The Double-White-Dwarf Merger Rate from ZTF

Using the Zwicky Transient Facility (ZTF), Burdge et al. (2020) discovered systems of eclipsing double white dwarfs (EDWDs) having orbital periods <1 hr. From the properties of 3 of the discovered systems, I estimate a merger rate of DWDs, per WD in the Galaxy, of $R_{\rm merge, WD}\approx8\times 10^{-12}$yr$^{-1}$, or a rate per unit stellar mass in the Galaxy, of $R_{\rm merge, M*}\approx4.8\times 10^{-13}$yr$^{-1}$M$_\odot^{-1}$.This likely somewhat underestimates the rate, because of several known effects that work against EDWD detection in ZTF. The derived merger rate is within the uncertainty range, $R_{\rm merge, M*} =(4.6-5.8)\times 10^{-13}$yr$^{-1}$M$_\odot^{-1}$, measured independently by Maoz et al. (2018) based on two samples of DWDs discovered via radial-velocity variations. Based on the expected period distribution of DWDs and their detectability, of order 100 additional eclipsing DWDs with periods >1 hr are discoverable in ZTF, with potential to significantly improve the merger-rate's measurement precision.