Abstract
Post-starburst (PSB) galaxies comprise a class of objects with strong traces of young stellar populations in their spectra, but no sign of ongoing star formation. Their star formation histories (SFH) indicate that over 70% of their flux is produced by stellar populations younger than 1.5 Gyr. Samples of PSB galaxies are usually generated by selecting spectra with strong Balmer absorption lines, but no detectable emission lines that characterize star formation (e.g. [OII]λ 3727 and Hα). The usual criterion to limit star formation is to limit the equivalent width of the [OII]λ 3727 absorption line (Goto el al. 2004). Post-starburst galaxies identified in the Sloan Digital Sky Survey (SDSS) only by requiring minimal [OII]λ 3727 emission generally exhibit weak but nonzero emission lines with typical ratios of Active Galaxy Nuclei (AGN) hosts. This suggests that most PSB galaxies may harbor ``low-ionization nuclear emission-line regions'' (LINERs) and, more rarely, Seyferts (Yan et al. 2008). In this research, we use the STARLIGHT spectral synthesis code (Cid Fernandes et al. 2005) to compute the fraction of light coming from young stellar populations, here denoted by LFYS, in a volume-limited sample from the SDSS DR7 catalog. We then classify as PSB those galaxies with LFYS larger than 70%, log([NII]λ 6584/Hα) higher than -0.4 and Hα equivalent width (EW Hα) smaller than 5 Å. These two last criteria select galaxies without current star formation (Cid Fernandes et al. 2011). When plotting this sample in the BPT diagram, we identify a high occurrence of LINER and Seyfert hosts, as found by Yan et al. (2008). However, using the WHAN diagram, we show that most of post-starburst galaxies with low emission lines are in fact passive galaxies, frequently misclassified as weak AGN hosts.
