Do spiral arms enhance star formation efficiency?

¹ Observatorio Astronómico Nacional (IGN), C/Alfonso XII, 3, 28014 Madrid, Spain
e-mail: m.querejeta@oan.es
² Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210, USA
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50157 Firenze, Italy
⁶ European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
⁷ Univ Lyon, Univ Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁸ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
⁹ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
¹⁰ Department of Astrophysical Sciences, Princeton University, 4 Ivy Ln., Princeton, NJ 08544, USA
¹¹ Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
¹² Department of Physics, University of Surrey, Guildford GU2 7XH, UK
¹³ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 75014 Paris, France
¹⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
¹⁵ Cosmic Origins Of Life (COOL) Research DAO, https://coolresearch.io
¹⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁷ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁸ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁹ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
²⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²¹ Department of Physics, Tamkang University, No.151, Yingzhuan Rd., Tamsui Dist., New Taipei City 251301, Taiwan
²² Leibniz-Institut for Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²³ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38406 Saint Martin d’Hères, France
²⁴ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 La Laguna, Spain
²⁵ Departamento de Astrofísica, Universidad de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Spain
²⁶ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
²⁷ Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada
²⁸ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
²⁹ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³¹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

Received: 26 February 2024
Accepted: 22 April 2024

Abstract

Spiral arms, as those of our own Milky Way, are some of the most spectacular features in disc galaxies. It has been argued that star formation should proceed more efficiently in spiral arms as a result of gas compression. Yet, observational studies have so far yielded contradictory results. Here, we examine arm/interarm surface density contrasts at ∼100 pc resolution in 28 spiral galaxies from the PHANGS survey. We find that the arm/interarm contrast in stellar mass surface density (Σ_⋆) is very modest, typically a few tens of percent. This is much smaller than the contrasts measured for molecular gas (Σ_mol) or star formation rate (Σ_SFR) surface density, which typically reach a factor of ∼2 − 3. However, Σ_mol and Σ_SFR contrasts show a significant correlation with the enhancement in Σ_⋆, suggesting that the small stellar contrast largely dictates the stronger accumulation of gas and star formation. All these contrasts increase for grand-design spirals compared to multi-armed and flocculent systems (and for galaxies with high stellar mass). The median star formation efficiency (SFE) of the molecular gas is 16% higher in spiral arms than in interarm regions, with a large scatter, and the contrast increases significantly (median SFE contrast 2.34) for regions of particularly enhanced stellar contrast (Σ_⋆ contrast > 1.97). The molecular-to-atomic gas ratio (Σ_mol/Σ_atom) is higher in spiral arms, pointing to a transformation of atomic to molecular gas. As a consequence, the total gas contrast (Σ_mol + Σ_atom) slightly drops compared to Σ_mol (median 4% lower, working at ∼kpc resolution), while the SFE contrast increases when we include atomic gas (median 8% higher than for Σ_mol). The contrasts show important fluctuations with galactocentric radius. We confirm that our results are robust against a number of effects, such as spiral mask width, tracers, resolution, and binning. In conclusion, the boost in the SFE of molecular gas in spiral arms is generally modest or absent, except for locations with exceptionally large stellar contrasts.