Some ultracompact dwarf galaxies (UCDs) have elevated observed dynamical V-band mass-to-light (M/LV) 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 centres of several UCDs, having a mass of a few to 15 per cent of the present-day stellar mass of the UCD. This central mass concentration has typically been interpreted as a supermassive black hole, but can in principle also be a subcluster 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 starbursting 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 theory. While the observed elevated M/LV ratios of the UCDs cannot be reproduced if the IMF is invariant and universal, the empirically derived IMF that 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/LV 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/LV ratios could have formed monolithically with significant remnant-mass components that are centrally concentrated, while those with small M/LV values may be merged star cluster complexes.

中文翻译：

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超致密矮星系是整体形成还是作为合并的星团复合体形成？

一些超致密矮星系 (UCD) 已经提高了观测到的动态 V 波段质光 (M/LV) 比，相对于假设标准初始质量函数 (IMF) 的恒星群体的预期。观测还显示，在几个 UCD 的中心存在一个致密的暗天体，其质量仅为当前 UCD 恒星质量的百分之几到 15%。这种中心质量浓度通常被解释为超大质量黑洞，但原则上也可以是恒星残骸的子团。我们探讨了 UCD 的以下两种形成情景：（i）整体坍塌和（ii）星团在星团复合体中的合并，如在大规模星爆区域中观察到的那样。根据综合银河 IMF 理论，我们假设 UCD 的不同初始恒星质量和 IMF 是普遍的或随着金属丰度和密度系统地变化，我们探索 UCD 在不同演化阶段的物理性质。如果 IMF 是不变的和普遍的，则观察到的 UCD 的 M / LV 比值升高无法重现，但根据经验得出的随密度和金属丰度系统变化的 IMF 与观察结果一致。结合黑暗残留物的 UCD 质量依赖性保留分数改善了这种一致性。此外，我们将 N 体模拟的结果应用于年轻的 UCD，并表明描述观察到的 M/LV 比率的相同初始条件再现了观察到的半质量半径与 UCD 的当前质量之间的关系。