Exploring the quasar disc-wind-jet connection with LoTSS and SDSS

1. Problem Statement

The paper addresses fundamental questions regarding the physics of active galactic nuclei (AGN), specifically quasars:

• The Jet-Wind Dichotomy: What determines whether a quasar launches a relativistic radio jet, a wide-angle accretion disc wind, or both?
• The Connection to Accretion: How do jet and wind properties correlate with fundamental quasar parameters, specifically black hole mass ($M_{BH}$) and the Eddington-scaled accretion rate ($\lambda_{Edd}$)?
• Observational Gaps: Previous studies have suggested correlations between radio loudness and emission line properties (e.g., C iv blueshift), but it remains unclear if these are intrinsic to the outflow mechanisms or artifacts of underlying distributions in mass and luminosity.

2. Methodology

Data Sources:

• Optical/UV: A sample of $\sim100,000$ quasars at redshift $1.5 < z < 2.65$ from the Sloan Digital Sky Survey (SDSS) Data Release 17 (DR17). Spectral properties (C iv, He ii, Mg ii lines) and black hole masses were derived using the reconstruction methods of Temple et al. (2023) and Rankine et al. (2020).
• Radio: Radio flux densities at 144 MHz from the LOFAR Two-metre Sky Survey (LoTSS) Data Release 2 (DR2).
• Sample Size: After cross-matching and quality cuts, the final sample contains $\sim108,000$ quasars, with $\sim15,000$ radio-detected sources.

Radio Loudness Classification:
To avoid the pitfalls of arbitrary cuts, the authors employed three methods to define "Radio Loud" (RL) and "Radio Quiet" (RQ) populations:

1. Classical Radio Loudness ($R$): Based on the ratio of radio to optical luminosity ($\log_{10} R > 2.5$).
2. Radio Luminosity Cut: A hard threshold of $L_{144} > 10^{26} \text{ W Hz}^{-1}$ to exclude star-formation-dominated sources.
3. Gaussian Mixture Model (GMM): A statistical two-component model fitted to the distribution of sources in the $L_{3000}$ (optical) vs. $L_{144}$ (radio) plane. This is the primary method used, as it is data-driven and provides probability weights for classification rather than binary cuts.

Population Matching:
To isolate the effects of jet/wind physics from fundamental parameters, the authors performed a 2D K-nearest neighbors matching of RL and RQ populations in the space of:

• $L_{3000}$ (rest-frame monochromatic luminosity at 3000 Å).
• $\text{FWHM}_{MgII}$ (Full Width at Half Maximum of the Mg ii line).
  This matching ensures that comparisons between RL and RQ sources are not biased by differences in black hole mass or Eddington fraction.

Tracers:

• Disc Winds: Inferred via the C iv $\lambda1549$ blueshift (velocity offset) and the Equivalent Width (EW) of C iv.
• Ionizing Flux/Spectral Energy Distribution (SED): Traced by the EW of He ii $\lambda1640$. A higher EW He ii indicates a "harder" ionizing spectrum (more photons $>54.4$ eV), while a lower EW suggests a "softer" SED conducive to line-driven winds.

3. Key Contributions

1. Statistical Rigor in Classification: The paper demonstrates that a Gaussian Mixture Model provides a more robust and unbiased classification of radio-loudness compared to traditional fixed cuts, particularly in handling the continuous nature of the radio-optical luminosity distribution.
2. Decoupling Mass/Accretion from Outflows: By rigorously matching populations in $M_{BH}$ and $\lambda_{Edd}$, the study proves that differences in emission line properties between RL and RQ quasars are intrinsic to the presence of jets, not merely secondary effects of mass or accretion rate distributions.
3. The "Missing Wedge" Phenomenon: The study identifies a specific region in parameter space (high $M_{BH}$ and high $\lambda_{Edd}$) where jet formation efficiency appears suppressed, a feature that depends on the classification method used.

4. Key Results

A. Jet Production Efficiency (RL Fraction):

• The fraction of radio-loud sources follows a non-linear "U-shape" with respect to the Eddington fraction ($\lambda_{Edd}$).
• Minimum: Jet production is least efficient at $\lambda_{Edd} \approx 0.1$.
• Maxima: Efficient jet launching occurs in two regimes:
  1. High Mass: $M_{BH} > 10^9 M_\odot$.
  2. High Accretion: $\lambda_{Edd} > 0.3$.
• Suppression: In the extreme regime where both $M_{BH} > 10^9 M_\odot$ and $\lambda_{Edd} > 0.3$, the RL fraction drops (depending on the classification method), suggesting a suppression of jet formation in the most massive, rapidly accreting systems.

B. Disc Wind vs. Jet Anti-Correlation:

• C iv Blueshift: Even after matching for mass and accretion, RQ quasars exhibit significantly larger C iv blueshifts (stronger disc winds) than RL quasars.
• The "Wedge": The region of highest C iv blueshift (strongest winds) is populated almost exclusively by RQ quasars. RL quasars are notably absent from this high-blueshift, high-mass/high-accretion region.
• Interpretation: There is a strong anti-correlation between the presence of powerful disc winds and the efficiency of jet launching.

C. Spectral Energy Distribution (SED) Differences:

• He ii Emission: RL quasars show significantly stronger He ii $\lambda1640$ emission (higher EW) compared to matched RQ quasars.
• SED Hardness: Stronger He ii implies a "harder" ionizing SED (more EUV/X-ray photons).
• Mechanism: The "harder" SED in RL sources likely over-ionizes the gas, preventing the line-driving mechanism required for strong disc winds. Conversely, RQ sources with "softer" SEDs allow for the formation of strong disc winds.
• Baldwin Effect: The study finds a different Baldwin effect (EW vs. Luminosity) for RL and RQ populations, further confirming distinct SED shapes.

5. Significance and Physical Implications

• Jet Launching Mechanisms: The results support a scenario where jet launching is efficient either via high black hole mass (potentially linked to the Blandford-Znajek mechanism requiring high spin) or high accretion rates (potentially linked to the Blandford-Payne mechanism or high magnetic flux accumulation).
• Wind-Jet Competition: The findings suggest a physical competition between winds and jets. Strong disc winds (driven by line forces in soft SEDs) may quench jet formation, or conversely, the presence of a jet (associated with a hard SED) may suppress the formation of strong disc winds.
• Black Hole Spin: The "harder" SED of RL sources is consistent with the spin paradigm, where rapidly spinning black holes allow the inner disc to extend closer to the event horizon, increasing EUV flux. However, the low overall RL fraction ($\sim10\%$) even in high-mass/high-accretion regimes suggests that spin alone is not the sole determinant; magnetic field configuration or other factors likely play a role.
• Unified View: The paper provides a unified framework linking accretion physics, black hole mass, and outflow modes, suggesting that the "radio loudness" of a quasar is a complex function of its ability to launch jets versus winds, mediated by its spectral energy distribution.

In conclusion, this study utilizes the largest combined optical-radio quasar sample to date to demonstrate that radio loudness is not just a function of mass or accretion rate, but is intrinsically linked to the spectral shape of the quasar, which dictates the coexistence (or mutual exclusion) of relativistic jets and disc winds.