Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)

IV. Extent of active galactic nucleus feedback on the interstellar medium

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, E-28014 Madrid, Spain
³ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
⁴ Center for Cosmology and Astro-Particle Physics, Ohio State University, Columbus, OH, USA
⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
⁹ Arkansas Center for Space and Planetary Sciences, University of Arkansas, 332 N. Arkansas Ave, Fayetteville, AR 72701, USA
¹⁰ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹¹ Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
¹² Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
¹³ National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
¹⁴ Institut für Theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle-Str 2, D-69120 Heidelberg, Germany
¹⁵ Department of Physics and Astronomy, Middle Tennessee State University, 1301 E. Main St Box 71 Murfreesboro, TN 37312, USA
¹⁶ IRAM, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France
¹⁷ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
¹⁸ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 December 2025
Accepted: 25 March 2026

Abstract

Context. Active Galactic Nuclei (AGN) are intertwined with galaxy evolution, injecting energy into the interstellar medium (ISM) and possibly regulating 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 and to what extent AGN influence the ISM.

Aims. M51 is a perfect case study of the boundary between where AGN feedback and star formation feedback dominate the ISM, hosting a low-luminosity type II Seyfert nucleus with a well-defined molecular and ionized outflow. We endeavor to characterize the spatial extent and dominant modes of AGN feedback in M51 utilizing multiple phases of the ISM.

Methods. Using integral field spectroscopy observations from VENGA of the central 3 kpc, we identified regions dominated by AGN ionization using an emission line ratio (ELR) function. We then combined this information with new observations of the dense molecular ISM in M51 from SWAN, including cloud-scale mapping of HCN(1–0), HNC(1–0), HCO⁺(1–0), and N₂H⁺(1–0). Both datasets allowed us to achieve ∼180 pc resolution, allowing for a clear demarcation of where AGN feedback dominates the ISM. We then tested how the ELR compares to other tracers of AGN activity, using both millimeter emission line ratios as well as X-ray observations from Chandra to assess the dominant mode of feedback.

Results. If we assume that N₂H⁺(1–0) 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 would be expected if they traced dense gas alone, implying excitation of these lines from AGN feedback. The ELR is better at selecting these regions compared to molecular tracers of AGN activity, such as HCN(1–0)/HCO⁺(1–0), which are heightened for a greater extent in M51. Some of the highest ELR values are also associated with fast shocks evident in the optical, which are concurrent with large HNCO(4–3)/CO(1–0) values that point to slow shocks near the nucleus. The presence of shocks and heightened N₂H⁺(1–0) near the nucleus indicate a potential dense molecular outflow, meaning heightened dense tracer emission could be partly due to larger abundance rather than excitation alone in this limited region.

Conclusions. 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.