干合并是干的，湿的，还是湿的？
Are Dry Mergers Dry, Moist, or Wet?
摘要：
我们对van Dokkum（2005）确定的干合并残留物和正在进行的干合并的红序星系样本进行了光谱分析。动力学，恒星星族吸收功能，和电离从发射线，推导。我们发现，大约有一半的样本显示强潮汐功能有年轻的恒星星族比控制样本在给定的速度色散。相反，潮汐尾较弱和/或正在合并的星系-除了一个星系-没有显示出这种年轻的成分。这似乎表明，在相当一部分椭圆星系中观测到的年轻恒星星族是由合并引发的恒星形成的结果。这个年轻的组成部分与年轻恒星（<2%质量）叠加在占主导地位的老（~11 Gyr）恒星星族上的轻微“结霜”相一致。就恒星星族而言，这些合并实际上是相当干燥的。然而，我们发现，合并残留物与年轻的恒星星族支持旋转，相反的预期，一个主要的干合并。这表明，少量的气体已经足以产生一个动态冷恒星成分。然而，有一半的星系与强大的潮汐扭曲，是缓慢旋转，并有恒星星族兼容的控制样品在给定的速度色散。值得注意的是，没有一个速度色散超过250 km/s的星系有一个独立于合并阶段的年轻恒星成分。
英文摘要：
We present a spectral analysis of a sample of red-sequence galaxies identified by van Dokkum (2005) as dry merger remnants and ongoing dry mergers. Kinematics, stellar population absorption features, and ionisation from emission lines, are derived. We find that approximately half of the sample showing strong tidal features have younger stellar populations than a control sample at a given velocity dispersion. Conversely, galaxies with weak tidal tails and/or ongoing mergers -- with the exception of one galaxy -- do not show this young component. This seems to indicate that the young stellar populations observed in a significant fraction of ellipticals is the consequence of star formation triggered by mergers. This young component is consistent with a light "frosting" of young stars (<2% by mass) superimposed upon a dominant, old (~11 Gyr), stellar population. In terms of stellar populations, these mergers are, in fact, fairly dry. We found, however, that merger remnants with young stellar populations are supported by rotation, contrary to the expectations of a major dry merger. This suggests that the small amount of gas involved has been sufficient to produce a dynamically cold stellar component. Half of the galaxies with strong tidal distortion, however, are slow rotating and have stellar populations compatible with the control sample at a given velocity dispersion. Remarkably, none of the galaxies with velocity dispersions in excess of 250 km/s have a young stellar component, independent of the merger stage.
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