Galaxy Survey On The Fly: Prospects of Rapid Galaxy Cataloging to Aid the Electromagnetic Follow-up of Gravitational-wave Observations

Galaxy catalogs are essential for efficient searches of the electromagnetic counterparts of extragalactic gravitational-wave (GW) signals with highly uncertain localization. We show that one can efficiently catalog galaxies within a short period of time with 1-2 meter-class telescopes such as the Palomar Transient Factory (PTF) or MDM, in response to an observed GW signal from a compact binary coalescence. We find that a rapid galaxy survey is feasible on the relevant time scale of $\lesssim 1$ week, maximum source distance of $>200$ Mpc and sky area of 100 deg$^2$. With PTF-like telescopes, even 1 day is sufficient for such a survey. This catalog can then be provided to other telescopes to aid electromagnetic follow-up observations to find kilonovae from binary coalescences, as well as other sources. We consider H$\alpha$ observations, which track the star formation rate and are therefore correlated with the rate of compact binary mergers. H$\alpha$ surveys are also able to filter out galaxies that are farther away than the maximum GW source distance. Rapid galaxy surveys that follow GW triggers could achieve $\sim90\%$ completeness with respect to star formation rate, which is currently unavailable. This will significantly reduce the required effort and enhance the immediate availability of catalogs compared to possible future all-sky surveys.