Outflows from active galactic nuclei: the BLR–NLR metallicity correlation

Abstract

The metallicity of active galactic nuclei (AGNs), which can be measured by\nemission line ratios in their broad and narrow line regions (BLRs and NLRs),\nprovides invaluable information about the physical connection between the\ndifferent components of AGNs. From the archival databases of the International\nUltraviolet Explorer, the Hubble Space Telescope and the Sloan Digital Sky\nSurvey, we have assembled the largest sample available of AGNs which have\nadequate spectra in both the optical and ultraviolet bands to measure the\nnarrow line ratio [N II]/H{\\alpha} and also, in the same objects, the\nbroad-line N V/C IV ratio. These permit the measurement of the metallicities in\nthe NLRs and BLRs in the same objects. We find that neither the BLR nor the NLR\nmetallicity correlate with black hole masses or Eddington ratios, but there is\na strong correlation between NLR and BLR metallicities. This metallicity\ncorrelation implies that outflows from BLRs carry metal-rich gas to NLRs at\ncharacteristic radial distances of ~ 1.0 kiloparsec. This chemical connection\nprovides evidence for a kinetic feedback of the outflows to their hosts. Metals\ntransported into the NLR enhance the cooling of the ISM in this region, leading\nto local star formation after the AGNs turn to narrow line LINERs. This\npost-AGN star formation is predicted to be observable as an excess continuum\nemission from the host galaxies in the near infrared and ultraviolet, which\nneeds to be further explored.\n