[Paper Review] Into the central 10 pc of the most distant known radio quasar. VLBI imaging observations of J1429+5447 at z=6.21

S. Frey, Z. Paragi+3 more|arXiv (Cornell University)|Jun 3, 2011

Galaxies: Formation, Evolution, Phenomena23 references17 citations

TL;DR

This study presents the first VLBI imaging of the most distant known radio quasar, J1429+5447 at z=6.21, revealing a compact, steep-spectrum radio structure on scales <100 pc. The low brightness temperature (≈10⁹ K) and steep spectrum (α = −1.0) indicate no significant relativistic beaming, suggesting these sources are young, compact symmetric objects rather than Doppler-boosted jets.

ABSTRACT

Context: There are about 60 quasars known at redshifts z&gt;5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs, and on the physical conditions in their environment. Aims: We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale, in order to compare the structural and spectral properties with those of other two z~6 radio-loud quasars, J0836+0054 (z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results: Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of &lt;100 pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a "universal" feature of the most distant radio quasars, it is essential to study more, yet to be discovered radio-loud active galactic nuclei at z&gt;6.

Motivation & Objective

• To investigate the compact radio structure of J1429+5447 at z=6.21 using high-resolution VLBI imaging.
• To compare the structural and spectral properties of this highest-redshift radio quasar with two other z≈6 radio-loud quasars (J0836+0054 and J1427+3312).
• To determine whether compact steep-spectrum radio emission is a universal feature in the most distant radio quasars.
• To assess the role of relativistic beaming in shaping the observed radio morphology of high-redshift quasars.
• To constrain the physical conditions and evolutionary state of the central engine and jet in early-universe AGNs.

Proposed method

• Performed Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 GHz and 5 GHz frequencies.
• Used Difmap software to model visibility data with circular Gaussian components to derive flux density, position, size, and brightness temperature.
• Calculated the spectral index α from flux densities at 1.6 GHz and 5 GHz using the power-law definition S ∝ ν^α.
• Estimated brightness temperature T_B from the fitted component size and flux density, assuming a circular Gaussian model.
• Computed the Doppler factor δ = T_B / T_B,equilibrium ≈ 0.02 using the equipartition brightness temperature T_B,equilibrium ≈ 5×10¹⁰ K.
• Applied cosmological distance and size scaling using a flat ΛCDM model with H₀=70 km s⁻¹ Mpc⁻¹, Ω_m=0.3, Ω_Λ=0.7.

Experimental results

Research questions

• RQ1 What is the milli-arcsecond-scale radio structure of the most distant known radio quasar J1429+5447 at z=6.21?
• RQ2 How do the spectral and structural properties of J1429+5447 compare with those of other z≈6 radio-loud quasars (J0836+0054 and J1427+3312)?
• RQ3 Is the compact steep-spectrum radio emission in high-redshift quasars indicative of relativistic beaming or youth of the source?
• RQ4 What is the inferred brightness temperature of the radio core, and what does it imply about the jet orientation and Lorentz factor?
• RQ5 Is compact steep-spectrum emission a common feature among the most distant radio-loud AGNs, suggesting a universal evolutionary phase?

Key findings

• The quasar J1429+5447 exhibits a compact but slightly resolved radio structure on scales <100 pc, with a total extent of less than 100 pc in the rest frame.
• The two-point spectral index between 1.6 GHz and 5 GHz is α = −1.0 ± 0.05, indicating a steep radio spectrum consistent with a compact steep-spectrum (CSS) source.
• The brightness temperature of the compact component is ≈1.3 × 10⁹ K, significantly lower than the equipartition value of ≈5 × 10¹⁰ K, indicating no strong relativistic beaming.
• The derived Doppler factor δ ≈ 0.02 implies a minimum Lorentz factor Γ ≳ 50, which is inconsistent with a face-on jet geometry and suggests a large viewing angle.
• The radio morphology and spectral properties are remarkably similar to those of the other two z≈6 radio quasars, J0836+0054 and J1427+3312, suggesting a common evolutionary state.
• The lack of Doppler-boosted emission across all three z≈6 quasars studied suggests that compact steep-spectrum radio emission may be a universal feature of young, compact symmetric objects at high redshift.