A NEW SURVEY FOR LOW-LUMINOSITY EMISSION-LINE GALAXIES

A survey for low-luminosity emission-line galaxies is described in whi ch deep CCD images have been obtained through five narrow-band (DELTA( lambda) = 80 angstrom) filters. The survey covers 0.7 square degrees t o a depth corresponding to a monochromatic magnitude M6600 = 21.2 in t he emission line plus continuum. The survey is sensitive to objects wi th strong Halpha emission out to a redshift of z = 0.054, as well as h igher redshift galaxies with [O III], Hbeta, or [O II] emission. Two h undred ninety-three emission-line galaxy candidates were selected base d on a (line plus continuum)/continuum ratio of 1.5 or greater. These candidates were observed spectroscopically with a multifiber spectrogr aph. Eleven low-redshift (z < 0.054) emission-line galaxies were confi rmed with moderate to strong Halpha emission and M(B) in the range - 1 7.8 to - 12-9. Thirty-six additional objects are emission-line galaxie s at higher redshift, including 11 in which only one emission line is seen. Nine of the low-redshift emission-line galaxies form an essentia lly complete sample, and two methods are explored to calculate the lum inosity function. While the range of luminosity sampled is small enoug h that trade-offs between slope and normalization of the luminosity fu nction are possible, the combination of this survey with the study of Salzer allows fairly restrictive limits on these two quantities. The b est-fit Schechter function to the combination of these two data sets h as slope alpha = - 1.35 and normalization, log phi = -3.15 mag-1 Mpc- 3. In heavy-element abundance, the objects found in this survey span t he range defined by known dwarf emission-line galaxies, with the most metal-poor object having an O/H abundance about 0.09 of the solar valu e. Despite the fact that the survey technique employed is sensitive to the presence of ultra-low metal-abundance galaxies, no such objects w ere found. One group of higher redshift emission-line galaxies, those with [O III] lambda5007 shifted into the Halpha region, has relevance to the interpretation of faint blue galaxy counts. These objects are e xamples of the type of galaxy that has been proposed to account for th e excess counts at B = 22 and fainter. We find that the number discove red in the survey is exactly that predicted by the local luminosity fu nction, i.e., there is no evidence for strong evolution in the number density of these objects, in disagreement with that interpretation.