On the Tully-Fisher Relation for Active Galaxies -- I: Evidence of Larger Scatter
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The pith
Active galaxies exhibit significantly larger scatter in the Tully-Fisher relation than inactive galaxies.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We present an investigation of the Tully-Fisher (TF) relation solely for galaxies hosting an active galactic nucleus (AGN). Using 22 galaxies with primary, z-independent distances, we find that active galaxies exhibit significantly larger scatter about all TF relations compared to each respective calibration for (largely) inactive galaxies. The larger scatter persists despite removal of the AGN contamination from the photometry of the Type 1 AGNs via careful surface brightness decompositions or employing SEDs to constrain the light contribution of the AGN. These results suggest that the influence of an AGN on its host galaxy's surface brightness may extend beyond the nucleus. We also calcula
What carries the argument
Comparison of scatter in the Tully-Fisher relation for AGN hosts versus inactive-galaxy calibrations, after AGN-light removal via surface-brightness decomposition or SED fitting.
If this is right
- TF distances to active galaxies overestimate primary distances by 5-10 percent in optical and near-infrared bands.
- Distances from the inverted baryonic TF relation overestimate primary distances by approximately 15 percent.
- Active galaxies should be removed from future samples used to model the local peculiar velocity field.
- The AGN's influence on surface brightness appears to extend beyond the nucleus itself.
Where Pith is reading between the lines
- Separate TF calibrations may be needed for AGN hosts if the larger scatter cannot be removed by improved photometry.
- Current TF-based peculiar-velocity maps could contain systematic offsets where active galaxies remain in the sample.
- Extending the analysis to galaxies with secondary distance indicators could test whether the bias persists at larger distances.
Load-bearing premise
The 22 galaxies with primary distances form a representative sample of AGN hosts and the AGN light removal methods fully capture any extended influence of the AGN on host galaxy surface brightness beyond the nucleus.
What would settle it
A larger sample of active galaxies with independent primary distances that shows scatter matching the inactive-galaxy calibrations after refined AGN-light subtraction.
Figures
read the original abstract
We present an investigation of the Tully-Fisher (TF) relation solely for galaxies hosting an active galactic nucleus (AGN). Using 22 galaxies with primary, z-independent distances, we find that active galaxies exhibit significantly larger scatter about all TF relations compared to each respective calibration for (largely) inactive galaxies. The larger scatter persists despite removal of the AGN contamination from the photometry of the Type 1 AGNs via 1) careful surface brightness decompositions or 2) employing SEDs to constrain the light contribution of the AGN. These results suggest that the influence of an AGN on its host galaxy's surface brightness may extend beyond the nucleus. We also calculate the percentage difference between TF and primary distances, and find that TF-based distances are biased towards overestimation of the primary distances to active galaxies by anywhere from 5-10 percent for the optical/near-infrared and approximately 15 percent for distances predicted from inverting the Baryonic TF (BTF) relation. As TF-based distances (especially the I-band) are relied on heavily for analysis and modeling of the local peculiar velocity (Vpec) field, we suggest that active galaxies be removed from future Vpec modeling samples.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript examines the Tully-Fisher (TF) relation for active galactic nuclei (AGN) hosts using a sample of 22 galaxies with primary, redshift-independent distances. It reports significantly larger scatter about standard TF relations (optical, near-infrared, and baryonic) compared to calibrations based on largely inactive galaxies. This excess scatter remains after AGN contamination is removed from Type 1 hosts via surface-brightness decompositions or SED modeling. The paper additionally quantifies a systematic bias in which TF distances overestimate primary distances by 5–10% (optical/NIR) to ~15% (BTF inversion) and recommends excluding active galaxies from future peculiar-velocity field analyses.
Significance. If the larger scatter is confirmed to be a general property of AGN hosts rather than a selection artifact, the result would affect the reliability of TF-based distances in local peculiar-velocity modeling and distance-ladder applications. The persistence of excess scatter after explicit AGN-light removal points to possible extended host-galaxy effects, which is a potentially important systematic for TF studies.
major comments (3)
- [Sample selection / data section] Sample selection (implicit in abstract and § on data): The central claim that active galaxies exhibit significantly larger TF scatter rests on the 22 galaxies with primary distances forming a representative draw from the AGN-host population used in existing TF calibrations. No quantitative comparison of stellar-mass, morphological, or AGN-luminosity distributions against the parent calibration samples is presented; if the primary-distance subsample is biased toward nearby, low-luminosity, or morphologically disturbed systems, the reported excess scatter could be an artifact of selection rather than an AGN effect.
- [Results / statistical analysis] Statistical comparison of scatter (results section): The abstract states that active galaxies show 'significantly larger scatter' but provides no description of the metric (standard deviation, rms residual, reduced χ²), whether measurement uncertainties are included, the treatment of outliers, or the statistical test (F-test, Levene’s test, etc.) used to establish significance. Without these details the quantitative claim cannot be evaluated.
- [AGN contamination removal methods] AGN-light removal validation (methods): The persistence of excess scatter after decompositions or SED fitting is used to argue that AGN influence extends beyond the nucleus. However, no quantitative assessment is given of residual AGN light at large radii or of how the decompositions were validated against mock data or multi-wavelength constraints; this directly affects the interpretation that the scatter is intrinsic to AGN hosts rather than incomplete subtraction.
minor comments (2)
- [Abstract] The abstract would be clearer if it reported the actual measured scatter values (with uncertainties) for both the AGN sample and the comparison calibrations.
- [Figures / tables] Figure captions and table footnotes should explicitly state whether the plotted TF residuals include measurement errors or are raw observed scatters.
Simulated Author's Rebuttal
We thank the referee for their constructive comments, which highlight important areas for clarification and strengthening of the manuscript. We address each major comment below with honest responses based on the available data and analysis in the paper.
read point-by-point responses
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Referee: Sample selection (implicit in abstract and § on data): The central claim that active galaxies exhibit significantly larger TF scatter rests on the 22 galaxies with primary distances forming a representative draw from the AGN-host population used in existing TF calibrations. No quantitative comparison of stellar-mass, morphological, or AGN-luminosity distributions against the parent calibration samples is presented; if the primary-distance subsample is biased toward nearby, low-luminosity, or morphologically disturbed systems, the reported excess scatter could be an artifact of selection rather than an AGN effect.
Authors: The 22 galaxies represent the complete set of AGN hosts with primary (redshift-independent) distances that meet the TF selection criteria; primary distances are scarce, so this is not a random subsample but the full available population for direct comparison. We agree a quantitative comparison to calibration samples (e.g., SFI++ or similar) would be useful and will add it in revision via a table or cumulative distribution plots for stellar mass, morphology, and AGN luminosity proxies. This will allow assessment of any biases while noting that the observed excess scatter is uniform across the sample. revision: partial
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Referee: Statistical comparison of scatter (results section): The abstract states that active galaxies show 'significantly larger scatter' but provides no description of the metric (standard deviation, rms residual, reduced χ²), whether measurement uncertainties are included, the treatment of outliers, or the statistical test (F-test, Levene’s test, etc.) used to establish significance. Without these details the quantitative claim cannot be evaluated.
Authors: We will expand the results section to specify that scatter is quantified as the rms residual about the TF fit (with measurement uncertainties included in quadrature), that no outliers were removed beyond initial sample cuts, and that significance was assessed via an F-test on the variances relative to the inactive calibration samples. These details were present in our internal analysis but omitted from the submitted text; we apologize for the oversight. revision: yes
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Referee: AGN-light removal validation (methods): The persistence of excess scatter after decompositions or SED fitting is used to argue that AGN influence extends beyond the nucleus. However, no quantitative assessment is given of residual AGN light at large radii or of how the decompositions were validated against mock data or multi-wavelength constraints; this directly affects the interpretation that the scatter is intrinsic to AGN hosts rather than incomplete subtraction.
Authors: The methods describe the surface-brightness decompositions and SED modeling used to subtract AGN light, but we agree quantitative validation is needed. We will add estimates of residual AGN flux at large radii (beyond ~2 Re) from the fits and any available multi-wavelength cross-checks. Full mock-data validation was not performed in the original analysis; if feasible with existing data we will include a brief assessment, otherwise we will explicitly note this limitation in the revised text. revision: partial
Circularity Check
No circularity: direct observational comparison of measured scatter
full rationale
The paper reports an empirical measurement of TF scatter in 22 AGN hosts with primary distances, compared against published calibrations for inactive galaxies. No derivation, equation, or prediction is constructed that reduces to a fitted parameter or self-citation by definition. The central result (larger scatter) is a direct statistical comparison of observed residuals; the sample selection and AGN subtraction methods are described as independent inputs rather than outputs of the analysis itself. External TF calibrations are treated as fixed benchmarks, not re-derived within the paper.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Prior TF calibrations for inactive galaxies provide an accurate baseline against which AGN-host scatter can be compared.
- domain assumption Primary, z-independent distances for the 22 galaxies are accurate reference standards.
Reference graph
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discussion (0)
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