宇宙钟：高选择星系的紧半径-速度关系
Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies
摘要：
高选择星系的最大可探测半径Rmax与自转速度呈线性关系。这一结果涵盖了测量的光学，紫外线和HI发射的星系跨越30的大小和速度的因素，从小矮不规则的最大的螺旋。因此，星系的行为就像时钟一样，在它们圆盘的最外围旋转一圈。一个大的光学选择样本的观测是一致的，这意味着这种关系是通用的低红移宇宙中的盘星系。从星系形成和演化的简单模型中，可以预期RV的线性关系。与目前宇宙的平均密度相比，Rmax内的总质量已经坍缩了37倍。采用标准的假设，我们发现一个平均晕自旋参数λ在0.020至0.035的范围内。λ的色散为0.16 dex，小于模拟的预期值。这可能是由于我们选择盘星系而不是所有晕的偏见。在Rmax处估计的恒星和原子气体的质量密度是相似的（~0.5 Msun/pc ^2），表明外盘是高度演化的。气体的消耗和恒星星族的建立时间尺度是数百个Gyr，因此恒星的形成并不是驱动目前外盘演化的动力。Rmax和圆盘尺度长度之间的估计比率与整体坍塌模型的长期预测一致。因此，目前还不清楚盘的范围是否来自持续的吸积，快速的初始崩溃，长期演化或其组合。
英文摘要：
HI-Selected galaxies obey a linear relationship between their maximum detected radius Rmax and rotational velocity. This result covers measurements in the optical, ultraviolet, and HI emission in galaxies spanning a factor of 30 in size and velocity, from small dwarf irregulars to the largest spirals. Hence, galaxies behave as clocks, rotating once a Gyr at the very outskirts of their discs. Observations of a large optically-selected sample are consistent, implying this relationship is generic to disc galaxies in the low redshift Universe. A linear RV relationship is expected from simple models of galaxy formation and evolution. The total mass within Rmax has collapsed by a factor of 37 compared to the present mean density of the Universe. Adopting standard assumptions we find a mean halo spin parameter lambda in the range 0.020 to 0.035. The dispersion in lambda, 0.16 dex, is smaller than expected from simulations. This may be due to the biases in our selection of disc galaxies rather than all halos. The estimated mass densities of stars and atomic gas at Rmax are similar (~0.5 Msun/pc^2) indicating outer discs are highly evolved. The gas consumption and stellar population build time-scales are hundreds of Gyr, hence star formation is not driving the current evolution of outer discs. The estimated ratio between Rmax and disc scale length is consistent with long-standing predictions from monolithic collapse models. Hence, it remains unclear whether disc extent results from continual accretion, a rapid initial collapse, secular evolution or a combination thereof.
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