确认z~2.3的大质量静止星系的显着致密性：早型星系不是在简单的整体坍缩中形成的
Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse
摘要：
Kriek等人（2006）使用深近红外光谱发现，在z = 2.3的大质量星系中，约有45%已经演化出恒星星族，并且很少或根本没有恒星形成。在这里，我们确定这些静止星系的大小使用深，高分辨率的图像与HST/NIC 2和激光导星辅助凯克/AO。考虑到它们的恒星质量中值是1.7x10^11太阳质量，这些星系非常小，有效半径中值是0.9 kpc。附近宇宙中类似质量的星系大小约为5 kpc，平均恒星密度比z~2.3星系低两个数量级。这些结果扩展了早期在z~1.5的工作，并证实了以前在z>2的研究，这些研究缺乏足够分辨率的光谱红移和成像来分辨星系。我们的研究结果表明，完全组装的早型星系在z~2.3时最多占K选择的静止星系总数的10%，有效地排除了它们形成的简单整体模型。星系必须在z~2.3之后通过干合并或其他过程显着演化，与分层模型的预测一致。
英文摘要：
Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11 Solar masses the galaxies are remarkably small, with a median effective radius of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5 kpc and average stellar densities which are two orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 which lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ~10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.
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