COMPARING Hα AND H I SURVEYS AS MEANS TO A COMPLETE LOCAL GALAXY CATALOG IN THE ADVANCED LIGO/VIRGO ERA

Identifying the electromagnetic counterparts of gravitational wave (GW) sources detected by upcoming networks of advanced ground-based interferometers will be challenging, due in part to the large number of unrelated astrophysical transients within the ∼10-100 deg2 sky localizations. A potential way to greatly reduce the number of such false positives is to limit detailed follow-up to only those candidates near galaxies within the GW sensitivity range of ∼200 Mpc for binary neutron star mergers. Such a strategy is currently hindered by the fact that galaxy catalogs are grossly incomplete within this volume. Here, we compare two methods for completing the local galaxy catalog: (1) a narrowband Hα imaging survey and (2) an H I emission line radio survey. Using Hα fluxes, stellar masses (M *), and star formation rates (SFRs) from galaxies in the Sloan Digital Sky Survey (SDSS), combined with H I data from the GALEX Arecibo SDSS Survey and the Herschel Reference Survey, we estimate that an Hα survey with a luminosity sensitivity of L Hα = 1040 erg s–1 at 200 Mpc could achieve a completeness of f HαSFR ≈ 75% with respect to total SFR, but only fM*⋆Hα approx. 33% with respect to M * (due to lack of sensitivity to early-type galaxies). These numbers are significantly lower than those achieved by an idealized spectroscopic survey due to the loss of Hα flux resulting from resolving out nearby galaxies and the inability to correct for the underlying stellar continuum. An H I survey with sensitivity similar to the proposed WALLABY survey on ASKAP could achieve fSFRHI ≈ 80% and fM⋆HI ≈ 50%, somewhat higher than that of the Hα survey. Finally, both Hα and H I surveys should achieve ∼> 50% completeness with respect to the host galaxies of short-duration gamma-ray bursts, which may trace the population of binary neutron star mergers.