GW190521 from the Merger of Ultra-Dwarf Galaxies

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