GW190521 from the Merger of Ultra-Dwarf Galaxies

TL;DR

This paper proposes that GW190521 resulted from the merger of central black holes in ultra-dwarf galaxies, offering a new formation channel that aligns with observed merger rates and challenges standard stellar evolution models.

Contribution

It introduces a novel formation scenario for GW190521 involving ultra-dwarf galaxy mergers, supported by merger rate analysis and cosmological considerations.

Findings

Merger rates of ultra-dwarf galaxies match GW190521 event rate.

Black hole masses in the mass gap can originate from dwarf galaxy mergers.

Time scales are consistent with black hole merger expectations.

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