1. AGN Origin of [O II] in Red Galaxies --- Implications for post-starburst galaxy studies (1) University of California, Berkeley (2) Lawrence Berkeley Laboratory (3)University of California, Santa Cruz Renbin Yan 1, Jeffrey Newman 2, Sandy Faber 3, Nick Konidaris 3, David Koo 3, Marc Davis 1

2. [O II] has been widely used as a star formation indicator at high redshift, especially in the study of post-starburst (E+A) galaxies. But AGN, especially LINERs, can also produce [O II] emission. So, how often and to what degree is [O II] emission contaminated by AGN or LINERs? How is the sample selection of post-starburst galaxies affected by [O II] from AGN? Can [O II] be used as a universal star- formation indicator? How often is it contaminated by AGNs? The Questions :

3. Post-starburst galaxies stand out in Hα but not in [O II] The Mystery

4. Data and Sample Selection We measured emission lines for ~200,000 galaxies from SDSS DR2 ‘main’ galaxy sample after a careful subtraction of the stellar continua. We used a volume limited sample of ~40,000 galaxies with 0.05< z < 0.10 and M 0.1r < -19.5.

5. Color- Magnitude Diagram showing the bimodality in rest-frame colors and our cuts Red Sample Blue Sample

6. Median [O II]/Hα Red1.4 Blue0.35 (w/o extinction correction) Bimodality in rest-frame color is reflected by bimodality in [OII]/H  ratio. Red SampleBlue Sample

7. Line-emitting red galaxies appear to be mostly AGN. [O II] emission is common among red galaxies. ---- Nearly half of the red galaxies have [O II] EW > 3A.

8. Post-starburst galaxies stand out in Hα but not in [O II] The Mystery

9. [O II] emission in post-starburst galaxies with minimal H  can be as strong as in some star-forming galaxies. The position of post- starburst galaxies (identified using H  ) in [O II]-Hα plot suggest they might be AGN or LINERs.

10. Conclusions 1) Galaxies display bimodality in [O II]/Hα ratios, which corresponds to the bimodality in rest-frame colors. 2) Line-emitting red galaxies have line ratios following those of AGN or LINERs. 3) Post-starburst galaxies identified in the SDSS dataset using Hα often exhibit significant [O II] EW; their position in [O II]-Hα plot suggest they might be AGN or LINERs. 4) [O II] emission in LINERs can be as strong as in star-forming galaxies. Locally, it is only a proxy for SFR in blue galaxies. 5) Post-starburst samples defined using [O II] will be very incomplete, especially if AGN rates and intensity were higher in the past. We recommend using Hb as an alternative for high redshift.