A RADIAL AGE GRADIENT IN THE GEOMETRICALLY THICK DISK OF THE MILKY WAY
A RADIAL AGE GRADIENT IN THE GEOMETRICALLY THICK DISK OF THE MILKY WAY
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DOI：
10.3847/0004-637x/831/2/139
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发表时间：
2016-09
期刊：
影响因子：
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通讯作者：
M. Martig;I. Minchev;M. Ness;M. Fouesneau;H. Rix
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作者：
M. Martig;I. Minchev;M. Ness;M. Fouesneau;H. Rix
在银河系中，厚盘可以用单个恒星的丰度、运动学或年龄来定义，或者用几何学来定义，如中平面上方的恒星。在附近的星系中，只能使用几何定义，厚盘似乎有很大的径向尺度长度，它们的红色表明它们是一致的老。银河系的几何厚盘也是径向延伸的，但它的化学性质远非均匀：α增强的恒星被限制在银河系内部。在模拟的星系中，老恒星集中在中心，几何厚度的圆盘也径向延伸。年轻的恒星群体在模拟的圆盘的外部区域闪耀，使这些恒星高出中平面。由此产生的几何厚盘因此显示出径向年龄梯度，从中心区域的老到外围的年轻。根据我们对APOGEE调查中大量巨星样本的年龄估计，我们现在可以测试银河系的这种情况。我们发现，银河系中几何定义的厚盘确实有一个很强的径向年龄梯度：红色团簇恒星的年龄中位数从内盘的109 Gyr到外盘的5 Gyr。我们提出，至少有一些附近的星系也可能有厚盘，是不均匀的年龄，和几何厚盘可能是复杂的结构，导致在其内部和外部的不同形成机制。
In the Milky Way, the thick disk can be defined using individual stellar abundances, kinematics, or age, or geometrically, as stars high above the midplane. In nearby galaxies, where only a geometric definition can be used, thick disks appear to have large radial scale lengths, and their red colors suggest that they are uniformly old. The Milky Way’s geometrically thick disk is also radially extended, but it is far from chemically uniform: α-enhanced stars are confined within the inner Galaxy. In simulated galaxies, where old stars are centrally concentrated, geometrically thick disks are radially extended, too. Younger stellar populations flare in the simulated disks’ outer regions, bringing those stars high above the midplane. The resulting geometrically thick disks therefore show a radial age gradient, from old in their central regions to younger in their outskirts. Based on our age estimates for a large sample of giant stars in the APOGEE survey, we can now test this scenario for the Milky Way. We find that the geometrically defined thick disk in the Milky Way has indeed a strong radial age gradient: the median age for red clump stars goes from ∼9 Gyr in the inner disk to 5 Gyr in the outer disk. We propose that at least some nearby galaxies could also have thick disks that are not uniformly old, and that geometrically thick disks might be complex structures resulting from different formation mechanisms in their inner and outer parts.