: Star clusters in IC 342, NGC 2403, and Holmberg II

We examine the star cluster populations in the three nearby (3.20–3.45 Mpc) galaxies IC 342, NGC 2403, and Holmberg II, observed as part of the Euclid Early Release Observations programme. Our main focus in this paper is old globular clusters (GCs), for which the wide field of view and excellent image quality of Euclid offer substantial advantages over previous works. With respect to IC 342, in particular, this is the first study of stellar clusters that goes beyond its nuclear cluster. After a selection process based on size and magnitude criteria, followed by visual inspection, we identified 111 old (≳1 Gyr) GC candidates in IC 342, 49 in NGC 2403 (of which 15 were previously known), along with 7 in Holmberg II. In addition, a number of younger and/or intermediate-age candidates were identified. The colour distributions of GC candidates in the two larger galaxies show hints of bimodality, with peaks at (IE−{HE)0 = 0.36 and 0.79 in both IC 342 and NGC 2403, corresponding to metallicities of [Fe/H] ≈ −1.5 and [Fe/H] ≈ −0.5. These results are similar to those reported for the metal-poor and metal-rich GC subpopulations in the Milky Way. The luminosity functions of our GC candidates exhibit an excess of relatively faint objects, relative to a canonical, approximately Gaussian GC luminosity function (GCLF). Although some contamination from background galaxies and younger clusters might be present in our GC samples, we argue that the excess could be at least partially real. In particular, we find this could be the case for IC 342, where the excess objects could be similar to those previously identified, for example, in M101 and NGC 6946. The specific frequency of old GCs in IC 342, as determined on the basis of the brighter half of the GCLF, appears to be unusually low with SN ≈ 0.2–0.3. The combined luminosity function of young and intermediate-age clusters in all three galaxies is consistent with a power-law distribution of dN/dL ∝ L−2.3 ± 0.1. The total numbers of young clusters brighter than M(IE) = −8 in NGC 2403 and Holmberg II are comparable with those found in their Local Group counterparts, namely, M33 and the Small Magellanic Cloud, respectively.