Stochastic angular momentum slews and flips and their effect on discs in galaxy formation models
Stochastic angular momentum slews and flips and their effect on discs in galaxy formation models
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DOI：
10.1093/mnras/stu1321
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发表时间：
2013-11
影响因子：
4.8
通讯作者：
N. Padilla;Salvador Salazar-Albornoz;S. Contreras;S. Cora;A. Ruiz
中科院分区：
文献类型：
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作者：
N. Padilla;Salvador Salazar-Albornoz;S. Contreras;S. Cora;A. Ruiz
在星系形成的半解析模型中，气态星系盘的角动量通常考虑到它们的主晕的角动量以及落向星系盘的物质的贡献。然而，对于角动量不均匀的物质的吸积影响却很少关注。这些影响是本文的主题，我们采用蒙特卡罗模拟在星系盘吸积物质时角动量方向的变化。蒙特卡罗模拟遵循了在千禧年II模拟中精确测量的暗物质亚晕中角动量旋转和翻转的概率，质量为M bbb10 10 h−1 M⊙。我们在半解析模型中的实现是这样做的，即假设作用于暗物质晕的翻转对冷重子是相同的；然而，我们假设在后一种情况下，翻转也需要一个困难的光盘增加其角动量，导致光盘变得更小，相对于一个没有翻转的情况下的余弦翻转的角度。这有几个结果使得恒星形成得更快，特别是在所有红移的低质量星系中。一般来说，小圆盘经历了一个快速冷气体质量增长的阶段，这是最大的翻转发生的时候，使它们更密集，增加了它们的恒星形成活动。因此，高红移恒星质量函数的高质量端达到更高的质量，与观测结果更吻合。有趣的是，星系盘的平均大小并没有受到这个模型的强烈影响，因为那些由于角动量翻转而没有增长的盘很快就会被盘的不稳定性破坏，只允许较大的盘存活下来。在我们的星系中，盘的大小与观测值在低红移和高红移种群中都表现出很好的一致性。我们还采用了在合并过程中触发恒星形成爆发的新条件。最大的翻转发生在合并中，这非常有效地阻止了光盘的大小增长。因此，星盘质量的增加转化为更高的星盘密度，这使得这个新的实现很自然地采用星盘不稳定性准则来评估合并中爆发的触发，而不是像原始模型那样基于质量比。新的实现将圆盘的平均寿命减少了2倍，同时仍然允许大型螺旋星系的现有圆盘的衰老。
The angular momentum of gaseous galactic discs in semi-analytic models of galaxy formation is usually followed considering the angular momentum of their host haloes as well as contributions by material that falls toward the discs. However, little attention is paid to the effects of accretion of material with missaligned angular momenta. These effects are the subject of this paper, where we adopt a Monte-Carlo simulation for the changes in the direction of the angular momentum of a galaxy disc as it accretes matter. The Monte-Carlo simulation follows the probability of angular momentum slews and flips in dark matter subhaloes accurately measured in the Millennium II simulation for masses M > 10 10 h −1 M⊙. Our implementation in a semi-analytic model is done such that the flip acting on the dark matter haloes is assumed to be the same for the cold baryons; however, we assume that in the latter case the flip also entails a difficulty for the disc to increase its angular momentum which causes the disc to become smaller relative to a no-flip case by the cosine of the angle of the flip. This has several consequences which make star formation occur faster, specially in low mass galaxies at all redshifts. In general, small discs suffer a phase of fast cold gas mass growth, which is when the largest flips take place making them denser and increasing their star formation activity. Therefore, the high mass end of the high redshift stellar mass function reaches higher masses, in better agreement with observations. Interestingly, the average galaxy disc size is not strongly affected by this model since the discs that do not grow due to angular momentum flips are quickly destroyed by disc instabilities, allowing only the larger discs to survive. The sizes of discs in our galaxies show good agreement with observed values in both the low and high redshift population. We also adopt a new condition for the triggering of bursts of star formation during mergers. The largest flips occur in mergers and these are very effective at impeding the disc from growing in size. Therefore, the increase in the mass of discs translates into a higher disc density, which makes it natural for this new implementation to adopt the disc instability criterion to evaluate the triggering of bursts in mergers instead of one based on mass ratios as in the original model. The new implementation reduces the average lifetimes of discs by a factor of � 2, while still allowing old ages for the present-day discs of large spiral galaxies.