The DEEP2 galaxy redshift survey: Redshift identification of single-line emission galaxies

We present two methods for determining spectroscopic redshifts of galaxies in the DEEP2 survey which display only one identifiable feature, an emission line, in the observed spectrum ("single-line galaxies"). First, we assume each single line is one of the four brightest lines accessible to DEEP2: Hα, [O III] λ5007, Hβ, or [O II] λ3727. Then, we supplement spectral information with BRI photometry. The first method, parameter-space proximity (PSP), calculates the distance of a single-line galaxy to galaxies of known redshift in (B - R), (R - I), R, λ _observed parameter space. The second method is an artificial neural network (ANN). Prior information, such as allowable line widths and ratios, rules out one or more of the four lines for some galaxies in both methods. Based on analyses of evaluation sets, both methods are nearly perfect at identifying blended [O II] doublets. Of the lines identified as Hα in the PSP and ANN methods, 91.4% and 94.2%, respectively, are accurate. Although the methods are not this accurate at discriminating between [O III] and Hβ, they can identify a single line as one of the two, and the ANN method in particular unambiguously identifies many [O III] lines. From a sample of 640 single-line spectra, the methods determine the identities of 401 (62.7%) and 472 (73.8%) single lines, respectively, at accuracies similar to those found in the evaluation sets.