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REVIEW 3 major objections 5 minor 1 cited by

Jet formation studies in AGN: a search for new targets

T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read Sixteen nearby radio galaxies are imaged on jet-launching scales (below $10^3$ Schwarzschild radii), more than doubling the known census.

desk verdict A solid, genuinely useful HSA survey of 16 radio galaxies at 1 cm and 7 mm, whose headline 'more than doubled' census claim is softer than it looks but whose target list and data are the real value. read the letter →

arxiv 2412.19268 v1 pith:FYSFUBNZ submitted 2024-12-26 astro-ph.HE

B. Boccardi , L. Ricci , E. Madika , V. Bartolini , U. Bach , P. Grandi , E. Torresi , T.P. Krichbaum
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J.A. Zensus
This is my paper · ORCID
classification astro-ph.HE
keywords AGNjetsverylongbaselineinterferometryradiogalaxiesjetformationSchwarzschildradiilow-excitationhigh-excitationbrightnesstemperature
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Jet formation in active galaxies has so far been imaged in only a handful of nearby objects, with M87 the outstanding case. This paper argues that the candidate pool can be substantially enlarged: it presents 1 cm and 7 mm VLBI images of sixteen faint radio galaxies chosen so that a 0.1 mas beam resolves the jet base on scales below $10^3$\text{--}$10^4$ Schwarzschild radii, and it detects every one. The central claim is that the number of sources imaged at $\lesssim10^3\,R_{\rm S}$ is more than doubled, with 3C 31, 3C 66B, and 3C 465 as the most promising low-excitation targets and 3C 452 as the most interesting high-excitation one. A reader should care because these targets connect jet-launching studies to different accretion modes and jet powers, and they are stepping stones for current and next-generation millimetre-VLBI arrays.

What carries the argument

The load-bearing conversion is angular resolution into Schwarzschild radii: with $z<0.1$ and $\log M_{\rm BH}\gtrsim8.5$, a 0.1 mas beam corresponds to roughly 85\text{--}2000 $R_{\rm S}$, placing the jet acceleration and collimation zone within reach. The High Sensitivity Array, combining the phased VLA, Effelsberg, and the VLBA, supplies the sensitivity needed to image these faint jets at 1 cm and 7 mm. Gaussian modelfitting with a signal-to-noise-based error and resolution-limit formalism then yields core brightness temperatures and jet-to-counter-jet ratios, from which the paper estimates intrinsic speeds and viewing angles.

What would settle it

Take the Table 1 black hole masses, divide each by two, and recompute the 7 mm resolutions in Schwarzschild radii; if that moves 3C 465 or any other ranked target above the 500 $R_{\rm S}$ threshold, the paper's headline comparison and target list would need revision.

Watch

Extended reading notes

Core claim

The paper reports dual-frequency High Sensitivity Array observations of sixteen radio galaxies at $z<0.1$ with log black hole masses above 8.5, designed so that a 0.1 mas beam reaches scales below $10^3$\text{--}$10^4$ $R_{\rm S}$. All sixteen were detected at 1 cm and 7 mm; eight show two-sided jet structures; several low-excitation jets show hints of limb-brightening; and core brightness temperatures sit below the roughly $5\times10^{10}$ K equipartition value, which the authors read as a jet base that is still magnetically dominated or Doppler de-boosted. The central result is that the number of sources imaged on scales $\lesssim10^3\,R_{\rm S}$ is more than doubled, with the best new LEG targets being 3C 31, 3C 66B, and 3C 465, each combining more than 50 mJy at 7 mm with resolution below 500 $R_{\rm S}$, while 3C 452 stands out among HEG for its highly symmetric, two-sided jet base.

Load-bearing premise

The claimed physical resolutions depend on the literature black hole masses; if those masses are systematically too high by even a factor of two, some sources drop out of the sub-$10^3\,R_{\rm S}$ group and the ranking of best targets weakens.

Editorial extensions

If this is right

  • 3C 31, 3C 66B, and 3C 465 become primary follow-up targets for millimetre-VLBI campaigns, since they combine flux density above 50 mJy with resolution below 500 $R_{\rm S}$ at 7 mm.
  • 3C 452 gives the high-excitation galaxy class a viable target for studying jet launching in a powerful FR II jet, complementing the LEG-dominated census.
  • The generally sub-equipartition core brightness temperatures imply that the jet base is still far from equipartition on these scales or is Doppler de-boosted, both of which bear on jet-formation models.
  • The eight two-sided jets provide jet-to-counter-jet ratios that can be combined with future spectral and kinematic data to map acceleration along the jet.
  • The expanded sample allows population studies comparing LEG and HEG jets over a broad range of radio power, rather than relying on M87 alone.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Extension: if a realistic factor-of-two uncertainty in the adopted black hole masses were propagated, some sources currently ranked below 500 $R_{\rm S}$ could move above that threshold; re-releasing Figure 2 with mass-uncertainty bands would settle how robust the target ranking is.
  • Extension: the limb-brightening hints seen in several LEG jets, if confirmed at higher resolution, would strengthen the spine-sheath picture of low-power jets and give future transverse-resolved images a concrete prediction to test.
  • Extension: because all sixteen sources were detected at two frequencies, a natural next step is spectral-index mapping between 1 cm and 7 mm to locate optically thick cores and core shifts, which could directly probe the acceleration region.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 5 minor

Summary. This paper presents 1 cm and 7 mm HSA VLBI observations of 16 nearby radio galaxies selected as potential targets for jet-formation studies. All 16 sources are detected at both frequencies, and the authors provide clean maps, modelfit components with errors following Lee et al. (2008), core brightness temperatures, and jet-to-counter-jet ratios for eight two-sided sources. On this basis they identify 3C 31, 3C 66B, and 3C 465 as the most promising LEG targets for high-resolution follow-up, and 3C 452 as an interesting HEG target. The paper's headline quantitative claim is that the number of sources imaged on scales ≲10^3 R_S is more than doubled by this study.

Significance. If accepted, the paper provides a useful, observationally validated target list for current and next-generation mm-VLBI facilities, and it roughly doubles the small census of jets imaged near the acceleration and collimation region. The observational material is substantial: all targets are detected at both frequencies, the images and modelfit tables are included in the appendices, and error estimates for component parameters are provided. The target-ranking logic is transparent and reproducible from the published tables. The main risk is not the data quality or the individual source notes but the robustness of the census-level 'more than doubled' claim and of the '<500 R_S' ranking for marginal sources, which depend on the assumed angular resolution and on the heterogeneously measured black hole masses.

major comments (3)
  1. [Sec. 4.1 / Fig. 2 and abstract] The quantitative claim that the number of sources imaged on scales ≲10^3 R_S is 'more than doubled' is not robust to the resolutions and masses that the paper itself provides. Fig. 2 uses an idealized 0.1 mas resolution, whereas the actual 7 mm clean-beam minor axes in Table 2 are 0.12–0.16 mas. Repeating the count with those measured beams moves 3C 264 (0.14 mas → ~1340 R_S), 3C 338 (0.13 mas → ~1050 R_S), and 4C 30.31 (0.12 mas → ~1000 R_S) above the 10^3 R_S threshold, reducing the genuinely new sources from eight to five and the after/before ratio from roughly 15/7 ≈ 2.1 to 12/7 ≈ 1.7. The factor-of-two black-hole mass uncertainty acknowledged in Sec. 4.1 (Barth et al. 2016; GRAVITY Collaboration et al. 2018) acts in the same direction, yet only a factor-ten worst case is quantified. Please present the per-source census, recompute Fig. 2 with the actual beams and a propagated mass uncertainty, and revise the abstract or explicitly state the assumptions under which the doubling holds.
  2. [Sec. 5 / Table 1] The ranking of 3C 465 as one of the three LEG targets with resolution <500 R_S at 7 mm is marginal under the paper's own uncertainties. With the adopted mass and 0.1 mas resolution the source is at 451 R_S, but with the factor-of-two mass variation discussed in Sec. 4.1 it moves to roughly 900 R_S, and with the actual 0.13 mas beam to roughly 590 R_S (or ~1170 R_S with both). Please give the resolution range for each ranked target, or state which rankings survive the mass uncertainty, so that the 'superb resolution (<500 R_S)' claim is not tied to one favorable mass estimate.
  3. [Sec. 3 / Appendix B] The VLA system-temperature problems required replacing non-sensical recorded values with elevation-based estimates, and the paper notes that amplitude calibration may carry 20–30% systematic uncertainties. Because the core brightness temperatures in Fig. 3 and the jet-to-counter-jet ratios in Table 3 enter the physical interpretation, please state explicitly how such systematics would shift T_B and R_J/CJ, or add a caveat to the affected quantitative comparisons such as the '13/16 sources have lower T_B at 7 mm' statement in Sec. 4.2.
minor comments (5)
  1. [Sec. 3] The phrase 'non-sense' should be 'non-sensical' for the recorded VLA system temperature values.
  2. [Appendix A.8 and A.11] The figure captions for 3C 264 and 4C 30.31 have inconsistent left/center/right labels; the third panel in each caption should be labelled 'Right' rather than repeating 'Left'.
  3. [Appendix B, Eq. (B.2)] The notation 'S fitp 2' is ambiguous; it should be written as (S/S_p^fit)^2 so that the error propagation formula is clear.
  4. [Table B.16 / Table 2] The beam parameters in Table B.16 differ from those in Table 2 because different weighting schemes are used; please state this explicitly near Table B.16 so the reader does not perceive an inconsistency.
  5. [Fig. 2 caption] The caption lists NGC 315 among the blue sample points, but NGC 315 was already published by the same group; please clarify in the caption or text whether the 'previously studied' count includes NGC 315, since this directly affects the claimed before/after census ratio.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper's new results are direct observational measurements and standard formula applications, with no fitted parameter renamed as a prediction and no load-bearing self-citation chain.

full rationale

The paper's derivation chain is self-contained and observational. Core brightness temperatures are computed directly from measured MODELFIT sizes and flux densities via the standard formula in Eq. (1), with uncertainties propagated from the stated Lee et al. (2008) error analysis; the assumed spectral index and Lorentz factor in Eq. (2) are explicit inputs used to interpret measured jet-to-counter-jet ratios, not quantities derived from the data. The target-selection claims are transparent applications of stated criteria (flux density thresholds and resolution in Schwarzschild radii) to the measured quantities and literature black-hole masses, not predictions derived from those criteria. The census claim 'the number of sources imaged on scales ≲ 10^3 R_S is more than doubled by our study' depends on the adopted 0.1 mas reference resolution and on literature BH masses, and the paper itself acknowledges that mass estimates can vary by factors of two or more in Sec. 4.1; this is a robustness concern about input assumptions, not a circular reduction of output to input. The self-citations (e.g., Boccardi et al. 2016b, 2017, 2021; Ricci et al. 2022) are used for previously published images, limb-brightening comparisons, and the NGC 315 analysis, all of which are independently published observational facts rather than unverified premises that force the present conclusions. No equation in the paper reduces a claimed result to its own definition, and no fitted parameter is subsequently relabeled as a prediction.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The analysis rests mainly on standard radio-interferometry calibration and on literature black hole masses. The only hand-set quantities are the spectral index alpha=-0.7 and Lorentz factor Gamma=1.4 used for jet orientation estimates, neither of which affects the main target ranking. No new particles or physical entities are introduced.

free parameters (2)
  • Spectral index alpha for jet-to-counterjet ratio = -0.7 (assumed, not fitted)
    Used in Eq. (2) to convert measured R_J/CJ into beta_min, theta_max, and theta_beta=0.7 in Table 3. The choice affects the derived orientation and speed values but not the target ranking.
  • Lorentz factor Gamma for viewing angle estimates = Gamma ~ 1.4 (beta = 0.7 c), assumed
    Adopted in Sect. 4.3 as an average for radio galaxies to derive the exact viewing angles in Table 3. The true Gamma could deviate, as the authors note for NGC 4278.
assumptions (4)
  • domain assumption LambdaCDM cosmology with H0 = 71 km/s/Mpc, Omega_M = 0.27, Omega_Lambda = 0.73
    Adopted in footnote 1 for physical scale conversion (pc/mas), which enters the resolution in Schwarzschild radii. The low redshifts make cosmology dependence weak.
  • domain assumption Equipartition brightness temperature of about 5 x 10^10 K marks the threshold between particle- and magnetic-dominated jets
    Used in Sect. 4.2 to conclude cores have not reached equipartition and/or are de-boosted. If the equipartition argument is invalid, this interpretation weakens.
  • domain assumption Equation (2), R_J/CJ = ((1 + beta cos theta)/(1 - beta cos theta))^(2-alpha), holds under intrinsically symmetric, constant-speed twin jets
    Used in Sect. 4.3 and Table 3 to convert measured ratios to beta_min, theta_max, and theta_beta=0.7. Asymmetric or accelerating jets invalidate the derived angles and speeds.
  • domain assumption Observed brightness temperature is related to intrinsic by T_obs = delta T_intr
    Used in Sect. 4.2 to interpret low core temperatures as de-boosting or magnetized emission. Standard Doppler formalism, but the interpretation depends on unknown intrinsic speeds and angles.

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Cite this review

Pith. "Pith review of Jet formation studies in AGN: a search for new targets." pith.science (2026). https://pith.science/paper/FYSFUBNZ

@misc{pith2026241219268,
  author       = {Pith},
  title        = {Pith review of: Jet formation studies in AGN: a search for new targets},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FYSFUBNZ}},
  note         = {Machine review of arXiv:2412.19268}
}
abstract

In recent years, the jet formation region in active galaxies has been imaged through mm-VLBI in few ideal targets, first and foremost M87. An important leap forward for understanding jet launching could be made by identifying a larger number of suitable objects, characterized by different accretion modes and jet powers. In this article, we present 1 cm and 7 mm VLBI data of a sample of 16 poorly explored radio galaxies, comprising both High-Excitation (HEG) and Low-Excitation Galaxies (LEG) and spanning a large range in radio power. The combination of the sources vicinity (z<0.1) with a large black hole mass ($\log{M_{\rm BH}}$>8.5) results in a high spatial resolution in units of Schwarzschild radii ($<10^3-10^4$ $R_{\rm S}$), necessary for probing the region where the jet is initially accelerated and collimated. We aim at identifying the best candidates for follow-up observations with current and future VLBI facilities. The observations were performed with the High Sensitivity Array, including Effelsberg and the phased-VLA, which has allowed us to characterize the sub-parsec properties of these faint jets and to estimate their core brightness temperature and orientation. The number of sources imaged on scales $\lesssim 10^3$ $R_{\rm S}$ is more than doubled by our study. All targets were detected at both frequencies, and several present two-sided jet structures. Several LEG jets show hints of limb-brightening. The core brightness temperatures are generally below the equipartition value, indicating that equipartition has not yet been reached and/or that the emission is de-boosted. Among LEG, we identify 3C31, 3C66B, and 3C465 as the most promising, combining a relatively high flux density (>50 mJy) with superb spatial resolution (<500 $R_{\rm S}$) at 7 mm. The powerful HEG 3C452 is interesting as well due to its highly symmetric, two-sided jet base.

Figures

Figures reproduced from arXiv: 2412.19268 by the authors.

Figure 1
Figure 1. Left: 7 mm HSA image of 3C 452 (FRII-HEG). The beam size and position angle are 0.29 × 0.16 mas, −10◦ . Center: 7 mm HSA image of 3C 31 (FRI-LEG). The beam size and position angle are 0.48 × 0.18 mas, −16◦ . Right: 1 cm HSA image of NGC 4278 (compact-LEG). The beam size and position angle are 0.61 × 0.23 mas, −8 ◦ . Contours start at -3σ and increase by a factor of two until allowed by the peak intensity. These maps… view at source ↗
Figure 2
Figure 2. Spatial resolution in RS for an angular resolution of 0.1 mas versus flux density at 7 mm. In blue we plot the sources in our sample, in red well known radio galaxies previously studied on these scales (Boccardi et al. 2021). For Cyg A, NGC 1052, and M 87 we assume the average 7 mm flux densities from Boccardi et al. (2016b), Baczko et al. (2019), Walker et al. (2018), respectively. For NGC 6251, we refer to Cheng e… view at source ↗
Figure 3
Figure 3. Distributions of the observed core brightness temperature at 1 cm and 7 mm. The values on the x-axis are displayed in logarithmic scale. In linear scale, the six bins span the ranges 0.150-0.339, 0.339-0.763, 0.763-1.720, 1.720-3.878, 3.878-8.741, 8.741-19.703 in units of 1010 K. likely due to the presence of transverse velocity gradients in the jet, with the fast central spine becoming faint for larger an￾gles due … view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Wigner interferometry

    astro-ph.IM 2026-08 conditional novelty 7.0 of 10

    Correlating Wigner functions of the received fields yields a coherence pattern with roughly twice the spatial frequency content of the standard visibility, pointing to a possible two-fold angular resolution gain.

Reference graph

Works this paper leans on

13 extracted references · 13 canonical work pages · cited by 1 Pith paper

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    Appendix A.2: 3C 33 3C33 is one the three HEGs in our sample, and it develops a classic FRII morphology on kilo-parsec scales

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    Left: 7 mm image with uniform weighting. Center: 1 cm image with uniform weighting. Right: 1 cm image with natural weighting. Article number, page 8 of 22 Boccardi B. at al.: Jet formation studies in AGN: a search for new targets Appendix A.3: 3C 66B 3C 66B is another FRI-LEG where the jet base can be imaged with very high spatial resolution, since an ang...

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    Left: 7 mm image with uniform weighting. Right: 1 cm image with uniform weighting. Left: 1 cm image with natural weighting. Article number, page 11 of 22 A&A proofs: manuscript no. main Appendix A.9: NGC4278 NGC 4278 is by far the closest (z =0.0021) and lowest power source ( Pt∼6× 1021 W/Hz) in our sample. It is a LEG showing a compact morphology on VLA ...

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    Top: 7 mm image with uniform (left) and natural weighting (right). Bottom: 1 cm image with uniform (left) and natural weighting (right). Appendix A.6: 4C 39.12 The FRI-LEG 4C 39.12 is one of the radio galaxies detected by the Fermi satellite at γ-ray energies (Ajello et al. 2020). Its one- sided jet oriented towards south-east, which we detect at both ban...

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    Top: 7 mm image with uniform (left) and natural weighting (right). Bottom: 1 cm image with uniform (left) and natural weighting (right). Article number, page 12 of 22 Boccardi B. at al.: Jet formation studies in AGN: a search for new targets Appendix A.11: 4C30.31 The FRI-LEG 4C30.31 shows at both bands a one-sided jet oriented in the south-west direction...

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    Top: 7 mm image with uniform (left) and natural weighting (right). Bottom: 1 cm image with uniform (left) and natural weighting (right). Article number, page 13 of 22 A&A proofs: manuscript no. main Appendix A.13: 3C 388 3C 388 is a LEG developing an FRII jet, and showing signs of restarted activity based on the detection of extended emission beyond the w...

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