GW190521 from the Merger of Ultradwarf Galaxies

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.

Figure 1

Extrapolation to low masses of the relation between the central BH mass and galaxy stellar mass from [30] in blue (the shaded region represents the $1\sigma$ uncertainty on the relation), with the masses of the GW190521 components from LIGO-Virgo (dashed lines) and the 90% CI for both components (gray lines).

Figure 2

Rate of merging ultradwarf galaxies in the mass range of interest for GW190521 as a function of redshift. The dashed line is the result using our fit for the number density evolution parameters; the solid line is the result assuming that the galaxy density is constant with redshift and equal to the one measured at $z=0$. The red lines represent the 90% CI from the rate estimate of events similar to GW190521 from [5]. The blue dashed parallel lines show the range of 90% CI redshift for GW190521. The shaded region shows the time period of 4 Gyr preceding the central redshift.

Figure 3

Dynamical friction timescale for BHs of mass $m_1=85M_{\odot }$ and $m_2=66M_{\odot }$ at different separations from the center of the galaxy remnant, having stellar mass $M_{\star }={10}^6{M}_{\odot }$ formed by the merger of two ultradwarf galaxies. The shaded region represents the typical half-light radius of nearby $M_{\star }\sim {10}^6{M}_{\odot }$ galaxies, and their velocity dispersion ranges between 5 and $10\mathrm{km}{\mathrm{s}}^{-1}$.