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arXiv 1411.1069astro-ph.GA
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卫星吸积在形成扩展星团中的无效性

The inefficiency of satellite accretion in forming extended star clusters

Paolo Bianchini, Florent Renaud, Mark Gieles, Anna Lisa Varri

摘要:

球状星团和矮星系之间的区别已经逐渐模糊,因为最近发现了几个扩展星团,其大小(20-30 pc)和光度(-6 < Mv < -2)与微弱的矮球状体相当。为了解释它们的稀疏结构,有人认为它们是作为矮星系卫星中的星团形成的,这些卫星后来吸积到银河系中。如果这些星系团形成于矮星系的中心,它们会在潮汐压缩的环境中演化,潮汐对维里平衡的贡献会变得很大,一旦被移除,就会导致超维里状态和随后的星系团膨胀。使用N-体模拟,我们表明,在这样一个极端的环境中形成的集群经历了相当大的扩张,由于吸积过程的外部潮汐场的剧烈变化。然而,我们表明,由于压缩潮汐的去除膨胀是不足以解释所观察到的扩展结构,因为从这个过程中产生的恒星系统总是比相应的集群,由于两体弛豫孤立膨胀更紧凑。我们的结论是,一个吸积的起源扩展球状星团是不可能解释他们的大空间范围内,而赞成的假设,这样的集群已经扩展在其形成的阶段。

英文摘要:

The distinction between globular clusters and dwarf galaxies has been progressively blurred by the recent discoveries of several extended star clusters, with size (20-30 pc) and luminosity (-6 < Mv < -2) comparable to the one of faint dwarf spheroidals. In order to explain their sparse structure, it has been suggested that they formed as star clusters in dwarf galaxy satellites that later accreted onto the Milky Way. If these clusters form in the centre of dwarf galaxies, they evolve in a tidally-compressive environment where the contribution of the tides to the virial balance can become significant, and lead to a super-virial state and subsequent expansion of the cluster, once removed. Using N-body simulations, we show that a cluster formed in such an extreme environment undergoes a sizable expansion, during the drastic variation of the external tidal field due to the accretion process. However, we show that the expansion due to the removal of the compressive tides is not enough to explain the observed extended structure, since the stellar systems resulting from this process are always more compact than the corresponding clusters that expand in isolation due to two-body relaxation. We conclude that an accreted origin of extended globular clusters is unlikely to explain their large spatial extent, and rather favor the hypothesis that such clusters are already extended at the stage of their formation.

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