The m_max-M_ecl relation in the LEGUS clusters

¹ Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14-16, 53115 Bonn, Germany
² Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno 611 37, Czech Republic
³ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
⁵ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovickách 2, 180 00 Praha 8, Czech Republic
⁶ Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), PO Box 11365-9161, Zanjan, Iran
⁷ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395 - 5531, Tehran, Iran

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Received: 25 August 2025
Accepted: 6 March 2026

Abstract

The relation between the maximum stellar mass in a very young cluster (m_max) and the total stellar mass of the cluster (M_ecl), known as the m_max − M_ecl relation, remains debated in the literature. To test the validity of this relation, we modelled young star clusters with masses between 10^2.5 and 10^5.0M_⊙ and ages of 1–4 Myr using the galIMF code, in which stellar masses are optimally sampled from a varying initial stellar mass function. We compared the results with literature observations of extragalactic young star clusters. We incorporated stellar evolution via PARSEC and COLIBRI tracks and computed Hα luminosities using the Pégase code. To account for dynamical ejections, we stochastically removed stars based on their spectral type, following previous N-body simulations. Additional sources of scatter, including uncertainties in age determination and contamination by field stars, were considered. Our results indicate that, under the assumptions explored here, optimal sampling is consistent with the extragalactic star cluster observations considered, whereas purely random sampling produces distributions that are not in agreement. These findings support a highly self-regulated interpretation of cluster formation in which stellar masses align optimally with the initial mass function rather than being drawn independently at random.