Low Metallicity Active Galactic Nuclei in eBOSS

Active galactic nuclei with sub-solar metallicities are extremely rare in the local universe due to the tendency of AGNs to reside in massive metal-rich hosts. However, such objects are interesting to study because they can potentially shed light on the early phases of black hole and host galaxy co-evolution. Low metallicity AGNs (LM-AGNs) are expected to be more common at higher redshifts, where galaxies are more metal-poor on average and both black holes and their hosts are growing rapidly. Here we use data from the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) to search for low metallicity AGNs out to z = 0.55. We select 29,650 AGNs using the standard Baldwin, Phillips and Terlevich (BPT) diagram and we define a sample of 44 extreme LM-AGNs by selecting objects with log([NII]/H-alpha) < -0.8. We use additional line diagnostics to eliminate a small number of star forming interlopers and to explore whether other processes besides low gas-phase metallicity could potentially produce the extreme line ratios we observe. We find that the LM-AGNs have surprisingly large stellar and gas velocity dispersions and they have some of the highest [O III] luminosities in the eBOSS AGN sample, suggesting that their host galaxies and black holes are likely to be massive. To explain the contradiction of massive host galaxies and low metallicity narrow line regions, we hypothesize that many of the LM-AGNs in our sample may be the result of mergers between low metallicity gas-rich dwarf galaxies and massive gas-poor AGN hosts. We consider the role that such AGN ‘refueling’ events could play in the growth of black holes over cosmic time.