arxiv: 2605.06842 · v1 · submitted 2026-05-07 · 🌌 astro-ph.GA

Recognition: no theorem link

Spectrally and spatially resolved (sub)millimeter HCN-to-HCO⁺ flux ratios in nearby ultraluminous infrared galaxies

Masatoshi Imanishi (1) , Yuri Nishimura (2) , Shunsuke Baba (3) , Kouichiro Nakanishi (1) , Takuma Izumi (1) ((1) NAOJ , (2) University of Tsukuba , (3) JAXA)

Authors on Pith no claims yet

Pith reviewed 2026-05-11 01:15 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords ultraluminous infrared galaxiesHCNHCO+molecular outflowsAGNALMAflux ratiosgalaxy nuclei

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The pith

Elevated HCN-to-HCO+ flux ratios in ULIRG nuclei originate from resolved outflows in seven galaxies and from AGN or unresolved outflows in nine others.

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

This paper maps the three-dimensional geometry of regions showing elevated HCN-to-HCO+ flux ratios in 18 nearby ultraluminous infrared galaxies using ALMA data at roughly 500 pc resolution. It visually sorts those elevated-ratio regions in position-position-velocity space into spherical-shell, spectrally distinct compact, and filled morphologies. The classification is then tied directly to physical mechanisms: resolved outflows for the shell type, AGN or unresolved outflows for the compact type, and AGN or confined outflows for the filled type. A sympathetic reader would care because these distinctions refine how we interpret the high infrared luminosities of ULIRGs as mixtures of starburst-driven winds, AGN activity, and molecular feedback rather than a single blended process.

Core claim

By examining spectrally and spatially resolved HCN-to-HCO+ flux ratios at J=2-1, 3-2, and 4-3, the authors classify elevated-ratio geometries in PPV space into spherical shell (spectrally and spatially distinct), spectrally distinct and spatially compact, and filled (spectrally filled and spatially compact) types. These correspond respectively to spatially resolved outflows, AGN and/or spatially unresolved outflows with separated velocity components, and AGN and/or spatially confined outflows. Signatures of the first type appear in seven ULIRGs and the second type in nine ULIRGs; in the resolved-outflow cases the elevated ratios preferentially appear at modest velocities relative to the full

What carries the argument

Visual classification of elevated HCN-to-HCO+ flux ratio geometries into spherical shell, spectrally distinct compact, and filled types within position-position-velocity space.

If this is right

In the seven galaxies with resolved-outflow signatures, spaxels showing elevated ratios preferentially sample modest-velocity rather than maximum-velocity outflow material.
The resolved-ratio approach supplies additional constraints on physical origin that spatially or velocity integrated measurements alone cannot provide.
Nine galaxies exhibit geometries consistent with AGN or spatially unresolved outflows.
The same classification can be applied to future higher-resolution data on more distant ULIRGs to track how outflow and AGN contributions evolve with redshift.

Where Pith is reading between the lines

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

The method may help separate starburst-driven feedback from AGN-driven feedback in statistical samples of infrared-luminous galaxies.
Cross-matching these ratio maps with independent outflow indicators could test whether the modest-velocity preference is a general feature of resolved molecular winds.
If the classification holds, many ULIRG nuclei may host co-existing resolved outflows and compact AGN contributions rather than a single dominant mechanism.

Load-bearing premise

The three visual geometry classes in PPV space correctly correspond to distinct physical origins of resolved outflow, AGN or unresolved outflow, and confined outflow.

What would settle it

Finding a ULIRG in which a spherical-shell geometry of elevated ratios shows no kinematic match to an outflow in independent tracers such as CO or OH line wings would falsify the mapping.

Figures

Figures reproduced from arXiv: 2605.06842 by (2) University of Tsukuba, (3) JAXA), Kouichiro Nakanishi (1), Masatoshi Imanishi (1), Shunsuke Baba (3), Takuma Izumi (1) ((1) NAOJ, Yuri Nishimura (2).

**Figure 1.** Figure 1: PPV plot of the (sub)millimeter HCN-to-HCO+ flux ratios at J=2–1, J=3–2, and J=4–3, with a velocity resolution of 20 km s−1 and a pixel scale of 0. ′′02 pixel−1 for IRAS 00091−0738. The HCN-to-HCO+ flux ratios are calculated in brightness temperature (K). (This is virtually identical to the ratio in flux density (Jy ∝ K × ν 2 ), because the HCN and HCO+ frequencies differ only by a factor of ∼1.006 at the … view at source ↗

**Figure 2.** Figure 2: Same as [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗

**Figure 3.** Figure 3: Same as [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗

**Figure 4.** Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗

**Figure 5.** Figure 5: Same as [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗

**Figure 6.** Figure 6: Same as [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗

**Figure 7.** Figure 7: Same as [PITH_FULL_IMAGE:figures/full_fig_p013_7.png] view at source ↗

**Figure 8.** Figure 8: Same as [PITH_FULL_IMAGE:figures/full_fig_p014_8.png] view at source ↗

**Figure 9.** Figure 9: Same as [PITH_FULL_IMAGE:figures/full_fig_p017_9.png] view at source ↗

read the original abstract

We present the results of our investigations of spectrally and spatially resolved (sub)millimeter HCN-to-HCO$^{+}$ flux ratios at J=2-1, J=3-2, and/or J=4-3 in 18 nearby ($z <$ 0.15) ultraluminous infrared galaxies (ULIRGs), using ALMA $\lesssim$0.2" ($\lesssim$500 pc) resolution data. The geometry of elevated HCN-to-HCO$^{+}$ flux ratios (with $>$3$\sigma$ detections for both molecular lines) in position-position-velocity (PPV) space is visually classified into (i) spherical shell (spectrally and spatially distinct), (ii) spectrally distinct and spatially compact, and (iii) filled (spectrally filled and spatially compact). These can naturally be explained by the elevation of the flux ratio due to (i) a spatially resolved outflow, (ii) an AGN and/or a spatially unresolved outflow with blueshifted and redshifted emission components, and (iii) an AGN and/or a spatially confined outflow with not clearly separated blueshifted and redshifted velocity components, respectively. Signatures of elevated HCN-to-HCO$^{+}$ flux ratios originated from (a) spatially resolved outflow and (b) AGN and/or spatially unresolved outflow are seen in seven and nine ULIRGs, respectively. In the former spatially resolved outflow-origin case, modest-velocity components relative to the maximum outflow velocity tend to be probed by spaxels with elevated HCN-to-HCO$^{+}$ flux ratios. The spectrally and spatially resolved HCN-to-HCO$^{+}$ flux ratios can provide additional information on the physical origin of the elevated flux ratios in nearby ULIRG nuclei, compared to previously conducted spatially integrated and/or velocity-integrated analyses.

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.

Desk Editor's Note private letter to a colleague

New ALMA resolved HCN/HCO+ maps in 18 ULIRGs extend prior work, but the PPV geometry classifications rest on untested visual assignments to physical origins.

read the letter

The main point is new ALMA data at sub-arcsecond resolution that map HCN-to-HCO+ flux ratios in position-position-velocity space across 18 nearby ULIRGs. They classify regions of elevated ratios into three visual categories—spherical shell, spectrally distinct compact, and filled—and tie these to resolved outflows, AGN or unresolved outflows, and confined outflows respectively. Seven objects show the resolved-outflow signature and nine the AGN/unresolved type. This moves past the integrated ratios that dominated earlier studies and supplies a more detailed view of where the high ratios appear in these nuclei. The observations themselves look like a solid addition for people tracking dense-gas tracers in luminous infrared galaxies. Multiple transitions are covered and the sample size is reasonable for this class of object. The abstract presents the results clearly without overclaiming. The soft spot is the step from observed geometry to physical mechanism. The paper states that the three categories “can naturally be explained by” the listed origins, yet supplies no radiative-transfer calculations, mock ALMA cubes, or direct comparisons to independent outflow or AGN tracers such as OH absorption or optical kinematics. Visual classification also leaves open how repeatable the categories would be between different inspectors and whether the same PPV morphology could arise from more than one process. Without those checks the reported counts of seven and nine remain interpretive rather than demonstrated. Minor issues include the lack of stated quantitative thresholds beyond the 3-sigma detection cut and no discussion of how noise or beam effects might influence the filled versus compact distinction. This work is aimed at specialists in extragalactic molecular gas and AGN feedback. The raw maps and the attempt at a spatially resolved diagnostic are worth referee time even if the interpretation needs tightening. I would send it to peer review with the expectation that the authors add some validation or at least clearer caveats on the classification step.

Referee Report

1 major / 2 minor

Summary. The manuscript presents ALMA observations of spectrally and spatially resolved HCN-to-HCO+ flux ratios (J=2-1, 3-2, and/or 4-3) in 18 nearby (z<0.15) ULIRGs at ≲0.2″ resolution. Elevated-ratio regions (>3σ detections) in PPV space are visually classified into three geometry types—spherical shell, spectrally distinct compact, and filled—which are interpreted as arising from (i) spatially resolved outflows, (ii) AGN and/or unresolved outflows, and (iii) AGN and/or confined outflows, respectively. The work reports such signatures in seven ULIRGs for resolved-outflow cases and nine for AGN/unresolved-outflow cases, and argues that the resolved analysis supplies additional physical-origin information beyond integrated studies.

Significance. If the visual-to-physical mapping can be placed on firmer quantitative footing, the approach supplies a new spatially-spectrally resolved diagnostic for separating outflow and AGN contributions in ULIRG nuclei. The use of multiple transitions and sub-arcsecond ALMA data is a clear strength relative to prior integrated analyses.

major comments (1)

[Abstract and classification section] Abstract and classification description: the central counts (7 ULIRGs with resolved-outflow signatures, 9 with AGN/unresolved-outflow signatures) rest on assigning the three visual PPV geometry categories to distinct physical mechanisms. The text states only that the geometries “can naturally be explained by” those origins, without radiative-transfer simulations, mock ALMA cubes, quantitative classification criteria, inter-observer tests, or cross-checks against independent tracers (e.g., OH absorption or [O III] kinematics). This renders the reported tallies interpretive rather than demonstrated.

minor comments (2)

[Abstract] The abstract and results do not define quantitative thresholds or velocity ranges for “modest-velocity components” or “elevated” ratios beyond the >3σ detection criterion, nor do they report uncertainties or reproducibility metrics for the visual classifications.
[Figures and methods] Figure captions and text should explicitly state the exact velocity integration ranges and spatial masking criteria used to construct the PPV diagrams shown for each source.

Simulated Author's Rebuttal

1 responses · 2 unresolved

We thank the referee for their constructive review and for highlighting the interpretive aspects of our visual classification. We address the major comment below and have revised the manuscript to clarify the basis and limitations of our approach.

read point-by-point responses

Referee: [Abstract and classification section] Abstract and classification description: the central counts (7 ULIRGs with resolved-outflow signatures, 9 with AGN/unresolved-outflow signatures) rest on assigning the three visual PPV geometry categories to distinct physical mechanisms. The text states only that the geometries “can naturally be explained by” those origins, without radiative-transfer simulations, mock ALMA cubes, quantitative classification criteria, inter-observer tests, or cross-checks against independent tracers (e.g., OH absorption or [O III] kinematics). This renders the reported tallies interpretive rather than demonstrated.

Authors: We agree that the reported counts of 7 and 9 ULIRGs derive from visual classification of PPV geometries and that the physical interpretations are suggested rather than quantitatively demonstrated. The manuscript already employs the phrasing 'can naturally be explained by' and 'visually classified' to signal this. In the revised version we have added an explicit paragraph in the classification section stating that the assignments are qualitative and visual, discussing the absence of radiative-transfer simulations or mock cubes in the present work, and noting that quantitative criteria and inter-observer tests lie beyond the current scope. Where literature cross-checks with OH absorption or [O III] kinematics exist for individual sources we have referenced them; a systematic comparison for the full sample is not available. Full radiative-transfer modeling and mock ALMA observations would require a separate, computationally intensive study and cannot be completed within the revisions to this observational paper. revision: partial

standing simulated objections not resolved

Conducting new radiative-transfer simulations and generating mock ALMA cubes to place the geometry-to-mechanism mapping on quantitative footing
Developing quantitative classification criteria and performing inter-observer reliability tests for the PPV geometries

Circularity Check

0 steps flagged

No circularity: purely observational classification of ALMA data features with no fitted parameters or self-referential derivations

full rationale

The paper reports direct measurements of HCN-to-HCO+ flux ratios from ALMA (sub)millimeter data in 18 ULIRGs and performs visual classification of elevated-ratio geometries in PPV space into three categories (spherical shell, spectrally distinct compact, filled). These are tallied as 7 ULIRGs showing resolved-outflow signatures and 9 showing AGN/unresolved-outflow signatures. The text states that the geometries 'can naturally be explained by' the listed physical origins but provides no equations, model fits, or derivations that reduce the classifications or counts to the input data by construction. No self-citations, ansatzes, or uniqueness theorems are invoked as load-bearing steps. The analysis is self-contained against the observed cubes and does not rename known results or smuggle assumptions via prior work by the same authors.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is observational and relies on standard astrophysical interpretations of molecular line ratios rather than new derivations or postulates.

axioms (1)

domain assumption Elevated HCN-to-HCO+ flux ratios trace regions influenced by AGN radiation or shocks in outflows
Invoked when linking the three geometric classes to physical origins; standard in extragalactic molecular astrophysics.

pith-pipeline@v0.9.0 · 5693 in / 1302 out tokens · 49146 ms · 2026-05-11T01:15:20.020346+00:00 · methodology

discussion (0)

Reference graph

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