GW190521 from the Merger of Ultradwarf 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 (BHs) from two ultradwarf galaxies of stellar mass ∼10^{5}-10^{6} M_{⊙}, which had themselves previously undergone a merger. The GW190521 components' masses of 85_{-14}^{+21} M_{⊙} and 66_{-18}^{+17} M_{⊙} 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≲z≲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 ≲4 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.