaka Komiyama, Puragra Guhathakurta, Jason S. Kalirai

摘要:

作为银河系恒星晕调查的一部分,我们利用Subaru/Suprime-Cam拍摄的深视场和宽视场V-和I-波段图像,研究了玉夫座星系群成员星系NGC 55中恒星晕北部外部的结构和恒星星族。通过对红巨星分支(RGB)恒星的色星等图(CMD)的分析,得到了一个基于RGB(TRGB)的到星系的距离模为(m-M)_0 = 26.58 +/- 0.11(d = 2.1 +/-0.1Mpc).从恒星密度图,我们检测到的非对称干扰,厚盘结构和两个金属贫过密的子结构在北部地区的NGC 55,这可能对应于合并残留物与分级形成的NGC 55的晕。此外,我们确定了一个弥漫的贫金属晕延伸到至少z ~ 16千秒差距从银道面。垂直于银道面的z方向的表面亮度剖面表明,NGC 55北部外部的恒星密度分布是由z <~ 6 kpc的局部等温盘和可能的弥散贫金属晕描述的,V波段表面亮度大于~ 32 mag arcsec^{-2},其中老的RGB星占主导地位。我们推导出这些结构的金属丰度分布(MD)的基础上的光度比较的RGB恒星的理论恒星演化模型。厚盘结构的MD分别为[Fe/H]峰~ -1.4和[Fe/H]平均~-1.7,而外层亚结构比厚盘结构表现出更多的贫金属特征。结合我们对M31星晕的研究结果,讨论了不同哈勃类型星晕形成过程的可能差异。

英文摘要:

As part of our survey of galactic stellar halos, we investigate the structure and stellar populations of the northern outer part of the stellar halo in NGC55, a member galaxy of the Sculptor Group, using deep and wide-field V- and I-band images taken with Subaru/Suprime-Cam. Based on the analysis of the color-magnitude diagrams (CMDs) for red-giant-branch (RGB) stars, we derive a tip of RGB (TRGB)-based distance modulus to the galaxy of (m-M)_0 = 26.58 +/- 0.11 (d = 2.1 +/- 0.1 Mpc). From the stellar density maps, we detect the asymmetrically disturbed, thick disk structure and two metal-poor overdense substructures in the north region of NGC55, which may correspond to merger remnants associated with hierarchical formation of NGC55's halo. In addition, we identify a diffuse metal-poor halo extended out to at least z ~ 16 kpc from the galactic plane. The surface-brightness profiles toward the z-direction perpendicular to the galactic plane suggest that the stellar density distribution in the northern outer part of NGC55 is described by a locally isothermal disk at z <~ 6 kpc and a likely diffuse metal-poor halo with V-band surface brightness of mu_V >~ 32 mag arcsec^{-2}, where old RGB stars dominate. We derive the metallicity distributions (MDs) of these structures on the basis of the photometric comparison of RGB stars with the theoretical stellar evolutionary models. The MDs of the thick disk structures show the peak and mean metallicity of [Fe/H]peak ~ -1.4 and [Fe/H]mean ~ -1.7, respectively, while the outer substructures show more metal-poor features than the thick disk structure. Combined with the current results with our previous study for M31's halo, we discuss the possible difference in the formation process of stellar halos among different Hubble types.

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