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2 Spectral classification

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$\alpha $ 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$\alpha $ recombination line. When H$\alpha $ 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$\alpha $ recombination emission lines, and were classified as non-ELGs. Stellar H$\alpha $ 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:

1.
Because of their very large permitted line widths, Seyfert 1 galaxies are easily recognizable. We classify 5 galaxies (Mrk 335, Mrk 352, Mrk 6, I Zw 26, and Mrk 50) with broad emission lines in their spectra as Seyfert 1 galaxies.

2.
To discriminate the Seyfert 2 from star-forming galaxies, we consider the intensity ratio (corrected for reddening, see Sect. 3.4) $R_{23} = (\hbox{[O~{\sc ii}]}3727 + \hbox{[O~{\sc iii}]}4959,5007)/\hbox{H$\beta$ }$ (Dessauges-Zavadsky et al. 2000) to the remaining 78 galaxies except for VII Zw 631. Dessauges-Zavadsky et al. (2000) showed empirically that galaxies with log $R_{23} \simeq 1.1 $ are mostly Seyfert 2. Based on this diagnostic, we classify 4 galaxies, Mrk 1, Mrk 622, Mrk 198, Mrk 270, as Seyfert 2. The classification of Seyfert 1 and Seyfert 2 agrees with the results of Véron-Cetty & Véron (2001).

3.
When neither [O  II]3727 nor [O  III]4959,5007 are available, we use the flux ratio of H$\alpha $ and [N  II]6583 to identify an AGN (Carter et al. 2001). If $\log(\hbox{[N~{\sc ii}]}6583/\hbox{H$\alpha$ })<-0.25$, the galaxy is classified as a star-forming galaxy (SFG, H  II-like); otherwise it is classified as an AGN. This allows us to classify VII Zw 631, which has $\log(\hbox{[N~{\sc ii}]}6583/\hbox{H$\alpha$ })= -0.07$, as an AGN.

To summarize, our sample consists of 13 non-ELGs; 10 AGNs; and 74 SFGs.


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