We present an alternative formation scenario for the gravitational wave event GW190521 that can be explained as the merger of central black holes (BHs) from two ultradwarf galaxies of stellar mass $\sim {10}^5–{10}^6{M}_{\odot }$, which had themselves previously undergone a merger. The GW190521 components’ masses of ${85}_{-14}^{+21}{M}_{\odot }$ and ${66}_{-18}^{+17}{M}_{\odot }$ challenge standard stellar evolution models, as they fall in the so-called mass gap. We demonstrate that the merger history of ultradwarf galaxies at high redshifts ($1\lesssim z\lesssim 2$) matches well the LIGO-Virgo inferred merger rate for BHs within the mass range of the GW190521 components, resulting in a likely time delay of $\lesssim 4\mathrm{Gyr}$ considering the redshift of this event. We further demonstrate that the predicted timescales are consistent with expectations for central BH mergers, although with large uncertainties due to the lack of high-resolution simulations in low-mass dwarf galaxies. Our findings show that this BH production and merging channel is viable and extremely interesting as a new way to explore galaxies’ BH seeds and galaxy formation. We recommend this scenario be investigated in detail with simulations and observations.

中文翻译：

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来自超矮星系合并的GW190521

我们提出了引力波事件GW190521的另一种形成情况，可以将其解释为来自两个恒星质量超矮星系的中央黑洞（BH）合并 $〜{10}^5–{10}^6{\mathrm{中号}}_{\odot }$，他们之前曾进行过合并。GW190521组件的质量${85}_{-14}^{+\mathrm{21岁}}{\mathrm{中号}}_{\odot }$ 和 ${66}_{-\mathrm{18岁}}^{+17}{\mathrm{中号}}_{\odot }$挑战标准恒星演化模型，因为它们属于所谓的质量差距。我们证明了在高红移下超矮星系的合并历史（$\mathrm{1个}\lesssim ž\lesssim \mathrm{2个}$）与GW190521组件的质量范围内的BH的LIGO-Virgo推断的合并率非常匹配，从而可能导致时间延迟 $\lesssim 4\mathrm{吉尔}$考虑此事件的红移。我们进一步证明了预测的时间尺度与中央BH合并的期望是一致的，尽管由于在低质量矮星系中缺乏高分辨率的模拟而具有很大的不确定性。我们的发现表明，作为探索星系的BH种子和星系形成的新方法，这种BH的产生和合并通道是可行的，并且非常有趣。我们建议对这种情况进行详细的模拟和观察。