A Common Origin for Quasar Extended Emission-Line Regions and Their Broad-Line Regions

TL;DR

This study finds a correlation between luminous extended emission-line regions and low-metallicity broad-line regions in quasars, suggesting a common external origin from galaxy mergers that supply gas to the central black hole.

Contribution

It provides the first direct observational evidence linking merger-driven external gas to both EELRs and BLRs in quasars, highlighting a shared origin.

Findings

EELRs are associated with low-metallicity BLRs

Both EELRs and BLRs have metallicities below solar

Gas from mergers can reach near the black hole (<1 pc)

Abstract

We present a correlation between the presence of luminous extended emission-line regions (EELRs) and the metallicity of the broad-line regions (BLRs) of low-redshift quasars. The result is based on ground-based [O III] 5007 narrow-band imaging and Hubble Space Telescope UV spectra of 12 quasars at 0.20 < z < 0.45. Quasars showing luminous EELRs have low-metallicity BLRs (Z < 0.6 Z_Solar), while the remaining quasars show typical metal-rich gas (Z > Z_Solar). Previous studies have shown that EELRs themselves also have low metallicities (Z < 0.5 Z_Solar). The correlation between the occurrence of EELRs and the metallicity of the BLRs, strengthened by the sub-Solar metallicity in both regions, indicates a common external origin for the gas, almost certainly from the merger of a gas-rich galaxy. Our results provide the first direct observational evidence that the gas from a merger can indeed be driven down to the immediate vicinity (< 1 pc) of the central black hole.