The starburst-AGN disconnection

TL;DR

This paper investigates weak emission line galaxies, revealing that many are old, massive systems with ionization driven by stellar populations rather than active galactic nuclei, challenging traditional classifications.

Contribution

It demonstrates that weak line galaxies are often old, massive, star-forming or LINER systems with ionization from stellar populations, not active black holes.

Findings

Weak line galaxies are often old, massive systems.

LINERs in these galaxies have ceased star formation long ago.

Ionization is consistent with stellar populations, not AGN activity.

Abstract

Optical studies of starbursts, AGN and their connections usually leave out galaxies whose emission lines are too weak to warrant reliable measurement and classification. Yet, weak line galaxies abound, and deserve a closer look. We show that these galaxies are either massive, metal rich star-forming systems, or, more often, LINERs. From our detailed stellar population analysis, we find that these LINERs have stopped forming stars long ago. Moreover, their ionizing radiation field is amazingly consistent with that expected from their old stellar populations alone. The black-hole in the centers of these massive, early-type galaxies is not active enough to overwhelm stellar ionization, and thus, despite their looks, they should not be called AGN.