Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)
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 N 2 H + (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 N 2 H + (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 N 2 H + (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.
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BibTeX
@article{thorp2026surveying,
title = {{Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)}},
author = {Mallory D Thorp and Antonio Usero and Frank Bigiel and Ina Galić and Smita Mathur and Sophia K Stuber and Jennifer A Rodriguez and Dario Colombo and Bret Lehmer and Eva Schinnerer and Amirnezam Amiri and Ashley T Barnes and Zein Bazzi and Guillermo A Blanc and Cosima Eibensteiner and Simon Glover and Laura A Lopez and Erik B Monson and Lukas Neumann and Jérôme Pety and Miguel Querejeta and Thomas G Williams},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = may,
doi = {10.1051/0004-6361/202558700},
url = {https://hal.science/hal-05761217v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)
AU - Mallory D Thorp
AU - Antonio Usero
AU - Frank Bigiel
AU - Ina Galić
AU - Smita Mathur
AU - Sophia K Stuber
AU - Jennifer A Rodriguez
AU - Dario Colombo
AU - Bret Lehmer
AU - Eva Schinnerer
AU - Amirnezam Amiri
AU - Ashley T Barnes
AU - Zein Bazzi
AU - Guillermo A Blanc
AU - Cosima Eibensteiner
AU - Simon Glover
AU - Laura A Lopez
AU - Erik B Monson
AU - Lukas Neumann
AU - Jérôme Pety
AU - Miguel Querejeta
AU - Thomas G Williams
PY - 2026
DA - 2026/05/25
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202558700
UR - https://hal.science/hal-05761217v1
AB - 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 N 2 H + (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 N 2 H + (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 N 2 H + (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.
SN - 0004-6361
SN - 1432-0746
ER -