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arxiv: 2606.17152 · v1 · pith:RCVLTVR4new · submitted 2026-06-15 · 🌌 astro-ph.GA

Hector Galaxy Survey: Optical IFU and Chandra Reveal a Low-Luminosity AGN Behind Extended LINER Emission

Kyuseok Oh , Gabriella Quattropani , Pratyush Kumar Das , Minje Beom , Joon Hyeop Lee , O\u{g}uzhan \c{C}ak{\i}r , Andrei Ristea , Hyunjin Jeong
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Jiwon Chung Michael J. Koss Scott M. Croom Mina Pak Matt S. Owers Sarah M. Sweet Jong Chul Lee Julia J. Bryant Sree Oh Jesse van de Sande Peixin Zhu Stefania Barsanti Madusha L. P. Gunawardhana Sujeeporn Tuntipong Robert Content Jon Lawrence Ayoan Salim Sadman Will Saunders Barnaby Norris Gurashish Singh Bhatia
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Pith reviewed 2026-06-27 03:09 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords low-luminosity AGNLINER emissionintegral field spectroscopyX-ray detectionphoton budgetextended emissionHector Galaxy Survey
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The pith

Optical mapping and X-ray data show a low-luminosity AGN powers extended LINER emission where stars fall short.

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

The paper studies one galaxy from the Hector survey that displays LINER-like line ratios across most of its mapped area. Tau maps that compare ionizing photons from post-AGB stars to the photons actually required by the gas reveal a deficit over most of the galaxy, including the nucleus even under optimistic assumptions. Archival Chandra observations detect a compact nuclear X-ray source whose luminosity matches expectations for a weak AGN. Line-ratio and kinematic tests find no dominant shock contribution. The combination demonstrates that an additional nuclear source is needed at least centrally and that a low-luminosity AGN supplies a substantial fraction of the ionizing budget.

Core claim

The central claim is that evolved stellar populations alone cannot account for the observed emission, that an additional nuclear ionizing source is required at least in the inner region, and that a weak LLAGN likely contributes to the ionizing budget, particularly in the inner region. This is shown by tau maps remaining below unity even in the most favorable post-AGB normalizations, by the presence of a compact X-ray source, and by the absence of shock signatures as the primary driver.

What carries the argument

Spatially resolved tau maps (Q_pAGB / Q_req) combined with a compact nuclear X-ray detection, used to test whether post-AGB stars suffice or whether a nuclear source is required.

If this is right

  • Extended LINER-like emission can conceal a substantial LLAGN contribution even when traditional optical and infrared AGN indicators are weak.
  • Spatially resolved photon-budget tests combined with X-ray constraints can effectively reveal such hidden activity.
  • The galaxy serves as a pilot validating the photon-budget framework for identifying nuclear sources in other systems.

Where Pith is reading between the lines

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

  • The same photon-budget plus X-ray approach could be applied to the rest of the Hector sample to estimate how often weak AGNs are missed by standard diagnostics.
  • If the method proves reliable, it would lower the apparent fraction of pure star-ionized LINERs and raise the fraction of composite or AGN-hosting systems at low redshift.
  • Kinematic data with higher spatial resolution could test whether any localized shock contribution alters the photon accounting in the outer regions.

Load-bearing premise

The tau maps correctly quantify a photon deficit under the chosen post-AGB normalizations and the compact X-ray source is an AGN rather than another nuclear phenomenon.

What would settle it

A revised post-AGB model that supplies enough photons across the inner region without a nuclear source, or a demonstration that the X-ray emission arises from a non-AGN process, would falsify the need for the LLAGN contribution.

Figures

Figures reproduced from arXiv: 2606.17152 by Andrei Ristea, Ayoan Salim Sadman, Barnaby Norris, Gabriella Quattropani, Gurashish Singh Bhatia, Hyunjin Jeong, Jesse van de Sande, Jiwon Chung, Jong Chul Lee, Jon Lawrence, Joon Hyeop Lee, Julia J. Bryant, Kyuseok Oh, Madusha L. P. Gunawardhana, Matt S. Owers, Michael J. Koss, Mina Pak, Minje Beom, O\u{g}uzhan \c{C}ak{\i}r, Peixin Zhu, Pratyush Kumar Das, Robert Content, Sarah M. Sweet, Scott M. Croom, Sree Oh, Stefania Barsanti, Sujeeporn Tuntipong, Will Saunders.

Figure 1
Figure 1. Figure 1: Emission-line flux maps (a–f), a DESI Legacy Surveys DR10 grz image (g; A. Dey et al. 2019), and BPT classification maps (h–j) and BPT diagnostic diagrams (k–m). Panels (a–f) use a common asinh stretch (Q = 0.001) with limits set by the 0.1–99.9 percentile of the combined (positive) flux distribution across the six lines; spaxels with S/N ≥ 3 are colored and those with S/N < 3 are gray (Point Spread Functi… view at source ↗
Figure 2
Figure 2. Figure 2: Ionized-gas velocity dispersion and spatially resolved BPT classifications. Left: Map of the line-of-sight ionized-gas velocity dispersion, σgas (in km s−1 ), from the Hector pipeline vdisp extension (corrected for instrumental resolution), shown for spaxels with S/Nline ≥ 3 in the BPT lines. Symbols indicate the [N ii]λ6583/Hα BPT classification: composite (purple circles), star-forming (blue triangles), … view at source ↗
Figure 3
Figure 3. Figure 3 [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Line ratio versus velocity dispersion test for shock excitation. Each panel shows the spaxel-by-spaxel relation between the ionized-gas velocity dispersion σgas and a low-ionization line ratio: log[N ii]λ6583/Hα (left), log[O i]λ6300/Hα (middle), and log[S ii]λλ6717, 6731/Hα (right). Spaxels are required to have S/N ≥ 3 in the relevant emission lines. Each panel reports Spearman’s rank correlation coeffici… view at source ↗
Figure 5
Figure 5. Figure 5: Chandra X-ray data and model fit. Left panel: Chandra X-ray image, including source (solid circle) and background (dashed annulus) markers. Right panel: extracted data with the applied fit vs. energy (top) and residuals (bottom) [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Spatially resolved EWHα and ionizing-photon budget diagnostics. The top-left panel shows the EWHα map. The three map panels in the left and middle columns show log τ ≡ log(QpAGB/Qreq) computed for each spaxel for three pAGB ionizing-photon normalizations, qpAGB = 1040.0 , 1040.5 , and 1041.0 photons s−1 M−1 ⊙ (low, fiducial, and high, respectively). Black contours overlaid on the three log τ map panels tra… view at source ↗
Figure 7
Figure 7. Figure 7: Comparison with various classes of emission-line galaxies with X-ray detections. Left panel: [N ii]λ6583/Hα BPT diagnostic diagram showing the position of our target (green filled circles, color-coded by projected distance from the galaxy center), compared to sources with different ionization mechanisms, including LINER-like galaxies and AGNs from the literature with X-ray detections (A. Constantin et al. … view at source ↗
Figure 8
Figure 8. Figure 8: Relationship between extinction-corrected [O iii]λ5007 luminosity and intrinsic (unabsorbed) 2–10 keV X-ray lu￾minosity for various AGN classes. Red-filled squares and blue open circles represent Swift-BAT hard X-ray selected AGNs with log(NH/cm−2 ) ≥ 22 (type 2) and log(NH/cm−2 ) < 22 (type 1), respectively (C. Ricci et al. 2017; K. Oh et al. 2022). Orange-filled triangles show Seyfert galaxies from F. Pa… view at source ↗
Figure 9
Figure 9. Figure 9: Example spectra extracted from representative spaxels, shown with best-fit models. The top-left panel displays the spatially resolved classification map from the [N ii]λ6583/Hα BPT diagnostics, overlaid on the grz color-composite image cutout, as shown in [PITH_FULL_IMAGE:figures/full_fig_p025_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: The [N ii]λ6583/Hα BPT diagnostic diagram with radiative shock model grids from M. G. Allen et al. (2008). SDSS background contours show OSSY galaxies (K. Oh et al. 2011). Hector spaxels with S/N ≥ 3 in [N ii]λ6583, Hα, [O iii]λ5007, and Hβ are overplotted; spaxels with EWHα < 3 ˚A are shown as small open symbols, while spaxels with EWHα ≥ 3 ˚A are highlighted with larger symbols and their [N ii]λ6583/Hα-… view at source ↗
Figure 11
Figure 11. Figure 11: Spatial comparison of the [S II] doublet-ratio selection with the emission-line morphology and gas kinematics. Left: log F(Hα) map (arbitrary flux scaling; shown only for relative morphology), displayed with a linear grayscale stretch between the 5th and 99th percentiles of the valid spaxels. Right: ionized-gas velocity-dispersion map, σgas, shown with a linear grayscale stretch between the 5th and 95th p… view at source ↗
read the original abstract

We present evidence that the Hector Galaxy Survey galaxy C901005481609968 ($z_{\rm cl}=0.0553$), which exhibits spatially extended LINER-like emission in optical integral-field spectroscopy (IFS), hosts a low-luminosity active galactic nucleus (LLAGN) that contributes substantially to its ionization budget. Although the galaxy is not selected as an AGN by mid-infrared AGN color criteria, archival Chandra data reveal a compact nuclear X-ray source with $\log L_{\rm X}\approx41.46$ erg/s, supporting the presence of an LLAGN. Spatially resolved emission-line diagnostics show LINER-like line ratios across most spaxels with $\mathrm{S/N} \geq 3$, while spatially resolved $\tau$ maps ($\tau \equiv Q_{\rm pAGB}/Q_{\rm req}$) indicate a widespread photon deficit ($\log\tau<0$ over most of the mapped region), even under the most optimistic pAGB normalizations, the nuclear region remains at $\tau < 1$. Line-ratio--kinematic tests find no evidence for shock-dominated excitation as the primary driver of the extended emission, although a localized or sub-dominant shock contribution cannot be ruled out with the present data. We use this galaxy as a pilot case because the combination of Hector IFS and an independent nuclear X-ray constraint provides a stringent validation of the spatially resolved photon-budget framework. Our results indicate that evolved stellar populations alone cannot account for the observed emission, that an additional nuclear ionizing source is required at least in the inner region, and that a weak LLAGN likely contributes to the ionizing budget, particularly in the inner region. Our results demonstrate that extended LINER-like emission can conceal a substantial LLAGN contribution even when traditional optical and infrared AGN indicators are weak, and that spatially resolved photon-budget tests combined with X-ray constraints can effectively reveal such hidden activity.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper presents a pilot study of galaxy C901005481609968 (z=0.0553) from the Hector Galaxy Survey, which shows spatially extended LINER-like emission in IFS data. It argues that evolved stellar populations alone cannot explain the ionization, as spatially resolved tau maps (tau = Q_pAGB / Q_req) indicate a photon deficit (log tau <0 over most of the region, including the nucleus) even under optimistic pAGB normalizations. Archival Chandra data reveal a compact nuclear X-ray source (log L_X ≈41.46 erg/s) interpreted as a weak LLAGN contributing to the budget, particularly centrally. Line-ratio-kinematic tests rule out shocks as the primary driver. The work validates the photon-budget framework using the independent X-ray constraint and concludes that extended LINER emission can conceal substantial LLAGN activity despite weak traditional AGN indicators.

Significance. If the central claims hold after addressing uncertainties, the result would show that spatially resolved photon-budget analysis combined with X-ray data can uncover hidden LLAGNs in galaxies with extended LINER emission, even when MIR and optical diagnostics fail. This has implications for the ionization mechanisms in LINER galaxies and the demographics of low-luminosity AGN. The independent archival X-ray measurement provides external grounding for the optical tau maps, which is a strength.

major comments (2)
  1. [Abstract] Abstract: The claim that evolved stellar populations cannot account for the emission (and thus a nuclear source is required) rests on tau maps showing log tau <0 even under optimistic pAGB normalizations. However, the manuscript supplies no quantitative error analysis or sensitivity tests to variations in the pAGB normalization factor (explicitly a free parameter in the reader's assessment), nor to potential underestimates in Q_req from extinction or aperture effects; this makes the robustness of the deficit unclear and is load-bearing for the central conclusion.
  2. [Abstract] Abstract: The interpretation of the compact Chandra source (log L_X ≈41.46 erg/s) as an LLAGN contributing to the ionizing budget is load-bearing but not independently secured. The paper notes the source is not MIR-selected as AGN, yet alternatives such as a nuclear X-ray binary or hot gas are not quantitatively ruled out via luminosity functions, spectral properties, or multiwavelength comparisons; this weakens the validation of the photon-budget framework.
minor comments (1)
  1. The manuscript would benefit from inclusion of full data tables for the tau maps, line ratios, and X-ray properties, along with explicit handling of uncertainties, to support reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thoughtful and constructive report. The two major comments identify important areas where the robustness of our central claims can be strengthened. We address each below and indicate the revisions we will make.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that evolved stellar populations cannot account for the emission (and thus a nuclear source is required) rests on tau maps showing log tau <0 even under optimistic pAGB normalizations. However, the manuscript supplies no quantitative error analysis or sensitivity tests to variations in the pAGB normalization factor (explicitly a free parameter in the reader's assessment), nor to potential underestimates in Q_req from extinction or aperture effects; this makes the robustness of the deficit unclear and is load-bearing for the central conclusion.

    Authors: We agree that the absence of explicit sensitivity tests leaves the photon-deficit conclusion less secure than it could be. In the revised manuscript we will add a dedicated subsection (and supplementary figures) that (i) varies the pAGB normalization factor over a factor of five around the fiducial value, (ii) propagates the dominant uncertainties in the extinction correction and in the adopted aperture for Q_req, and (iii) shows the resulting range of log tau maps. These tests will quantify how far the deficit persists under conservative assumptions and will be referenced in the abstract. revision: yes

  2. Referee: [Abstract] Abstract: The interpretation of the compact Chandra source (log L_X ≈41.46 erg/s) as an LLAGN contributing to the ionizing budget is load-bearing but not independently secured. The paper notes the source is not MIR-selected as AGN, yet alternatives such as a nuclear X-ray binary or hot gas are not quantitatively ruled out via luminosity functions, spectral properties, or multiwavelength comparisons; this weakens the validation of the photon-budget framework.

    Authors: We acknowledge that a single archival Chandra detection does not by itself exclude a nuclear X-ray binary or a compact hot-gas component. In revision we will (i) compare the observed luminosity and hardness ratio to the expected XRB luminosity function for a galaxy of this stellar mass and SFR, (ii) note that the source is unresolved at Chandra resolution while hot gas in early-type galaxies is typically more extended, and (iii) add a short paragraph discussing the multi-wavelength context (weak MIR, strong central LINER, photon deficit). These additions will make the LLAGN interpretation more balanced while preserving the paper’s main point that the X-ray detection provides an independent consistency check on the optical tau maps. revision: partial

Circularity Check

0 steps flagged

No significant circularity: photon-budget maps and archival X-ray data are independent of the target conclusion

full rationale

The paper computes tau = Q_pAGB/Q_req maps from standard stellar population models (with explicit optimistic normalizations) and observed emission-line fluxes, then compares to an independent archival Chandra X-ray detection (log L_X ≈ 41.46). Neither step fits parameters to the final claim nor reduces the deficit conclusion to a self-citation or redefinition. Line-ratio-kinematic tests are likewise external to the result. The central claim therefore rests on externally grounded inputs rather than construction from its own outputs.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Abstract-only review; free parameters and axioms inferred from stated methods but cannot be exhaustively audited without full text.

free parameters (1)
  • pAGB normalization factor
    Optimistic normalizations invoked to compute tau; values not specified in abstract.
axioms (2)
  • domain assumption LINER-like line ratios trace photoionization rather than shocks or other mechanisms as primary driver
    Invoked when interpreting spatially resolved diagnostics and ruling out shocks.
  • domain assumption Compact nuclear X-ray source indicates LLAGN activity
    Used to link Chandra detection to the ionizing source.

pith-pipeline@v0.9.1-grok · 6032 in / 1245 out tokens · 32638 ms · 2026-06-27T03:09:42.307656+00:00 · methodology

discussion (0)

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