Azimuthal offsets in spiral arms of nearby galaxies

¹ Observatorio Astronómico Nacional (IGN), C/Alfonso XII, 3 E-28014, Madrid, Spain
² Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9 B-9000, Gent, Belgium
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1 D-85748, Garching, Germany
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71 53121 Bonn, Germany
⁵ European Southern Observatory, Karl-Schwarzschild Straße 2 D-85748, Garching bei München, Germany
⁶ Univ Lyon, Univ Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval, France
⁷ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str 2 D-69120, Heidelberg, Germany
⁸ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205 D-69120, Heidelberg, Germany
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹¹ National Radio Astronomy Observatory, 800 Bradbury SE, Suite 235, Albuquerque, NM 87106, USA
¹² Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210, USA
¹³ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040, Spain
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17 D-69117, Heidelberg, Germany
¹⁵ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁶ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
¹⁸ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁹ Department of Physics, University of Connecticut, Storrs, CT 06269, USA
²⁰ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
²¹ IRAP, OMP, Université de Toulouse, 9 Av. du Colonel Roche, BP 44346 F-31028, Toulouse cedex 4, France
²² AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²³ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14 D-69120, Heidelberg, Germany
²⁴ Purple Mountain Observatory, No. 10 Yuanhua Road, Qixia District 210023, Nanjing, China
²⁵ Department of Physics, Tamkang University, No.151, Yingzhuan Rd., Tamsui Dist., New Taipei City 251301, Taiwan
²⁶ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n E-38205, La Laguna, Spain
²⁷ Departamento de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Francisco Sánchez s/n E-38206, La Laguna, Spain
²⁸ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
²⁹ Faculty of Global Interdisciplinary Science and Innovation, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
³⁰ Department of Astrophysical Sciences, Princeton University, 4 Ivy Ln., Princeton, NJ 08544, USA

^⋆ Corresponding author.

Received: 30 June 2025
Accepted: 4 August 2025

Abstract

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 Hα 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 Hα peaks lie ahead of CO peaks in the direction of galactic rotation. With this convention, 14 galaxies show mean positive CO-Hα 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.