HST/ACS DIRECT AGES OF THE DWARF ELLIPTICAL GALAXIES NGC 147 AND NGC 185
HST/ACS DIRECT AGES OF THE DWARF ELLIPTICAL GALAXIES NGC 147 AND NGC 185
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矮椭圆星系 NGC 147 和 NGC 185 的 HST/ACS 直接年龄
DOI：
10.1088/0004-637x/811/2/114
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发表时间：
2015
期刊：
影响因子：
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通讯作者：
P. Guhathakurta
中科院分区：
文献类型：
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作者：
M. Geha;D. Weisz;A. Grocholski;A. Dolphin;R. V. D. Marel;P. Guhathakurta
基于哈勃太空望远镜对NGC147和NGC185局部星系群的观测，我们给出了任何矮小椭圆(De)星系的最深光学光度测量。我们的F606W和F814W颜色-星等图是第一个到达去星系中最古老的主序列转折以下的图，使我们能够确定这些系统中完整的恒星形成历史。ACS场位于距银河系中心约∼1.5有效半径处，以避免光度拥挤。虽然这两个星云星座都有明确的中老年恒星存在的证据，但与NGC185相比，NGC147的平均年龄要年轻∼4-5 Gyr。在NGC147中，只有40%的恒星在12.5Gyr前(z∼5)，大部分剩余的恒星群形成于5到7Gyr之间。相比之下，70%的恒星是在12.5Gyr之前在NGC185中形成的，其余大部分恒星是在8到10 Gyr之前形成的。这两个星云区域的恒星形成已经停止了至少3个回转。先前在NGC185中心区域的观测表明，早在100Myr前就有恒星形成的证据，并且有一个随半径变化的强烈的金属丰度梯度。这意味着NGC185的中心和我们的ACS场之间缺乏径向混合。NGC147中没有径向梯度，这表明我们推断的SFH更能代表其全球历史。我们对恒星形成历史中推断的差异的解释是，与NGC147相比，NGC185更早进入M31环境。
We present the deepest optical photometry for any dwarf elliptical (dE) galaxy based on Hubble Space Telescope Advanced Camera for Surveys (ACS) observations of the Local Group dE galaxies NGC 147 and NGC 185. Our F606W and F814W color–magnitude diagrams are the first to reach below the oldest main sequence turnoff in a dE galaxy, allowing us to determine full star formation histories in these systems. The ACS fields are located roughly ∼1.5 effective radii from the galaxy center to avoid photometric crowding. While both ACS fields show unambiguous evidence for old and intermediate age stars, the mean age of NGC 147 is ∼4–5 Gyr younger as compared to NGC 185. In NGC 147, only 40% of stars were in place 12.5 Gyr ago (z ∼ 5), with the bulk of the remaining stellar population forming between 5 to 7 Gyr. In contrast, 70% of stars were formed in NGC 185 prior to 12.5 Gyr ago with the majority of the remaining population forming between 8 to 10 Gyr ago. Star formation has ceased in both ACS fields for at least 3 Gyr. Previous observations in the central regions of NGC 185 show evidence for star formation as recent as 100 Myr ago, and a strong metallicity gradient with radius. This implies a lack of radial mixing between the center of NGC 185 and our ACS field. The lack of radial gradients in NGC 147 suggests that our inferred SFHs are more representative of its global history. We interpret the inferred differences in star formation histories to imply an earlier infall time into the M31 environment for NGC 185 as compared to NGC 147.