The nuclear star cluster of M 74: A fossil record of the very early stages of a star-forming galaxy

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n, E-38206 La Laguna, Spain
³ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
⁵ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁶ Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, ACT 2611, Australia
⁷ Universität Heidelberg, 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 Observatorio Astronómico Nacional (IGN), C/Alfonso XII, 3, E-28014 Madrid, Spain
¹⁰ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹¹ Astronomy Department, The Ohio State University, Columbus, Ohio, USA
¹² Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, Ohio, USA

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Received: 11 November 2025
Accepted: 2 March 2026

Abstract

Nuclear star clusters (NSC) are dense and compact stellar systems with sizes of a few parsecs located at galactic centers. Their properties and formation mechanisms seem to be tightly linked to the evolution of the host galaxy, with potentially different formation channels for late- and early-type galaxies (LTGs and ETGs). While most observations target ETGs, here we focus on the NSC in M 74 (NGC 628), a relatively massive and gas-rich star-forming spiral galaxy included in the PHANGS survey. We analyzed the central arc minute of the PHANGS-MUSE mosaic, in which the NSC is not spatially resolved. We analyzed the NSC stellar populations in a point spread function (PSF) aperture and compared it to the host galaxy. Within the PSF size, the NSC is contaminated by the host galaxy light. We performed a two-dimensional spectro-photometric decomposition of the MUSE cube, employing a modified version of the C2D code, to disentangle the NSC from its host. This method provided different data cubes for the NSC and the host galaxy, allowing for their comparison in a PSF aperture, as well as a spatially resolved analysis of the host. Our results show a very old and metal-poor NSC, in contrast to the surrounding regions. While similar properties have been found in NSCs hosted by galaxies of different masses and/or morphological types from M 74, they are somewhat unexpected for a relatively massive star-forming spiral galaxy. The spatially resolved stellar populations of the host galaxy display much younger (light-weighted) ages and higher metallicities, especially in the central region (∼500 pc) surrounding the NSC. This suggests that this NSC formed a long time ago and evolved passively until today without any further growth. No significant amounts of gas would have reached the very central region in the past 8 Gyr.