Hα dots: A catalog of faint emission-line objects discovered in narrowband images

During a wide-field narrowband Hα imaging survey, we noted the presence of numerous isolated emission-line point sources in the data. These objects could represent ultra-low-luminosity galaxies at low-redshift (detection via Hα), isolated extragalactic H II regions associated with the galaxy targeted by the original observation, or background galaxies or QSOs where strong emission lines (most often [O III] λ5007) redshift into our narrowband filter. We have carried out a systematic search for these "Hα dots" in over 200 15 × 15 arcmin fields. To date we have cataloged 61 candidate emission-line sources in roughly 11.7deg ². The sample has a median R magnitude of 19.5, and detects objects as faint as R = 22.6. Follow-up spectroscopy reveals that 85% of the candidates are bona fide emission-line objects, with roughly 60% of the real sources being lower-redshift objects (detection via Hα) and 40% being higher-redshift objects detected via [O III] emission or some other emission line. Here we present the results of our initial survey and follow-up spectroscopy. We use our sample to study the properties (including star-formation rates and metal abundances) of low-luminosity star-forming galaxies in the nearby universe and of low-metallicity star-forming galaxies at z 0.33.