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R. Yan (University of California,Berkeley), DEEP2 Team
We investigate the emission-line properties of galaxies with red rest-frame colors (compared to the g-r color bimodality) using spectra from Data Release 2 of the Sloan Digital Sky Survey. Emission lines are detected in more than half of the red galaxies. We focus on the relationship between two emission lines commonly used as star formation rate indicators: H\alpha and [OII] \lambda3727. Since [OII] is the principal proxy for H\alpha at z~1, the correlation between them is critical for comparison between low-z and high-z galaxy surveys. We find strong bimodality in [OII]/H\alpha ratio in the SDSS sample, which closely corresponds to the bimodality in rest-frame color. Based on standard line diagnostics, we find that most of the line-emitting red galaxies would be classified as LINERs, with a small fraction of Seyferts and some intermediate objects. [OII] emission in LINERs can be as strong as that in star-forming galaxies. Thus, without a careful pre-selection of galaxy samples to remove red/quiescent galaxies, it can not be used directly as a proxy for star formation rate. Lack of [OII] emission is generally used to indicate lack of star formation when post-starburst galaxies are selected at high redshift. Our results imply, however, that these samples have been cut on LINER/AGN properties as well as star formation, and therefore may provide seriously incomplete sets of post-starburst galaxies. In fact, post-starburst galaxies identifed in SDSS by requiring minimal H\alpha equivalent width often have strong [OII] emission, which suggest they harbor AGNs.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.