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

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] $λ$3727 emission line in quasars, based on a kinematically consistent decomposition of [O II] and the high-ionization [Ne V] $λ$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.

Figures (4)

• Figure 1: Sample and spectral fitting methodology.Left panel: Distribution of the full sample in the $L_{\rm bol}$--$\lambda_{\rm Edd}$ parameter space. The apparent linear features formed by data points in the upper-left and lower-right corners arise from the truncation applied to the Mgii FWHM values in ws22, and the affected sources represent only a negligible portion of the sample. Right panel: The composite spectrum of the full sample illustrating the continuum and Fe II subtraction procedure. In the main panel, the Fe II fitting includes three free parameters: the velocity dispersion and two independent amplitudes for the wavelength ranges 2200–3100 Å and 3250–3500 Å respectively, with the interval 3100–3250 Å excluded from the fit zfchen2022. The thick orange horizontal line indicates the fitting window for the power-law continuum. Green and red lines mark the wavelength ranges for two separate iron templates with independent amplitudes. The fitted continuum and iron components are shown in corresponding colors. The [O II] $\lambda\lambda$3726,3729 doublet region is indicated by a gray dashed box in the main panel, with a zoomed-in view shown in the upper-right inset where a single iron template (green line) is used.
• Figure 2: Kinematically consistent decomposition of [O II] and [Ne V] emission lines for the full sample. Both panels show dual x-axes in wavelength and velocity, with the velocity axes aligned for direct comparison. The observed profile is shown in blue, and each line is decomposed into narrow (orange), "broad (green)", and "very broad" (red) Gaussian components, whose widths and velocity shifts are tied between [O2] and [Ne5]. In the [O2] doublet (upper panel), the $\lambda3726$ and $\lambda3729$ lines are indicated by dotted and solid curves, respectively. For each component and for the total model (gray), 100 randomly drawn MCMC posterior samples are overplotted as semi-transparent curves to illustrate the fitting uncertainties. In the lower panel, a scaled [O III] emission-line profile (light gray) is overplotted as a reference (see §\ref{['sec:discussionA']}).
• Figure 3: [O2] and [Ne5] line profiles and flux ratios as a function of bolometric luminosity.Upper and middle panels: Observed [O2] and [Ne5] line profiles in five bolometric-luminosity bins, normalized to the same integrated flux to facilitate comparison of line widths and shapes. Lower panel: [O II]/[Ne V] flux ratios for narrow component (blue), broad components (orange), and total flux (green). Previous total-flux measurements from vdb2001 (star) and zfchen2022 (triangles) are shown for comparison. The horizontal dotted line marks the flux ratio of 0.5 predicted by CLOUDY for AGN photoionization.
• Figure 4: Star formation rates as a function of quasar bolometric luminosity. Star formation rates derived from host [O II] luminosity are shown without dust correction (blue circles), and with constant dust corrections of $A_V = 1.0$ (dashed line), $1.3$ (dash-dotted line), and $2.2$ (blue dotted line) mag, as well as with a bolometric-luminosity-dependent dust correction based on the $M_{\rm BH}$–$M_\star$ relation (purple dotted line). Horizontal error bars indicate the bin width. For comparison, independent SFR estimates for quasar hosts from far-infrared measurements serjeant2009, radio continuum measurements macfarlane2021, and CO ($J=5$–$4$) measurements for three luminous quasars at $z = 2$silverman2026 are also shown.