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The paper shows that three z>4 quasars previously classified as compact radio sources actually possess extended kpc-scale radio structures, with two resembling bent FR II galaxies and one resembling a compact steep-spectrum source.

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2026-08-01 02:35 UTC pith:3Z4LLBUG

load-bearing objection Worth refereeing: three z>4 quasars with new kpc-scale radio morphologies, but J1548+3335's CSS classification depends on excluding a 62-kpc SW lobe without independent evidence. the 1 major comments →

arxiv 2607.25378 v1 pith:3Z4LLBUG submitted 2026-07-28 astro-ph.GA

From compact jets to extended lobes: radio morphologies of distant quasars at z > 4

K. \'E. Gab\'anyi , M. Kunert-Bajraszewska , A. Krauze , S. Frey , M. Krezinger , Z. Paragi , H. Cao , T. An
show 4 more authors
L. I. Gurvits K. Perger T. Sbarrato K. Rozgonyi
This is my paper
classification astro-ph.GA
keywords high-redshift quasarsradio jetsFanaroff-Riley type IIcompact steep-spectrum sourcesvery long baseline interferometrylow-frequency radio imagingradio-loud AGNgalaxy evolution
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper combines low-frequency, arcsecond-resolution imaging with mas-resolution very long baseline interferometry (VLBI) to reveal the true radio morphologies of three z>4 radio quasars. It argues that two of them, J0813+3508 and J1231+3816, are not compact core-dominated objects but bent, Fanaroff–Riley II-like systems with kpc-scale lobes and hotspots, with projected sizes of roughly 75 and 85 kpc. The third, J1548+3335, shows a one-sided core–jet structure of about 6.7 kpc (or up to ~13 kpc if a counter-jet of similar extent is assumed), with a flat-spectrum core and a steep-spectrum hotspot. The paper also identifies the previously detected mas-scale VLBI features as either AGN cores, based on positional coincidence with optical positions, or, in one case, a hotspot within a lobe. If these interpretations are correct, mature, extended radio structures were already present within the first ~1.6 Gyr of cosmic history, and VLBI-only views systematically underestimate the sizes of such quasars.

Core claim

The central discovery is that the three quasars J0813+3508, J1231+3816, and J1548+3335, previously detected only as compact mas-scale radio sources, each have significant extended radio emission when observed at ~144 MHz with sub-arcsecond resolution. J0813+3508 and J1231+3816 show multiple steep-spectrum components (lobes/hotspots) arranged in bent C- and U-shaped morphologies, with projected linear sizes of roughly 75 and 85 kpc; these resemble FR II radio galaxies distorted by environmental interactions. J1548+3335 shows a one-sided jet-like structure of ~6.7 kpc (up to ~13 kpc if a symmetric counter-jet is assumed), with the flat-spectrum component at the AGN core and a steep-spectrum ho

What carries the argument

The argument is carried by multi-frequency, multi-resolution radio imaging: merging international low-frequency (144 MHz) observations with intermediate-resolution GHz imaging and re-imaged archival VLBI data. The method works because extended steep-spectrum emission is invisible to high-resolution GHz VLBI but appears at low frequency; matching the positions and spectral indices of components across resolutions lets the authors assign each compact VLBI feature to either the AGN core or a lobe hotspot. Spectral-index differences (flatter core, steeper lobes) and the radio power–linear size diagram are the diagnostic tools that ground the morphological classification.

Load-bearing premise

The classification rests on assuming that the extended low-frequency radio components truly belong to the target quasars and, for J1548+3335, that a jet of similar size exists on the opposite side of the nucleus; the paper itself cautions that the ~62-kpc southwestern component may be an unrelated foreground galaxy.

What would settle it

A spectroscopy or photometric redshift of the southwestern radio component of J1548+3335 would settle the matter: a foreground galaxy at z≈0.31 falsifies the 62-kpc association, while a matching redshift confirms the large size. Similarly, an image of J1548+3335 deep enough to detect a counter-jet at <0.1 mJy beam^-1 would test the one-sided-jet interpretation, and arcsecond-scale imaging at 144 MHz that resolves the putative lobes of the other two quasars into multiple background sources would falsify their FR II classification.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • z>4 radio quasars can be FR II-like systems with tens-of-kpc lobes, so compactness inferred from VLBI alone is not a reliable guide to their physical size.
  • A VLBI-detected compact feature does not always mark the AGN core; it can be a hotspot in a lobe, so core identification needs positional or variability cross-checks.
  • One of the targets falls in the CSS region of the power–size diagram, showing that compact steep-spectrum sources already existed at z>4 (within the first ~1.3 Gyr).
  • Low-frequency high-resolution observations are essential to recover rest-frame ~0.8–0.9 GHz emission that is resolved out by GHz VLBI; without them the source sizes and classifications are underestimated.
  • The bent morphologies and arm-length asymmetries imply strong environmental influence or jet reorientation, linking these quasars to forming clusters or proto-clusters.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the ~62-kpc southwestern LOFAR component of J1548+3335 is eventually proven to belong to the quasar rather than a foreground galaxy, this would move the source out of the CSS class and make it comparable to the largest known z>4 jet, so the size classification is an immediate testable extension.
  • The study suggests that censuses of z>4 radio quasars built from VLBI catalogues systematically underestimate extended flux; cross-matching low-frequency surveys with VLBI samples should reveal more hidden FR II lobes.
  • The single-sided jet in J1548+3335 with a jet-to-counter-jet ratio >348 and no Doppler-boosted core could be explained by an asymmetric environment rather than orientation; future high-resolution imaging of the counter-side can distinguish these models.
  • If these objects are genuinely in the FR II/CSS classes, their jet kinetic powers (~2–4×10^38 W) imply jet-production efficiencies of a few percent, which can be compared with black-hole growth models at z>4.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

1 major / 5 minor

Summary. This paper presents multi-frequency, multi-resolution radio observations (international LOFAR at 144 MHz, e-MERLIN at 1.5/5 GHz, archival EVN, and survey data) of three z>4 radio-loud quasars with previously known mas-scale VLBI features. The authors report that J0813+3508 and J1231+3816 possess bent, kpc-scale morphologies with multiple components and hotspots, interpreted as FR II-like radio galaxies with extreme jet bending; J1548+3335 shows a one-sided core-jet structure of ~6.7 kpc, with an assumed counter-jet giving a largest linear size of ~13 kpc, placing it among CSS sources in the P-D diagram. They identify the previously detected VLBI components as AGN cores or, in one case, a hotspot embedded in an extended jet. The paper discusses radio powers, jet kinetic powers, Eddington ratios, X-ray properties, and environmental interactions.

Significance. If the morphological identifications hold, the paper provides rare evidence that some z>4 quasars are not merely compact VLBI cores but host kpc-scale jets/lobes detectable at low frequency, bridging mas-scale and arcsecond-scale radio structure. The multi-frequency, multi-resolution dataset is a clear strength: calibration steps are transparent, noise levels are stated, and component fitting is documented. The two bent FR II-like sources are convincing and constitute an important result. The third object's CSS classification is, however, conditional on the non-association of a 61.6-kpc southwestern LoTSS component, as detailed below. The use of external P-D and jet-power relations is appropriate and not circular.

major comments (1)
  1. [Sect. 5.3, 5.4, Conclusions] The CSS classification of J1548+3335 and its P-D placement (Fig. 8) are load-bearing for the abstract's characterization of this source and for the Sect. 6 conclusion that a CSS quasar exists at z>4. This classification is reached only after excluding the LoTSS-DR2 component SW at 9.18 arcsec (~61.6 kpc, PA≈−114°, aligned with the inner jet). The paper states that the association "cannot be firmly established" but presents no independent evidence against it: no optical/IR counterpart, no photometric redshift, no spectral index, and the non-detections in e-MERLIN/VLASS/high-resolution LOFAR are attributed to the component being resolved out. If SW is a lobe of J1548, the largest projected size becomes ~62 kpc and the source would move from the CSS locus to the FRII-like region, invalidating one of the three main results. The abstract and Sect. 6 should be qualified, or the authors should
minor comments (5)
  1. [Sect. 5.2 vs. 5.4] The largest linear size of J0813+3508 is quoted as ~75 kpc in Sect. 5.2 but ~70 kpc in Sect. 5.4. Please harmonize the value and define how it is measured.
  2. [Sect. 5.4] Typo: "J1231+3806" should be "J1231+3816".
  3. [Sect. 4.3.2 / 5.3] The spectral index used in the jet-to-counter-jet ratio (Eq. 2) is α_W = −0.61, from a fit that includes LOFAR data, whereas the e-MERLIN-only fit gives α_W = −1.13±0.03. The choice should be justified because it changes the derived βcosθ lower limit and the inferred inclination range.
  4. [Sect. 4.2.2 / Fig. D.2] The integrated spectral fit for J0813+3508 includes both TGSS and LoTSS points at nearly the same frequency that differ by a factor of ~2. Given the paper's own discussion of variability, the fit should either exclude one of these points or explicitly account for the systematic uncertainty this introduces.
  5. [Sect. 5.3] The orientation/inclination estimate uses the assumption of a symmetric external medium, which the paper itself questions because of the distorted shape of component W. The quoted 30°–35° inclination and the deprojected arm-length should be presented as illustrative rather than as secure measurements.

Circularity Check

0 steps flagged

No significant circularity: the paper is an observational morphology study whose derived sizes, powers, and classifications rest on direct interferometric data and external literature relations, not on fitted targets or self-citation chains.

full rationale

All load-bearing derivations are independent measurements or external relations. The LOFAR/e-MERLIN/EVN images are direct interferometric data; component flux densities and spectral indices are fitted to visibilities/images, not to the target conclusions. Size estimates use measured angular separations and an adopted cosmology. The 13 kpc size for J1548 assumes a counter-jet of similar extent (Sect. 5.3) - an explicit, testable assumption, not a quantity fitted to make the CSS classification true; the paper also reports the unassociated SW LoTSS component and gives its 61.6 kpc alternative. Jet power, P-D placement, core dominance, and jet orientation use published formulas (Rusinek et al. 2017; Scheuer & Readhead 1979; Wright 2006) and literature comparisons; no equation reduces to a fitted parameter. Prior VLBI data are cited as independent observations; self-citations (Krezinger et al. 2022; Kunert-Bajraszewska et al. 2025) provide source selection and a comparison diagram, but the central morphological claims are not derived from them by construction. The paper explicitly flags its own limitations, including the unproven SW association and the counter-jet assumption, which are caveats about association and projection, not circular reasoning. No self-definitional, fitted-prediction, or self-citation-chain circularity is present.

Axiom & Free-Parameter Ledger

2 free parameters · 5 axioms · 0 invented entities

The central claims rest on standard radio-astronomy assumptions: the cosmology, power-law spectra, positional association of VLBI features with the AGN, and two explicit assumptions about J1548+3335 (comparable counter-jet, symmetric environment) that the authors flag. Spectral indices are measured from data rather than imposed. No new physical entities are introduced.

free parameters (2)
  • Component spectral indices (power-law slopes) = multiple; e.g. J1231 C/S/W = -0.58/-1.12/-0.93
    Fitted to multi-frequency flux densities; used to classify components as flat-spectrum cores or steep-spectrum lobes. These are measured results, not ad hoc parameters.
  • Jet-power normalization f = 10
    Adopted in Eq. (4) for FR II lobes following Rusinek et al. (2017); sets the Ljet scale in Table 2 but does not affect the morphological classification.
axioms (5)
  • domain assumption ΛCDM cosmology with H0=70 km/s/Mpc, Ωm=0.27, ΩΛ=0.73
    Adopted at the start of Sect. 1; all angular-scale and luminosity-distance conversions depend on it.
  • domain assumption Power-law synchrotron spectra with S∝ν^α and extrapolation across frequency decades
    Used throughout Sects. 4 and 5; the 74-MHz turnover in J1548 shows curvature exists, so two-point spectral indices should be read as approximations.
  • domain assumption VLBI/Gaia positional coincidence identifies the radio core with the optical nucleus in J0813+3508
    Sect. 5.2: 'The 5-GHz peak position ... is consistent, within errors, with the Gaia DR3 optical position.' Astrometric errors or source structure could misplace the centroid.
  • ad hoc to paper J1548+3335 has a counter-jet of similar extent to the detected jet
    Sect. 5.3: 'Assuming a counter-jet of similar extent to the detected jet, we estimate a largest linear size of ~13 kpc.' Without this, the CSS classification is insecure.
  • ad hoc to paper External medium is similar on both sides of J1548+3335 in the jet-to-counter-jet analysis
    Sect. 5.3 after Eq. (2); the authors note the complex shape of component W 'questions the initial assumption.'

pith-pipeline@v1.3.0-alltime-deepseek · 28093 in / 13470 out tokens · 145673 ms · 2026-08-01T02:35:48.665724+00:00 · methodology

0 comments
read the original abstract

High-redshift quasars play an essential role in studying the growth and evolution of supermassive black holes and active galactic nuclei (AGN). Radio-loud quasars additionally enable us to investigate the interactions between the jets and their environment. We aimed to reveal the radio morphology of three radio quasars at redshifts z>4 that contain milliarcsecond (mas) scale compact radio features according to previous very long baseline interferometry (VLBI) observations, but show significant flux density at arcsecond scales, indicating the presence of extended radio structure that cannot be sampled by the highest-resolution observations. We analysed radio interferometric data obtained at various angular resolutions and multiple frequencies, including observations made by the international Low-Frequency Array and the enhanced Multi-Element Remotely-Linked Interferometer Network. We also re-imaged archival European VLBI Network observations of our targets. Two quasars (J0813+3508 and J1231+3816) exhibit complex radio structures with hotspots, extended to tens of kpc. They resemble Fanaroff--Riley II-type radio galaxies with extremely bent jet morphology. The third object (J1548+3335) shows a one-sided structure of $\lesssim 10$ kpc size. There was no sign of relativistic boosting at its previous mas-scale resolution radio observations. Its radio power and (inferred) linear size derived from the lower-resolution observations are similar to the known high-power compact steep-spectrum sources. We found that the previously detected mas-scale compact radio features are related to the centres of the AGN or in one case to one of the hotspots in an extended lobe.

Figures

Figures reproduced from arXiv: 2607.25378 by A. Krauze, H. Cao, K. \'E. Gab\'anyi, K. Perger, K. Rozgonyi, L. I. Gurvits, M. Krezinger, M. Kunert-Bajraszewska, S. Frey, T. An, T. Sbarrato, Z. Paragi.

Figure 1
Figure 1. Figure 1: Radio images of J1231+3816. White contours show the high-resolution LOFAR image, with contours starting at ±0.3 mJy beam−1 (±5σ of the off-source image noise) and increasing by a factor of 2. The peak intensity is 28.6 mJy beam−1 , and the restoring beam (white hatched ellipse shown in the lower right corner of the left panel) is 367 mas×242 mas with PA = −1 ◦ . Left panel: The colour scale and cyan contou… view at source ↗
Figure 2
Figure 2. Figure 2: Radio images of J0813+3508. Top: White contours show the high-resolution LOFAR image, with the lowest levels at ±0.8 mJy beam−1 (corresponding to ±4σ off-source noise level); higher contours increase by a factor of 2. The peak brightness is 42.7 mJy beam−1 . The restoring beam, shown as a hatched white ellipse in the lower right corner, is 368 mas × 272 mas with PA = 130◦ . The third-epoch VLASS image is d… view at source ↗
Figure 4
Figure 4. Figure 4: High-resolution LOFAR (colour scale) and 5-GHz e [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The low-resolution LoTSS-DR2 image (white con [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: The low-resolution LoTSS-DR2 image (white con [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: The radio power vs. linear size (P–D) diagram reproduced from Kunert-Bajraszewska et al. (2025), including various types of [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗

discussion (0)

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