On the Origin of [Oii] Emission in Red‐Sequence and Poststarburst Galaxies

Abstract

We investigate the emission-line properties of galaxies with red rest-frame colors using spectra from SDSS DR2. 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: Ha 6563 and [OII] 3727. There is a strong bimodality in [OII]/Ha ratio in the full SDSS sample which closely corresponds to the bimodality in rest-frame color. Nearly all of the line-emitting red galaxies have line ratios typical of various types of AGN -- most commonly LINERs, a small fraction of transition objects and, more rarely, Seyferts. The [OII]-Ha EW diagram reveals the continuity between quiescent galaxies with no detectable emission and galaxies with LINER-like line ratios, which altogether form a LINER/Quiescent sequence. A straight line in the [OII]-Ha EW plane divides these galaxies from the remainder. [OII] equivalent widths in LINER- and AGN-like galaxies can be as large as in star-forming galaxies. Thus, unless objects with AGN/LINER-like line ratios are excluded, [OII] emission cannot 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 AGN properties as well as star formation, and therefore may provide seriously incomplete sets of post-starburst galaxies. Furthermore, post-starburst galaxies identifed in SDSS by requiring minimal Ha equivalent width generally exhibit weak but nonzero line emission with ratios typical of AGNs; few of them show residual star formation. This suggests that most post-starbursts harbor AGNs/LINERs