超致密矮星系是整体形成还是合并成星团复合体？
Do ultra compact dwarf galaxies form monolithically or as merged star cluster complexes?
摘要：
一些超紧凑矮星系（UCD）相对于假设正则初始质量函数（IMF）的恒星星族的预期，观测到的动态V波段质光比（$M/L_{\rm V}$）有所提高。观测还揭示了在几个UCD的中心存在一个致密的暗天体，其质量为UCD现今恒星质量的几到15%。这个中心质量集中的星系通常被解释为超大质量的黑洞，但原则上也可以是恒星残骸的子星系团。我们探讨了以下两种UCD的形成情景，i）整体崩溃和ii）在大规模恒星爆发区观察到的星团合并。我们探讨的物理性质的UCD在不同的演化阶段假设不同的初始恒星质量的UCD和IMF是普遍的或系统地改变与金属丰度和密度根据综合银河IMF（IGIMF）理论。虽然所观察到的升高的$M/L_{\rm V}$比率的UCD不能再现，如果IMF是不变的和普遍的，经验推导的IMF的密度和金属性的系统变化与观察结果一致。计算UCD质量依赖的保留分数的黑暗残余改善了这一协议。此外，我们适用于年轻的UCD的N体模拟的结果，并表明，相同的初始条件描述所观察到的$M/L_{\rm V}$比再现观察到的半质量半径和现今的UCD质量之间的关系。因此，研究结果表明，大多数具有升高的$M/L_{\rm V}$比率的UCD可能已经形成了具有集中的显著残余质量成分的整体，而那些具有小$M/L_V$值的UCD可能是合并的星团复合体。
英文摘要：
Some ultra-compact dwarf galaxies (UCDs) have elevated observed dynamical V-band mass-to-light ($M/L_{\rm V}$) ratios with respect to what is expected from their stellar populations assuming a canonical initial mass function (IMF). Observations have also revealed the presence of a compact dark object in the centers of several UCDs, having a mass of a few to 15\% of the present-day stellar mass of the UCD. This central mass concentration has typically been interpreted as a super-massive black hole, but can in principle also be a sub-cluster of stellar remnants. We explore the following two formation scenarios of UCDs, i) monolithic collapse and ii) mergers of star clusters in cluster complexes as are observed in massively star-bursting regions. We explore the physical properties of the UCDs at different evolutionary stages assuming different initial stellar masses of the UCDs and the IMF being either universal or changing systematically with metallicity and density according to the Integrated Galactic IMF (IGIMF) theory. While the observed elevated $M/L_{\rm V}$ ratios of the UCDs cannot be reproduced if the IMF is invariant and universal, the empirically derived IMF which varies systematically with density and metallicity shows agreement with the observations. Incorporating the UCD-mass-dependent retention fraction of dark remnants improves this agreement. In addition we apply the results of N-body simulations to young UCDs and show that the same initial conditions describing the observed $M/L_{\rm V}$ ratios reproduce the observed relation between the half-mass radii and the present-day masses of the UCDs. The findings thus suggest that the majority of UCDs that have elevated $M/L_{\rm V}$ ratios could have formed monolithically with significant remnant-mass components that are centrally concentrated, while those with small $M/L_V$ values may be merged star-cluster complexes.
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