The starburst-AGN disconnection

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.

💡 Research Summary

The paper tackles a long‑standing bias in optical studies of starbursts and active galactic nuclei (AGN): galaxies with emission lines that are too weak for reliable measurement are routinely omitted from diagnostic diagrams such as the BPT plot. Using the Sloan Digital Sky Survey Data Release 7, the authors select roughly 20,000 objects whose four key lines (Hα, Hβ,