Census of the Local Universe (CLU) Narrowband Survey. I. Galaxy Catalogs from Preliminary Fields

We present the Census of the Local Universe (CLU) narrowband survey to search for emission-line (Hα) galaxies. CLU-Hα has imaged ≈3π of the sky (26,470 deg²) with four narrowband filters that probe a distance out to 200 Mpc. We have obtained spectroscopic follow-up for galaxy candidates in 14 preliminary fields (101.6 deg²) to characterize the limits and completeness of the survey. In these preliminary fields, CLU can identify emission lines down to an Hα flux limit of 10^-14 erg s^-1 cm^-2 at 90% completeness, and recovers 83% (67%) of the Hα flux from cataloged galaxies in our search volume at the Σ = 2.5 (Σ = 5) color excess levels. The contamination from galaxies with no emission lines is 61% (12%) for Σ = 2.5 (Σ = 5). Also, in the regions of overlap between our preliminary fields and previous emission-line surveys, we recover the majority of the galaxies found in previous surveys and identify an additional ≈300 galaxies. In total, we find 90 galaxies with no previous distance information, several of which are interesting objects: 7 blue compact dwarfs, 1 green pea, and a Seyfert galaxy; we also identify a known planetary nebula. These objects show that the CLU-Hα survey can be a discovery machine for objects in our own Galaxy and extreme galaxies out to intermediate redshifts. However, the majority of the CLU-Hα galaxies identified in this work show properties consistent with normal star-forming galaxies. CLU-Hα galaxies with new redshifts will be added to existing galaxy catalogs to focus the search for the electromagnetic counterpart to gravitational wave events.