关于M33的CLUES：三角座星系外围的反向径向恒星年龄梯度
CLUES about M33: the reversed radial stellar age gradient in the outskirts of Triangulum galaxy
摘要：
HST/ACS沿M33长轴的观测表明，其恒星的平均年龄随着离星系中心距离的增加而减小。这种行为与椎间盘的由内向外生长相一致。然而，在观测到的最外层区域，在r $\simeq$ 11.6 kpc处，发现了这种梯度的逆转，在这个半径之外发现了很高比例的老恒星。在这项工作中，我们调查这样的恒星年龄梯度反转的起源，通过使用模拟M33模拟约束本地UniversE模拟（CLUES）。模拟的M33在质量、自转速度、表面亮度方面与观测到的相似，并且与观测报告中的相似，显示了大半径的恒星年龄转变。我们证明，这种逆转主要是由于恒星吸积从老卫星星系，在较小程度上，恒星迁移的原位恒星。旧的吸积星，形成时间tf < 4 Gyrs，在运动学上是热的，可以通过它们的高速色散和它们没有旋转支撑轨道的事实与原地星区分开来。未来，获得M33外围恒星的运动学信息将有助于验证这一假设。
英文摘要：
HST/ACS observations along the major axis of M33 show that the mean age of its stars decreases with increasing distance from the galaxy center. Such a behavior is consistent with an inside-out growth of the disc. However, in the outermost observed field, at r$\simeq$11.6 kpc, a reversal of this gradient is detected, with old stars found in high percentages beyond this radius. In this work we investigate the origin of such a reversal in stellar age gradient, by using a simulated M33 analogue from the Constrained Local UniversE Simulations (CLUES). The simulated M33 is similar to the observed one in terms of mass, rotation velocity, surface brightness and, similar to what has been reported in observations, shows a stellar age turnaround at large radii. We demonstrate that this reversal is mostly a result of stellar accretion from old satellite galaxies and, to a lesser extent, of stellar migration of in-situ stars. The old accreted stars, with formation times tf < 4 Gyrs, are kinematically hot and can be differentiated from the in-situ stars by their high velocity dispersion and the fact that they do not have rotationally-supported orbits. In the future, obtaining kinematic information of the stars in the outskirt of M33 will help to verify this scenario.
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