arxiv: 2604.15438 · v1 · submitted 2026-04-16 · 🌌 astro-ph.GA

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Surveying the Whirlpool at Arcseconds with NOEMA (SWAN). IV. Extent of active galactic nucleus feedback on the interstellar medium

Mallory D. Thorp , Antonio Usero , Frank Bigiel , Ina Gali\'c , Smita Mathur , Sophia K. Stuber , Jennifer A. Rodriguez , Dario Colombo

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Bret Lehmer Eva Schinnerer Amirnezam Amiri Ashley Barnes Zein Bazzi Guillermo A. Blanc Cosima Eibensteiner Simon Glover Laura A. Lopez Erik B. Monson Lukas Neumann Jerome Pety Miguel Querejeta Thomas G. Williams

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Pith reviewed 2026-05-10 09:57 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords active galactic nucleiAGN feedbackinterstellar mediummolecular gasM51dense gas tracersjet-ISM interactionSeyfert nucleus

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The pith

In M51, AGN feedback excites HCN and similar molecules via jet-driven soft X-rays over at least 180 parsecs.

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

The paper maps AGN-dominated regions in M51 using an emission line ratio function from optical integral field spectroscopy and compares them directly to high-resolution NOEMA maps of dense molecular tracers. It finds that HCN, HNC, and HCO+ lines are brighter in these regions than expected from dense gas traced by N2H+ alone, indicating extra excitation by AGN processes. This pattern, together with shock tracers and nuclear outflow signatures, supports a two-stage sequence in which mechanical jet-ISM interactions first generate soft X-rays that then excite the molecules. The result shows AGN feedback can alter the excitation state of the interstellar medium on scales of hundreds of parsecs, offering a concrete way to connect nuclear activity to changes in the dense gas that fuels star formation.

Core claim

Using an emission line ratio function to select AGN-ionized regions and cross-matching with 180-pc resolution maps of HCN(1-0), HNC(1-0), HCO+(1-0), and N2H+(1-0), the study shows that AGN regions exhibit excess emission in the first three lines relative to what N2H+ predicts for dense gas alone. This excess is attributed to excitation by soft X-rays. The highest line-ratio values coincide with optical and molecular shock tracers, consistent with a dense molecular outflow in the nucleus. All tracers together indicate a two-stage feedback process in which jet-ISM mechanical interactions produce the soft X-rays that excite the molecules, while chemical abundance changes from entrained densegas

What carries the argument

The emission line ratio (ELR) function from VENGA spectroscopy that demarcates AGN-dominated ISM regions at 180 pc scales, cross-compared against SWAN maps of dense-gas tracers.

If this is right

AGN influence on ISM excitation extends at least to the 180 pc resolution limit in M51.
Mechanical jet feedback precedes and enables the radiative soft-X-ray excitation stage.
The ELR diagnostic identifies AGN-affected gas more cleanly than the molecular ratio HCN/HCO+.
Dense gas in the nuclear outflow may raise tracer abundances, but excitation dominates the observed enhancements.
All AGN-activity tracers converge on the same two-stage sequence rather than conflicting.

Where Pith is reading between the lines

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

The same staged process may operate in other low-luminosity Seyfert systems and could be tested with matched optical-mm datasets.
Combining optical ionization maps with mm-wave dense-gas lines offers a practical way to separate excitation from abundance changes in AGN environments.
If the two-stage picture holds, AGN feedback can modify dense-gas properties across hundreds of parsecs without requiring immediate full quenching of star formation.

Load-bearing premise

N2H+ traces only the amount of dense cold gas and is not itself boosted by AGN shocks, ionization, or outflows.

What would settle it

If maps at higher resolution or with additional tracers show the line excesses scaling directly with total gas column density rather than with ionization or shock indicators, the excitation-by-X-rays claim would be falsified.

Figures

Figures reproduced from arXiv: 2604.15438 by Amirnezam Amiri, Antonio Usero, Ashley Barnes, Bret Lehmer, Cosima Eibensteiner, Dario Colombo, Erik B. Monson, Eva Schinnerer, Frank Bigiel, Guillermo A. Blanc, Ina Gali\'c, Jennifer A. Rodriguez, Jerome Pety, Laura A. Lopez, Lukas Neumann, Mallory D. Thorp, Miguel Querejeta, Simon Glover, Smita Mathur, Sophia K. Stuber, Thomas G. Williams, Zein Bazzi.

**Figure 1.** Figure 1: Map of the ELR [NII] function over the SWAN field of view of M51, with HCN emission line contours shown in black for reference. The BPT diagram used to establish the ELR is shown in the top right, with values ranging from 0-0.25 for star-forming regions as defined by the Kauffmann et al. (2003) criteria (dashed green line) and values exceeding 0.5 for AGN regions (above the Kewley et al. (2001) criteria,… view at source ↗

**Figure 2.** Figure 2: Dense gas relations with N2H + , with alternative dense gas tracers on the x axis, color-coded by the ELR function. Only points with a measurable ELR, which have a S/N greater than three for both lines, are included as colored points. Gray points are the remainder of pixels where the ELR is not measured, and the S/N of one or both emission lines is less than 3. The ELR accurately distinguishes “problem reg… view at source ↗

**Figure 3.** Figure 3: Dense gas relation of N2H + vs. HCN as shown in [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗

**Figure 4.** Figure 4: Heat matrix for how well different tracers of AGN activity (x axis) correlate with line ratios from SWAN (y axis). Kendall tau correlation coefficients are shown in each box, and set the color scheme (with blue being highly correlated, red being highly anticorrelated, and yellow showing no correlation). p-values are shown in parentheses below. The strongest correlation for each SWAN line ratio is shown in … view at source ↗

**Figure 5.** Figure 5: 3D BPT diagrams, with axes corresponding to the velocity dispersion of Hα (Hα σv in km/s, the distance from the center of the galaxy in kiloparsecs, and the ELR function. Points are color-coded by relevant line ratios. Top: (1.) HCN/CO ratios are largest where the ELR is largest, with a secondary dependency on Hα σv , clear evidence of AGN feedback leading to excitation of HCN. (2.) N2H + /HCN ratios drop … view at source ↗

**Figure 6.** Figure 6: Variety and extent of AGN impact as a function of the chosen indicator. Contours of the 6 cm radio continuum shown for comparison in purple. The pixel that hosts the AGN is circled in black, position provided by Hagiwara & Edwards (2015). Top: (1.) N2H + /HCN ratio, demonstrating where the two dense gas tracers diverge (2.) HCN/HCO+ ratio, traditionally used to distinguish PDRs from XDRs. Generally a ratio… view at source ↗

**Figure 7.** Figure 7: 0.5–8 keV Chandra spectrum of the nucleus of M51 with the best-fit model. The dotted lines in the soft (0.5—2 keV) band show the two thermal components. In the hard (2–8 keV) band the strong Fe Kα is seen which is the tell-tale sign of the reflection dominated continuum. The direct AGN continuum is completely obscured; the reflected continuum is shown with the dotted line. more apec which provided our bes… view at source ↗

read the original abstract

Active Galactic Nuclei (AGN) are intertwined with galaxy evolution, injecting energy into the interstellar medium (ISM) that could regulate star formation as a galaxy evolves. However, the phenomena through which we observe AGN are multiphase and multiscale, which can lead to conflicting results for how significantly AGN influence the ISM. We endeavor to characterize the spatial extent and dominant modes of AGN feedback in M51, which hosts a low-luminosity Seyfert nucleus and multi-phase outflow. We identified regions dominated by AGN ionization using an emission line ratio (ELR) function constructed from VENGA integral field spectroscopy. We then investigated how AGN feedback influences the ISM using cloud-scale mapping of dense molecular gas tracers HCN(1-0), HNC(1-0), HCO+(1-0), and N2H+(1-0) provided by SWAN. This combined dataset has a resolution of 180pc, providing a clear demarcation of where AGN feedback dominates the ISM. If we assume that N2H+ is the best tracer of dense, cold gas in SWAN, then AGN-dominated regions defined by the ELR all have greater emission in (1-0) transitions in HCN, HNC, and HCO+ than expected if they traced dense gas alone, implying excitation of these lines from the AGN. The ELR better selects these regions than molecular tracers of AGN activity like HCN/HCO+. The highest ELR values are also associated with optical and molecular shock tracers (HNCO/CO), indicating a potential dense molecular outflow in the nucleus that agrees with the heightened N2H+ emission in this limited region. All tracers of AGN activity point to a "two-stage" feedback scenario, whereby mechanical feedback from the jet-ISM interaction spurs soft X-ray emission that excites molecules such as HCN. Dense gas entrenched in a molecular outflow may also lead to a greater chemical abundance of multiple tracers measured with SWAN, but to a lesser extent than excitation from AGN feedback.

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

The paper maps AGN feedback extent in M51 by overlaying optical ELR regions on SWAN molecular maps and argues for a two-stage jet-to-X-ray process, but the excess HCN/HNC/HCO+ signal depends on N2H+ being a clean dense-gas baseline.

read the letter

The main point is that this work combines VENGA emission-line ratios with the existing SWAN NOEMA maps of HCN, HNC, HCO+, and N2H+ to show where AGN ionization dominates the ISM in M51 at 180 pc scales. They find that ELR-selected AGN regions show brighter HCN(1-0), HNC(1-0), and HCO+(1-0) than expected from N2H+(1-0) alone, which they interpret as soft X-ray excitation on top of density effects, plus a nuclear molecular outflow traced by shocks and elevated N2H+.

Referee Report

1 major / 2 minor

Summary. The paper claims that in M51, AGN-dominated regions identified via an emission line ratio (ELR) function from VENGA integral-field spectroscopy exhibit excess emission in HCN(1-0), HNC(1-0), and HCO+(1-0) relative to N2H+(1-0) when mapped at 180 pc resolution with NOEMA SWAN data. Assuming N2H+ traces dense cold gas without AGN influence, this excess is interpreted as soft X-ray excitation from AGN feedback. The work concludes that all tracers support a two-stage feedback scenario in which jet-ISM mechanical interaction drives soft X-ray emission that excites molecules, with additional evidence for a dense molecular outflow from shock tracers (HNCO/CO) and elevated nuclear N2H+.

Significance. If the quantitative line-ratio comparisons and the N2H+ baseline hold, the result would provide a concrete observational example of the spatial extent and dominant mode of low-luminosity AGN feedback on dense molecular gas in a nearby galaxy. The multi-tracer approach at cloud scales, combining optical ELR selection with mm-wave mapping of four dense-gas species plus shock indicators, offers a useful template for distinguishing mechanical versus radiative feedback effects and for testing whether ELR outperforms molecular ratios such as HCN/HCO+ for region selection. The explicit two-stage scenario supplies a falsifiable prediction that can be checked in other systems.

major comments (1)

[Abstract] Abstract: The central claim that AGN regions show greater HCN, HNC, and HCO+ emission than expected from dense-gas tracing alone rests on the assumption that N2H+ is the least-affected dense-gas tracer. The same abstract reports heightened N2H+ emission in the nucleus linked to the molecular outflow, indicating that AGN feedback can alter N2H+ as well. If N2H+ is enhanced across other ELR-selected regions (via shocks, X-rays, or abundance changes), the apparent excess in the other lines becomes an artifact of the normalization rather than evidence for differential excitation. No quantitative test (N2H+ intensity ratios between ELR bins, comparison to shock/chemistry models, or error budget on the excess) is described to confirm N2H+ remains a stable baseline.

minor comments (2)

[Abstract] The abstract states the resolution as 180 pc but does not specify the synthesized beam size, weighting scheme, or how the NOEMA data were combined with VENGA; these details belong in the methods section for reproducibility.
[Abstract] The ELR function is introduced without listing the specific optical lines or the functional form used; a brief equation or reference to the prior VENGA paper would improve clarity.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for identifying a key point that requires clarification regarding our use of N2H+ as a baseline tracer. We address this concern directly below and will incorporate revisions to strengthen the presentation of the analysis.

read point-by-point responses

Referee: [Abstract] Abstract: The central claim that AGN regions show greater HCN, HNC, and HCO+ emission than expected from dense-gas tracing alone rests on the assumption that N2H+ is the least-affected dense-gas tracer. The same abstract reports heightened N2H+ emission in the nucleus linked to the molecular outflow, indicating that AGN feedback can alter N2H+ as well. If N2H+ is enhanced across other ELR-selected regions (via shocks, X-rays, or abundance changes), the apparent excess in the other lines becomes an artifact of the normalization rather than evidence for differential excitation. No quantitative test (N2H+ intensity ratios between ELR bins, comparison to shock/chemistry models, or error budget on the excess) is described to confirm N2H+ remains a stable baseline.

Authors: We acknowledge the referee's point that the nuclear N2H+ enhancement must be reconciled with the assumption of N2H+ as a stable baseline. The manuscript explicitly states that this enhancement is localized to the nuclear region and associated with the molecular outflow traced by shock indicators (HNCO/CO), while the broader ELR-selected AGN-dominated regions at 180 pc resolution do not exhibit comparable N2H+ excess. The excess in HCN, HNC, and HCO+ is measured relative to N2H+ in these regions, supporting differential excitation. To address the lack of explicit quantitative tests, we will add in the revised manuscript: (i) direct N2H+ intensity ratio comparisons across ELR bins with error bars, (ii) an error budget on the reported excesses, and (iii) clarification in the abstract and methods that the N2H+ enhancement is confined to the nucleus. We do not perform new shock/chemistry model comparisons in this work, as the multi-tracer spatial mapping and ELR selection provide the primary evidence, but the added quantitative tests will allow readers to evaluate the baseline assumption directly. revision: yes

Circularity Check

0 steps flagged

No circularity; central result is observational comparison conditional on explicit assumption

full rationale

The paper defines AGN-dominated regions independently via an ELR function from VENGA integral-field spectroscopy, then compares SWAN molecular-line intensities (HCN, HNC, HCO+, N2H+) within those regions. The inference that HCN/HNC/HCO+ show excess emission relative to N2H+ is presented explicitly as conditional on the assumption that N2H+ traces dense cold gas without AGN influence; this assumption is stated outright rather than derived from any equation or prior result in the paper. No fitted parameters are relabeled as predictions, no self-citations supply load-bearing uniqueness theorems, and no ansatz or renaming reduces the claimed two-stage feedback scenario to its inputs by construction. The analysis therefore remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim depends on one domain assumption about which molecular line best traces undisturbed dense gas and on the interpretive step that excess line emission must be AGN-driven excitation rather than abundance or optical-depth effects.

axioms (1)

domain assumption N2H+(1-0) is the best tracer of dense, cold gas among the observed lines
Invoked to establish the baseline expectation against which HCN, HNC, and HCO+ enhancements are measured.

pith-pipeline@v0.9.0 · 5781 in / 1371 out tokens · 31888 ms · 2026-05-10T09:57:28.695773+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

5 extracted references · 1 canonical work pages

[1]

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work page Pith review doi:10.1086/499298 2006