GW190521 from the Merger of Ultra-Dwarf Galaxies

Abstract

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 $85_{-14}^{+21}{M}_{\odot }$ 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 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.

8 pages, 4 figures

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