On the Origin of [oii] Emission in Red Sequence and Post-starburst Galaxies

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 4 of the Sloan Digital Sky Survey (SDSS). 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: Hα and [O II] λ3727. Since [O II] is the principal proxy for Hα at z ∼ 1, the correlation between them is critical for comparison between low-z and high-z galaxy surveys. We find a strong bimodality in [O II]/Hα ratio in the SDSS sample, which closely corresponds to the bimodality in rest-frame color. Based on standard line ratio diagnostics, most (nearly all of the) line-emitting red galaxies have line ratios typical of various types of Active Galactic Nuclei (AGN) — most commonly “low-ionization nuclear emission-line regions” (LINERs), a small fraction of “transition objects” (TOs) and, more rarely, Seyferts. Only ∼ 6% of red galaxies have line ratios resembling star-forming galaxies. A straight line in the [O II]-Hα EW diagram separates LINER-like galaxies from other categories, provides an effective classification tool complementary to standard line ratio diagnostics. Quiescent galaxies with no detectable emission lines and those galaxies with LINER-like line ratios combine to form a single, tight red sequence in color-magnitude-concentration space. Other than modest differences in the luminosity range they span, these two classes are only distinguished from each other by line strength. We also find that [O II] equivalent widths in LINERand AGN-like galaxies can be as large as that in star-forming galaxies. Thus, unless objects with AGN/LINER-like line ratios are excluded, [O II] emission cannot be used directly as a proxy for star formation rate; this is a particular issue for red galaxies. Lack of [O II] 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 Hα 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-starburst galaxies may harbor AGNs/LINERs. Subject headings: galaxies: active — galaxies: evolution — galaxies: fundamental parameters — galaxies: ISM — galaxies: statistics