  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 square degree 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 narrow-band H-alpha imaging survey; and (2) an HI emission line radio
survey. Using H-alpha fluxes, stellar masses (M_star), and star formation rates
(SFR) from galaxies in the Sloan Digital Sky Survey (SDSS), combined with HI
data from the GALEX Arecibo SDSS Survey and the Herschel Reference Survey, we
estimate that a H-alpha survey with a luminosity sensitivity of L_H-alpha =
1e40 erg/s at 200 Mpc could achieve a completeness of f_SFR ~ 75% with respect
to total SFR, but only f_Mstar ~ 33% with respect to stellar mass (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
H-alpha flux resulting from resolving out nearby galaxies and the inability to
correct for the underlying stellar continuum. An HI survey with sensitivity
similar to the proposed WALLABY survey on ASKAP could achieve f_SFR ~ 80% and
f_Mstar ~ 50%, somewhat higher than that of the H-alpha survey. Finally, both
H-alpha and HI 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.
