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Azimuthal offsets in spiral arms of nearby galaxies

Spiral arms play a central role in disc galaxies, but their dynamical nature remains a long-standing open question. Azimuthal offsets between molecular gas and star formation are expected if gas crosses spiral arms, as predicted by quasi-stationary density wave theory. In this work, we measure offse...

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Main Authors: Querejeta, Miguel (Author) , Meidt, Sharon E. (Author) , Cao, Yixian (Author) , Colombo, Dario (Author) , Emsellem, Eric (Author) , García-Burillo, Santiago (Author) , Klessen, Ralf S. (Author) , Koch, Eric W. (Author) , Leroy, Adam K. (Author) , Ruiz-García, Marina (Author) , Schinnerer, Eva (Author) , Smith, Rowan (Author) , Krieger, Sophia K. (Author) , Thorp, Mallory (Author) , Williams, Thomas G. (Author) , Boquien, Médéric (Author) , Dale, Daniel A. (Author) , Faesi, Chris (Author) , Gleis, Damian R. (Author) , Grasha, Kathryn (Author) , Hughes, Annie (Author) , Jiménez-Donaire, María J. (Author) , Kreckel, Kathryn (Author) , Liu, Daizhong (Author) , Neumann, Justus (Author) , Pan, Hsi-An (Author) , Pinna, Francesca (Author) , Razza, Alessandro (Author) , Saito, Toshiki (Author) , Sun, Jiayi (Author) , Usero, Antonio (Author)
Format: Article (Journal)
Language:English
Published: 12 September 2025
In: Astronomy and astrophysics
Year: 2025, Volume: 701, Pages: 1-21
ISSN:1432-0746
DOI:10.1051/0004-6361/202556175
Online Access:Verlag, kostenfrei, Volltext: https://doi.org/10.1051/0004-6361/202556175
Verlag, kostenfrei, Volltext: https://www.aanda.org/articles/aa/abs/2025/09/aa56175-25/aa56175-25.html
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Author Notes:Miguel Querejeta, Sharon E. Meidt, Yixian Cao, Dario Colombo, Eric Emsellem, Santiago García-Burillo, Ralf S. Klessen, Eric W. Koch, Adam K. Leroy, Marina Ruiz-García, Eva Schinnerer, Rowan Smith, Sophia Stuber, Mallory Thorp, Thomas G. Williams, Médéric Boquien, Daniel A. Dale, Chris Faesi, Damian R. Gleis, Kathryn Grasha, Annie Hughes, María J. Jiménez-Donaire, Kathryn Kreckel, Daizhong Liu, Justus Neumann, Hsi-An Pan, Francesca Pinna, Alessandro Razza, Toshiki Saito, Jiayi Sun, and Antonio Usero
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Summary:Spiral arms play a central role in disc galaxies, but their dynamical nature remains a long-standing open question. Azimuthal offsets between molecular gas and star formation are expected if gas crosses spiral arms, as predicted by quasi-stationary density wave theory. In this work, we measure offsets between CO and H<i>α<i/> peaks in radial bins for 24 galaxies from the PHANGS survey that display a well-delineated spiral structure. The offsets exhibit substantial scatter, implying that star formation is not exclusively initiated at a coherent spiral shock. We define offsets such that positive values mean H<i>α<i/> peaks lie ahead of CO peaks in the direction of galactic rotation. With this convention, 14 galaxies show mean positive CO-H<i>α<i/> offsets, typically of a few hundred parsecs. In four of these 14 galaxies (17% of the total), offsets become smaller with increasing radius, as expected for a single quasi-stationary spiral density wave. Ten galaxies (42%) show positive mean offsets but no clear correlation with radius, which is compatible with multiple overlapping modes. In the remaining ten galaxies (42%), we find no significantly positive offsets, which could point to transient dynamical spirals or material arms, where gas and stars co-rotate with the spiral perturbation. Across the full sample, we find mostly positive offsets between CO peaks and the gravitational potential minimum, confirming that gas often crosses the spiral perturbation. For the four galaxies with clear positive offsets and a radial trend, we derived pattern speeds in good agreement with the literature. Overall, our results suggest that even well-delineated spirals in the local Universe can arise from a variety of underlying dynamical mechanisms.
Item Description:Gesehen am 20.01.2026
Physical Description:Online Resource
ISSN:1432-0746
DOI:10.1051/0004-6361/202556175

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