Study of a complete sample of Hα emission-line galaxies from the UCM survey

The Universidad Complutense de Madrid survey is a long-term project with the aim of finding and analyzing star forming galaxies using the Hα line as the tracer for star formation processes. In order to obtain a representative and complete sample of the population detected, spectroscopic observations were carried out for the full sample of Hα emission-line galaxy (ELG) candidates of the UCM lists 1 and 2. The ELGs types most commonly found (47%) are intermediate to low-luminosity objects with a very intense star-formation region which dominates the optical energy output of the galaxy. This kind of ELGs is similar to the galaxy population detected in the blue objective-prism surveys. And what is more important, a second population (43%) of star-forming galaxies with low ionization or high extinction properties has been found. This ELGs group is detected neither in the blue (University of Michigan survey, Case survey) nor in other surveys (Kiso, IRAS, Markarian) using other selection techniques

An automatic approach for selecting new Hα emission-line galaxies

The most unambiguous way to discover new emission-line galaxies (ELGs) is directly by the presence of their lines, using objective-prism plates of adequate resolution. The first survey using this technique was developed by Smith in 1975 with the 0.6 m CTIO Curtis Schmidt Telescope. The Universidad Complutense de Madrid (UCM) is carrying out a survey of ELGs with the Schmidt Telescope at Calar Alto (Almería, Spain) using the presence of Ha in emission in IIIa-F prism plates as selection criterion. The observational procedure and results are described in Rego et al. 1989; Zamorano et al. 1990; Zamorano et al. 1993. The standard procedure is to scan the plates by eye, with a low-power binocular microscope. The use of different subjective criteria in the visual inspection can produce biased samples. Moreover, the visual scan does not produce any quantitative measure but only a sample of candidates. Therefore, in order to obtain an observer-independent sample and to extract the largest amount of information from the original plates, 2 plates have been scanned with the MAMA machine 1 (Guibert & Moreau 1991; Moreau 1992) with the aim of developing an automatic procedure to detect ELGs spectra