A NEW SURVEY FOR LOW-LUMINOSITY EMISSION-LINE GALAXIES

A survey for low-luminosity emission-line galaxies is described in which deep CCD images have been obtained through five narrow-band (DELTA(lambda) = 80 angstrom) filters. The survey covers 0.7 square degrees to a depth corresponding to a monochromatic magnitude M6600 = 21.2 in the emission line plus continuum. The survey is sensitive to objects with strong Halpha emission out to a redshift of z = 0.054, as well as higher redshift galaxies with [O III], Hbeta, or [O II] emission. Two hundred ninety-three emission-line galaxy candidates were selected based on a (line plus continuum)/continuum ratio of 1.5 or greater. These candidates were observed spectroscopically with a multifiber spectrograph. Eleven low-redshift (z < 0.054) emission-line galaxies were confirmed with moderate to strong Halpha emission and M(B) in the range - 17.8 to - 12-9. Thirty-six additional objects are emission-line galaxies at higher redshift, including 11 in which only one emission line is seen. Nine of the low-redshift emission-line galaxies form an essentially complete sample, and two methods are explored to calculate the luminosity function. While the range of luminosity sampled is small enough that trade-offs between slope and normalization of the luminosity function are possible, the combination of this survey with the study of Salzer allows fairly restrictive limits on these two quantities. The best-fit Schechter function to the combination of these two data sets has 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 the 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 value. Despite the fact that the survey technique employed is sensitive to the presence of ultra-low metal-abundance galaxies, no such objects were 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 examples of the type of galaxy that has been proposed to account for the excess counts at B = 22 and fainter. We find that the number discovered in the survey is exactly that predicted by the local luminosity function, i.e., there is no evidence for strong evolution in the number density of these objects, in disagreement with that interpretation.