Tracing Star Formation in Quasar Hosts via [O II] $\lambda$3727: A Kinematically Consistent Approach

TL;DR

This paper introduces a kinematic decomposition method to isolate star formation signals in quasar host galaxies using [O II] emission, revealing a strong correlation between star formation rate and quasar luminosity.

Contribution

A new kinematic approach to separate AGN and star formation contributions in [O II] emission in quasars, enabling more reliable star formation measurements.

Findings

[O II] emission is mainly due to star formation in quasars.

Mean star formation rates correlate tightly with quasar luminosity.

Luminosity is more strongly linked to star formation than SMBH mass or Eddington ratio.

Abstract

Measuring star formation in quasar host galaxies is crucial for understanding the coevolution of supermassive black holes (SMBHs) and galaxies, yet remains observationally challenging due to severe contamination from active galactic nucleus (AGN) emission. In this work, we present a new method to robustly isolate the AGN contribution to the [O II] $\lambda$3727 emission line in quasars, based on a kinematically consistent decomposition of [O II] and the high-ionization [Ne V] $\lambda$3426 line. We find that the [O II] emission in quasars is primarily dominated by star formation, with only a weak AGN contribution, and thus can be reliably used as a tracer of star formation in quasar hosts. Applying this technique to a large sample of Sloan Digital Sky Survey quasars, we derive mean SFRs as a function of bolometric luminosity. We find a tight correlation between mean SFR and luminosity. Further analysis, assuming a constant dust extinction correction to [O II] emission, shows that luminosity is the primary parameter most strongly associated with star formation, rather than SMBH mass or Eddington ratio. This supports the scheme in which star formation and black hole accretion are closely linked through their common dependence on the cold gas supply.