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Detection of an orthogonal alignment between parsec scale AGN jets and their host galaxies

T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read This paper finds that parsec-scale AGN jets appear statistically perpendicular to the projected minor axes of their host galaxies, connecting scales from sub-parsec to kiloparsec.

desk verdict A careful large-sample search for a jet–host alignment that finds a real candidate signal, but the headline p-value appears only after selection cuts that the null does not yet control for. read the letter →

arxiv 2411.09099 v1 pith:OZ5MHBW5 submitted 2024-11-14 astro-ph.GA astro-ph.CO

D. Fernández Gil , J. A. Hodgson , B. L'Huillier , J. Asorey , C. Saulder , K. Finner , M. J. Jee , D. Parkinson
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F. Combes
This is my paper · ORCID
classification astro-ph.GAastro-ph.CO
keywords AGNjetsVLBIhostgalaxyalignmentmorphologysupermassiveblackholespositionanglesradiogalaxies
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

The paper sets out to test whether the direction of the relativistic jet launched near a supermassive black hole carries any information about the optical shape of its host galaxy. The authors compare VLBI jet position angles for more than six thousand AGN against galaxy shapes from several optical surveys, using the survey least affected by systematics as the main sample. In the subset of sources with measured spectroscopic redshifts and reliable shape measurements, they find a weak but significant excess of jets oriented perpendicular to the projected minor axis of the galaxy, with $p \leq 0.01$ in the primary sample. No alignment appears when the full sample is used without the redshift and quality cuts. If the effect is real, it implies that the black-hole–disk–jet system and the kiloparsec-scale stellar body of the galaxy are not independently oriented.

What carries the argument

The central object is the angular separation $\Delta PA = |PA_{\rm jet} - PA_{\rm minor}|$ wrapped into $[0^\circ, 90^\circ]$, measuring how close the jet is to perpendicular to the galaxy's projected minor axis. Jet position angles come from VLBI images with formal errors; optical minor axes come from elliptical model fits to galaxy images. The null hypothesis is built by reshuffling the jet and optical position-angle distributions 1000 times, producing a reference band and a statistic $Q$; a Kolmogorov–Smirnov test against a uniform distribution is used as a binning-independent check. The load-bearing cuts are a semi-minor axis larger than $1.3$ arcsec, position-angle errors below $22.5^\circ$, survey-specific 'good case' morphological flags, and the presence of a spectroscopic redshift. A projection simulation using a cosmological hydrodynamical simulation shows that projection alone cannot produce the observed peak unless the stellar spin is closely related to the galaxy plane.

What would settle it

Compute the same $\Delta PA$ histogram for sources that have only photometric redshifts while applying the identical shape-quality cuts; if the perpendicular alignment is a real astrophysical effect it should survive, whereas if it disappears the reported signal is a selection artifact tied to spectroscopic-redshift availability.

Watch

Extended reading notes

Core claim

The central claim is that the parsec-scale jet direction is statistically tied to the kiloparsec-scale optical shape of its host galaxy: jets preferentially point along the projected minor axis of the optical image, i.e., perpendicular to the major axis. The evidence comes from the distribution of the angle difference $\Delta PA$ between the jet position angle and the closest minor-axis position angle, which peaks toward small $\Delta PA$ only after restricting to sources with spectroscopic redshifts and well-measured, non-spherical shapes. The reported significance is $p \leq 0.01$ for the primary optical survey sample (and $p < 0.05$ in the conclusion), with consistent results from two-bin, five-bin, and Kolmogorov–Smirnov tests. The paper interprets the absence of signal in the unrestricted sample as a resolution and selection effect rather than as evidence against a physical connection.

Load-bearing premise

The load-bearing premise is that the subsample with measured spectroscopic redshifts and reliable shapes is not biased in a way that manufactures the angle correlation, and the analysis does not correct for the number of redshift, quality, and binning choices examined.

Editorial extensions

If this is right

  • A genuine pc–kpc alignment would connect sub-parsec jet physics to kiloparsec-scale stellar structure, supporting the idea that supermassive black holes and their host galaxies co-evolve.
  • The signal's appearance only in the spectroscopic-redshift, well-shaped subsample implies that source resolution and shape reliability control the detectability of the alignment in current data.
  • The projection simulations indicate that the observed peak is stronger than projection effects alone can produce, so a physical relation between the black-hole spin axis and the galaxy's angular momentum is the natural explanation.
  • Larger samples from upcoming wide-area radio and optical surveys should sharpen the significance and test whether the alignment persists at higher redshift and in different galaxy types.

Reading between the lines

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

  • Because several redshift, quality, and binning choices were examined, the reported $p \leq 0.01$ likely overstates the evidence unless corrected for multiple comparisons; re-running the same statistic on an independent sample would settle the matter.
  • If the perpendicular alignment is real, it suggests that the jet's launch direction is not determined purely by local accretion-disk dynamics but is correlated with the galaxy-scale angular momentum distribution, with consequences for feedback and merger models.
  • A clean test would be to split the sample by host morphology and radio luminosity; the merger-based interpretation predicts the alignment should be strongest in elliptical galaxies formed by major mergers and weaker in disk galaxies.
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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. The manuscript cross-matches roughly 6273 VLBI-detected AGN with optical survey catalogues to compare parsec-scale jet position angles with the projected kpc-scale shapes of their host galaxies. After applying resolution and quality cuts, the authors use a Monte Carlo reshuffling null, KS tests, and robustness checks on binning and magnitude cuts to search for an excess of small Delta PA between the VLBI jet and the optical minor axis. They find no alignment in the full DESI LS sample (Fig. 3a-c, p5 ~ 0.55-0.62), but report a weak signal with p <= 0.01 in the subset with measured spectroscopic redshifts (Fig. 3d-l). The same analysis is applied to a combined sample and to individual surveys, and an EAGLE projection model is used to argue that the observed Delta PA distribution is consistent with a ~33-degree scatter about a close 3D spin-minor-axis relation. The central claim is that the pc-scale jet direction is statistically tied to the kpc-scale projected minor axis of the host galaxy over three orders of magnitude in scale, at least for sources with well-measured shapes and spectroscopic redshifts.

Significance. If the result is robust, it would be an interesting and rare observational link between the innermost AGN jet direction and the large-scale stellar morphology of the host galaxy, with implications for merger-driven galaxy evolution and SMBH-galaxy co-evolution. The study is based on a large VLBI sample, uses independent jet and optical catalogues, and ships its cross-matching code and data publicly. The statistical machinery is carefully presented: the reshuffling null preserves the marginal PA distributions, the binning and magnitude checks are useful, and the EAGLE projection exercise is a sensible way to illustrate projection effects. The main weakness is that the headline signal appears only after selecting sources with spectroscopic redshifts and good shapes, and the analysis does not yet rule out the possibility that the signal is introduced by selection correlations with redshift, declination, or survey systematics rather than by an intrinsic alignment. The cross-survey comparison is also weaker than the DESI LS result alone, so the robustness of the central claim is not yet established.

major comments (3)
  1. [Fig. 3 and Methods 'Quantifying the alignment signal' (Eqs. 6-7)] The headline signal is introduced by the spectroscopic-redshift selection: Fig. 3a-c are consistent with the null (p5 = 0.62, 0.58, 0.55; pks ~ 0.09-0.11), while Fig. 3d-l reach p5 <= 0.01. The Q-statistic null reshuffles the two PA lists within the selected subsample (Eqs. 6-7), which preserves the marginal PA distributions but removes any joint dependence of jet PA and optical PA on a third variable. Spectroscopic-redshift availability in OCARS is not random: it correlates with redshift, magnitude, and survey footprint, and the paper itself documents a VLBI north-south elongation effect and non-uniform optical PAs in the section 'Underlying distribution of the galaxy and jet PAs'. A residual common dependence of the two PAs on declination or redshift within the spec-z/good-case subsample would therefore produce a Delta PA excess under the null. Please provide declination- and redshift-stratified null tests (or an equivalent conditional analysis) to close this selection channel.
  2. [Fig. 3 and Supplementary Figs. M3-M8] No multiple-testing correction is applied across the 12 panels of Fig. 3, the alternative bin numbers in M7, the magnitude cuts in M8, and the survey-by-survey and combined analyses in M3-M6. With the top row null and p-values of order 0.01 in the spec-z rows, a look-elsewhere effect is a plausible alternative to an astrophysical alignment. The authors should report the effective number of independent tests or demonstrate a pre-specified hierarchy of cuts in which the signal is present; the qualitative statement that 'the overall picture' is unchanged by binning does not provide a quantitative correction.
  3. [Supplementary Figs. M3-M6 and 'Survey by survey analysis'] The cross-survey comparison does not currently support a universal signal. DESI LS spec-z rows give p5 ~ 0.005-0.01, but the combined sample in M3 gives much weaker p-values (e.g., M3d p5 = 0.05, M3l p5 = 0.08), DES and SkyMapper in M5-M6 are consistent with the null in most rows, and the only independent significant detection is SDSS at z < 0.1 with N = 52 (M4j-l, p5 = 0.006). The text explains that DESI LS is used because it is less affected by systematics, but a quantitative error budget for why the other surveys lose the signal is needed before the survey dependence can be attributed to systematics rather than to the same selection channel that may be at work in DESI LS.
minor comments (5)
  1. [Conclusion] The conclusion states that the jet is 'oriented perpendicularly to the projected optical shape of the host galaxy with a p-value < 0.05' without the qualifiers 'in sources with measured spectroscopic redshifts' and 'weak'; please align this wording with the abstract and with the null results in the top row of Fig. 3.
  2. [Abstract and Fig. 1] The phrase 'perpendicularly to the projected optical shape' is ambiguous: the measured Delta PA is relative to the projected minor axis, so the signal is an alignment with the minor axis (equivalently a perpendicular orientation to the galaxy disk plane), not a perpendicular orientation to the projected shape as a whole. Consider rewording for clarity.
  3. [Fig. 3 caption and Table 1] The Fig. 3 caption repeats the sentence 'Each panel corresponds to a specific selection of sources' twice, and the KiDS row of Table 1 appears to be missing a column value; the table should be formatted consistently.
  4. [Fig. 3 and Fig. M7] pks values of 0.000 (e.g., Fig. 3g) should be reported as p < 0.001, given that the p-value resolution is set by 1000 reshufflings.
  5. [Fig. 4] The EAGLE scatter amplitude epsilon = 0.33 is chosen after the fact to match the observed 68% interval; the main text does acknowledge this, but the Fig. 4 caption should explicitly mark panels (c) and (d) as illustrative rather than as a quantitative fit to the data.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the alignment claim is a direct permutation-test comparison of independent VLBI and optical catalogues, with no fitted parameter or self-citation chain defining the result.

full rationale

The central claim is derived by direct statistical comparison of two independent observables: VLBI jet position angles from the Astrogeo/Plavin catalogue and optical galaxy position angles from DESI LS, using a reshuffling null that randomly re-pairs the observed PA lists. No equation in the paper defines the alignment signal in terms of a fitted parameter; the reported p-values come from the Q statistic measured against 1000 reshuffled realisations (Eqs. 4-7), so the detection is not equivalent to its input by construction. The EAGLE projection scenarios, including the epsilon = 0.33 scatter, are explicitly illustrative: the paper states 'we should be careful not to over-interpret', and the simulation is used only to interpret the physical meaning of the measured distribution, not to produce or fit the p-value. The few self-citations present (e.g., L'Huillier et al. 2017 for the reshuffling approach, Hodgson et al. for VLBI-related background) are methodological or contextual and are not load-bearing for the detection. The paper's own caveats about possible resolution effects, the weakness of the signal, and the need for future follow-up are limitations rather than circular steps. The main robustness concern, namely that the signal appears only after spectroscopic-redshift and shape-quality cuts, is a potential selection-effect or multiple-testing issue, not a circularity of derivation; the analysis preserves the observed marginals in the null but does not model selection jointly with redshift or sky position. That concern belongs to statistical validity, not to whether the derivation reduces to its inputs. Overall, the derivation is self-contained and not circular.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The central claim is an empirical measurement; the ledger contains hand-chosen cuts and domain assumptions rather than fitted physical constants. The EAGLE scatter epsilon is the only numeric knob and it is not used to generate the detection.

free parameters (3)
  • EAGLE projection scatter amplitude epsilon = 0.33
    Chosen by hand to reproduce the observed 68% median spread of 33 degrees in the DESI LS good-case Delta PA distribution (Methods, EAGLE simulation projection effects). Used only in a plausibility scenario, not to fit the detection p-value.
  • Position angle error cut = 22.5 degrees
    Hand-chosen cut; sources with larger PA errors are excluded from one of the quality-cut columns. This is a selection threshold, not a model parameter, but it affects which subset produces the low p-values.
  • Semi-minor axis cut = 1.3 arcsec
    Hand-chosen based on the median DESI LS seeing; all 'criterion 0' samples use it. Not fitted, but determines the resolved-galaxy sample.
assumptions (4)
  • domain assumption VLBI jet position angle is a proxy for the orientation of the SMBH-accretion disk system.
    Invoked in the introduction and conclusion, e.g. 'the pc-scale jet (which may be a proxy for the SMBH-accretion disk system)'; jet bends and frequency-dependent structures mean the mapping is imperfect, as the authors acknowledge.
  • domain assumption Projection effects cannot create the alignment alone.
    Stated in the interpretation section: 'Projection effects could be complex, but they cannot create such an alignment alone.' The EAGLE projection experiment is used to support this but does not prove it.
  • domain assumption The 1 arcsec cross-match tolerance identifies the correct host galaxy.
    Assumed in the cross-matching; optical positions can be offset by dust or jet emission, as the authors note in their discussion of VLBI-Gaia offsets.
  • domain assumption Spectroscopic redshift availability is independent of the angle difference.
    The main signal appears only in the spectroscopic-redshift subsample; if spec-z selection correlates with orientation or PA systematics, the result is biased. This is the load-bearing selection assumption.

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Cite this review

Pith. "Pith review of Detection of an orthogonal alignment between parsec scale AGN jets and their host galaxies." pith.science (2026). https://pith.science/paper/OZ5MHBW5

@misc{pith2026241109099,
  author       = {Pith},
  title        = {Pith review of: Detection of an orthogonal alignment between parsec scale AGN jets and their host galaxies},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OZ5MHBW5}},
  note         = {Machine review of arXiv:2411.09099}
}
abstract

The relationship between galaxies and their supermassive black holes (SMBHs) is an area of active research. One way to investigate this is to compare parsec-scale jets formed by SMBHs with the projected shape of their kiloparsec-scale host galaxies. We analyse Very Long Baseline Interferometry (VLBI) images of Active Galactic Nuclei (AGN) and optical images of their host galaxies. We compare the inner-jet position angle in VLBI-detected radio sources with the optical shapes of galaxies as measured by several large optical surveys. In total 6273 galaxy-AGN pairs were found. We carefully account for the systematics of the cross-matched sources and find that Dark Energy Spectroscopic Instrument Legacy Imaging Surveys data (DESI LS) is significantly less affected by them. Using DESI LS, with which 5853 galaxy-AGN pairs were cross-matched, we find a weak but significant alignment signal (with a p-value $\leqslant$ 0.01) between the parsec-scale AGN jet and the kpc projected minor axis of the optical host galaxy in sources with measured spectroscopic redshifts. Our results show that the observed source properties are connected over 3 orders of magnitude in scale. This points towards an intimate connection between the SMBH, their host galaxies and their subsequent evolution.

Figures

Figures reproduced from arXiv: 2411.09099 by the authors.

Figure 1
Figure 1. Demonstration of how the angle difference between the VLBI jet PA and the projected minor [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Footprints of the optical surveys and the VLBI sample used. Each survey is represented by a [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Histograms of all VLBI sources with a cross-match in DESI LS and a semi-minor axis [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: ∆PA between the projected spin axis of the stellar component and the semi-minor axis of the [PITH_FULL_IMAGE:figures/full_fig_p008_4.png]

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Pith tools

Reviewed August 12, 2026 · model on record in the stance chip above.