[Paper Review] The dependence of the fraction of radio luminous quasars on redshift and its theoretical implications

Katarzyna Rusinek, M. Sikora|arXiv (Cornell University)|Sep 15, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy136 references4 citations

TL;DR

This study investigates the redshift dependence of the radio-loud quasar fraction (RLF), correcting for biases in black hole mass and Eddington ratio. Using homogeneous quasar samples in two redshift bins, it finds that the RLF increases with cosmic time, primarily due to the slower decline of mixed mergers (elliptical + spiral) compared to pure spiral mergers, supporting the magnetically arrested disk (MAD) model for jet formation via the cosmic battery mechanism.

ABSTRACT

While radio emission in quasars can be contributed to by a variety of processes (involving star forming regions, accretion disk coronas and winds, and jets), the powering of the radio loudest quasars must involve very strong jets, presumably launched by the Blandford-Znajek mechanism incorporating the magnetically arrested disk (MAD) scenario. We focus on the latter and investigate the dependence of their fraction on redshift. We also examine the dependence of the radio-loud fraction (RLF) on BH mass ($M_{ m BH}$) and Eddington ratio ($\lambda_{ m Edd}$) while excluding the redshift bias by narrowing its range. In both these investigations we remove the bias associated with: (1) the diversity of source selection by constructing two well-defined, homogeneous samples of quasars (first within $0.7 \leq z < 1.9$, second within $0.5 \leq z < 0.7$); (2) a strong drop in the RLF of quasars at smaller BH masses by choosing those with BH masses larger than $10^{8.5} M_{\odot}$. We confirm some previous results showing the increase in the fraction of radio-loud quasars with cosmic time and that this trend can be even steeper if we account for the bias introduced by the dependence of the RLF on BH mass whereas the bias introduced by the dependence of the RLF on Eddington ratio is shown to be negligible. Assuming that quasar activities are triggered by galaxy mergers we argue that such an increase can result from the slower drop with cosmic time of mixed mergers than of wet mergers.

Motivation & Objective

• To investigate the redshift dependence of the radio-loud fraction (RLF) of quasars while correcting for selection biases.
• To assess the influence of black hole mass (MBH) and Eddington ratio (λEdd) on the RLF, isolating their effects from redshift bias.
• To test whether the observed increase in RLF with cosmic time can be explained by the merger-driven formation of magnetically arrested disks (MADs).
• To evaluate the role of galaxy merger types—specifically mixed (gE + Sp) vs. pure spiral (Sp + Sp)—in shaping the RLF evolution.
• To explore theoretical implications for MAD formation mechanisms, particularly the 'cosmic battery' scenario, in explaining radio loudness in quasars.

Proposed method

• Constructed two homogeneous, well-defined quasar samples: one in 0.7 ≤ z < 1.9 and another in 0.5 ≤ z < 0.7, minimizing selection function biases.
• Selected only quasars with MBH > 10^8.5 M⊙ to eliminate the strong bias from low-mass black holes on the RLF.
• Used the Kaplan-Meier estimator (via the Python package lifelines) to compute survival functions and RLF evolution with redshift.
• Applied spectral index corrections (⟨αr⟩ = 0.7, ⟨αo⟩ = 0.5) to convert radio and optical luminosities to the standard Kellermann radio loudness parameter RK.
• Modelled the RLF evolution under the assumption that quasar activity is triggered by galaxy mergers, comparing wet (spiral-spire) and mixed (gE + Sp) merger rates.
• Evaluated theoretical scenarios for MAD formation, including the 'cosmic battery' and in-situ formation, to explain the observed RLF trends.

Experimental results

Research questions

• RQ1 How does the radio-loud fraction (RLF) of quasars evolve with redshift when corrected for biases in black hole mass and Eddington ratio?
• RQ2 What is the relative contribution of black hole mass and Eddington ratio to the observed RLF evolution?
• RQ3 Can the increasing RLF with cosmic time be explained by the differing merger rate evolution of mixed (elliptical + spiral) versus pure spiral mergers?
• RQ4 Which MAD formation mechanism—'cosmic battery', advection of poloidal fields, or survival from low-accretion phases—is most consistent with the observed RLF trends?
• RQ5 How do the host galaxy properties (e.g., giant ellipticals) and environment (dark matter halo mass) correlate with the RLF, and what do they imply for jet formation?

Key findings

• The fraction of radio-loud quasars increases with cosmic time, and this trend is steeper than previously reported when accounting for the bias introduced by black hole mass.
• The dependence of the RLF on Eddington ratio was found to be negligible, indicating that accretion rate alone does not drive radio loudness.
• The observed increase in RLF with cosmic time is best explained by the slower decline of mixed mergers (giant elliptical + spiral) compared to pure spiral mergers.
• The majority of radio-loud quasars are hosted by giant elliptical galaxies, and they reside in denser environments with more massive dark matter halos than radio-quiet quasars.
• The results support the 'cosmic battery' scenario for MAD formation, particularly in systems where the black hole is in a corotating or counterrotating configuration post-merger.
• The RLF in the 0.5 ≤ z < 0.7 sample is approximately 0.59 × (Lν1.4 / Lνi), and in the 0.7 ≤ z < 1.9 sample it is about 0.12 × (Lν144 / Lνi), after spectral index corrections.