Post-starburst galaxies can be identified via the presence of prominent Hydrogen Balmer absorption lines in their spectra .
We present a comprehensive study of the origin of strong Balmer lines in a volume-limited sample of 189 galaxies with 0.01 < z < 0.05 , \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) > 9.5 and projected axis ratio b / a > 0.32 .
We explore their structural properties , environments , emission lines and star formation histories , and compare them to control samples of star-forming and quiescent galaxies , and simulated galaxy mergers .
Excluding contaminants , in which the strong Balmer lines are most likely caused by dust-star geometry , we find evidence for three different pathways through the post-starburst phase , with most events occurring in intermediate-density environments : ( 1 ) a significant disruptive event , such as a gas-rich major merger , causing a starburst and growth of a spheroidal component , followed by quenching of the star formation ( 70 % of post-starburst galaxies at 9.5 < \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) < 10.5 and 60 % at \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) > 10.5 ) ; ( 2 ) at 9.5 < \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) < 10.5 , stochastic star formation in blue-sequence galaxies , causing a weak burst and subsequent return to the blue sequence ( 30 % ) ; ( 3 ) at \log ( \mbox { M } _ { \star } / \mbox { M } _ { \odot } ) > 10.5 , cyclic evolution of quiescent galaxies which gradually move towards the high-mass end of the red sequence through weak starbursts , possibly as a result of a merger with a smaller gas-rich companion ( 40 % ) .
Our analysis suggests that AGN are ‘ on ’ for 50 \% of the duration of the post-starburst phase , meaning that traditional samples of post-starburst galaxies with strict emission line cuts will be at least 50 \% incomplete due to the exclusion of narrow-line AGN .