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

TL;DR

This study re-evaluates galaxy classifications by incorporating emission-line equivalent widths, revealing the evolving proportions of star-forming, retired, and AGN-hosting galaxies across stellar masses and redshifts, and estimating AGN lifetimes.

Contribution

It introduces the use of emission-line equivalent widths for better galaxy classification and provides demographic estimates of AGN lifetimes without relying on radiative properties.

Findings

Retired galaxies increase as redshift decreases in each mass bin.

AGN lifetimes are estimated to be 1-5 Gyr for detectable AGN.

Star formation is inhibited for 0.1-1 Gyr after AGN activation.

Abstract

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 $M_\star > 10^{11.5} M_\odot$ have formed all their stars at redshift larger than 0.4. The population of AGN hosts is never dominant for galaxy masses larger than $10^{10} M_\odot$. 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 $10^{10}$--$10^{12} M_\odot$. 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 $\sim$ 0.1 Gyr in galaxies with masses around $10^{10} M_\odot$, $\sim$ 1 Gyr in galaxies with masses around $10^{11} M_\odot$.