Most of our BCG sample was selected from the previous neutral hydrogen studies of blue compact galaxies by Gordon & Gottesman (1981). This study has focused on the Haro, Markarian, and Zwicky lists of galaxies and hence objects were selected on the basis of a blue color, UV-excess or compactness, but not on the basis of emission line strength (Smoker et al. 2000). The optical spectral observations of these galaxies show a range in spectral properties; from galaxies with absorption-line spectra to narrow emission line objects classified as SFGs.
In order to study our sample galaxies in detail, we use emission lines to classify the sample spectra into three types: non-emission line galaxy (non-ELG), low-luminosity active galactic nuclei (AGN) and star-forming galaxy (SFG). Our classification scheme is outlined below.
Because the H
recombination line is easily detected in optical
spectrum and only weakly affected by dust and underlying stellar
absorption, we first separated the spectra into two broad categories,
emission line galaxy and non-emission line galaxy, using the H
recombination line. When H
is detected in emission, we classified
the galaxy as an emission line galaxy, otherwise as a non emission
line galaxy. 13 of 97 BCGs spectra have no H
recombination
emission lines, and were classified as non-ELGs. Stellar H
absorption lines are, in fact, prominent in all of these spectra.
Next we classify the remaining 84 emission line spectra into active galactic nuclei and star-forming galaxies based on the next 3 steps:
Copyright ESO 2002