Z~1以来椭圆星系的尺度演化：膨胀与小合并
Dissecting the size evolution of elliptical galaxies since z~1: puffing up vs minor merging scenarios
摘要：
我们利用两个目视分类的样本探讨了椭圆星系局部质量-尺寸关系的建立。在低红移时，我们从SDSS中收集了2，656个椭圆星系的子样本，而在高红移时（直到z~1），我们从GOODS的HST/ACS图像中提取了228个物体的样本。我们研究中的所有星系都有光谱数据，使我们能够确定恒星成分的年龄和质量。使用化石记录的信息包含在我们的本地样本的恒星星族，我们没有发现任何证据的年龄隔离在一个给定的恒星质量取决于星系的大小。在固定的动力学质量下，我们样本的两个极端年龄四分位数的大小差异只有<9%。因此，当地的证据并不支持这样一种情况，即现在的质量-大小关系是通过自下而上的序列逐步建立的，其中较老的星系占据了这种关系的下部，自形成以来一直保持不变。我们发现，至少从z~1开始，大小的趋势对恒星星族的年龄不敏感。这一结果支持了恒星质量-尺寸关系在z~1处形成的观点，所有星系都分布在一个大约相当于目前尺寸分布的1/2的区域。事实上，在大小的演变是独立的恒星年龄，以及没有增加的关系，红移的散射不支持膨胀的机制。然而，观测证据在现阶段不能否定次合并假说。我们已经作出了一个估计的小合并事件的数量，使高Z星系的本地关系与所观察到的大小演变兼容。由于z=0.8，如果合并质量比为1：3，我们估计约3+-1次小合并，如果比例为1：10，我们得到约8+-2次事件。
英文摘要：
We have explored the buildup of the local mass-size relation of elliptical galaxies using two visually classified samples. At low redshift we compiled a subsample of 2,656 elliptical galaxies from SDSS, whereas at higher redshift (up to z~1) we extracted a sample of 228 object from the HST/ACS images of the GOODS. All the galaxies in our study have spectroscopic data, allowing us to determine the age and mass of the stellar component. Using the fossil record information contained in the stellar populations of our local sample, we do not find any evidence for an age segregation at a given stellar mass depending on the size of the galaxies. At a fixed dynamical mass there is only a <9% size difference in the two extreme age quartiles of our sample. Consequently, the local evidence does not support a scenario whereby the present-day mass-size relation has been progressively established via a bottom-up sequence, where older galaxies occupy the lower part this relation, remaining in place since their formation. We find a trend in size that is insensitive to the age of the stellar populations, at least since z~1. This result supports the idea that the stellar mass-size relation is formed at z~1, with all galaxies populating a region which roughly corresponds to 1/2 of the present size distribution. The fact that the evolution in size is independent of stellar age, together with the absence of an increase in the scatter of the relationship with redshift does not support the puffing up mechanism. The observational evidence, however, can not reject at this stage the minor merging hypothesis. We have made an estimation of the number of minor merger events necessary to bring the high-z galaxies into the local relation compatible with the observed size evolution. Since z=0.8, if the merger mass ratio is 1:3 we estimate ~3+-1 minor mergers and if the ratio is 1:10 we obtain ~8+-2 events.
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