Retired galaxies: not to be forgotten in the quest of the star formation - AGN connection

We propose a fresh look at the Main Galaxy Sample of the Sloan Digital Sky Survey by packing the galaxies in stellar mass and redshift bins. We show how important it is to consider the emission-line equivalent widths, in addition to the commonly used emission-line ratios, to properly identify retired galaxies (i.e. galaxies that have stopped forming stars and are ionized by their old stellar populations) and not mistake them for galaxies with low-level nuclear activity. We find that the proportion of star-forming galaxies decreases with decreasing redshift in each mass bin, while that of retired galaxies increases. Galaxies with M-* > 10(11.5) M-circle dot have formed all their stars at redshift larger than 0.4. The population of AGN hosts is never dominant for galaxy masses larger than 10(10)M(circle dot). We warn about the effects of stacking galaxy spectra to discuss galaxy properties. We estimate the lifetimes of active galactic nuclei (AGN) relying entirely on demographic arguments - i.e. without any assumption on the AGN radiative properties. We find upper-limit lifetimes of about 1-5 Gyr for detectable AGN in galaxies with masses between 10(10)-10(12) M-circle dot. The lifetimes of the AGN-dominated phases are a few 10(8) yr. Finally, we compare the star formation histories of star-forming, AGN and retired galaxies as obtained by the spectral synthesis code STARLIGHT. Once the AGN is turned on, it inhibits star formation for the next similar to 0.1 Gyr in galaxies with masses around 10(10)M(circle dot), similar to 1 Gyr in galaxies with masses around 10(11) M-circle dot.