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 Yan1, Jeffrey Newman2, Sandy Faber3, Nick Konidaris3, David Koo3, Marc Davis1

[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 :

Post-starburst galaxies stand out in Hα but not in [O II]

The Mystery

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 M0.1r< -19.5 .

Color-Magnitude Diagram showing the bimodality in rest-frame colors and our cuts

Red Sample

Blue Sample

Median [O II]/Hα

Red 1.4

Blue 0.35

(w/o extinction correction)

Bimodality in rest-frame color is reflected by bimodality in [OII]/H ratio.

Red SampleBlue Sample

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.

Post-starburst galaxies stand out in Hα but not in [O II]

The Mystery

• [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.

Conclusions1) 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.