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

S. S. Larsen, A. M. N. Ferguson, J. M. Howell, F. Annibali, J. -C. Cuillandre, L. K. Hunt, A. Lançon, T. Saifollahi, D. Massari, M. N. Le, N. Aghanim, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, V. Capobianco, C. Carbone, J. Carretero, S. Casas, M. Castellano, G. Castignani, S. Cavuoti, K. C. Chambers, A. Cimatti, C. Colodro-Conde, G. Congedo, C. J. Conselice, L. Conversi, Y. Copin, F. Courbin, H. M. Courtois, M. Cropper, A. Da Silva, H. Degaudenzi, G. De Lucia, H. Dole, M. Douspis, F. Dubath, X. Dupac, S. Dusini, S. Escoffier, M. Fabricius, M. Farina, F. Faustini, S. Ferriol, S. Fotopoulou, M. Frailis, E. Franceschi, S. Galeotta, K. George, B. Gillis, C. Giocoli, P. Gómez-Alvarez, J. Gracia-Carpio, A. Grazian, F. Grupp, S. V. H. Haugan, H. Hoekstra, W. Holmes, F. Hormuth, A. Hornstrup, K. Jahnke, M. Jhabvala, E. Keihänen, S. Kermiche, A. Kiessling, R. Kohley, B. Kubik, K. Kuijken, M. Kümmel, M. Kunz, H. Kurki-Suonio, A. M. C. Le Brun, S. Ligori, P. B. Lilje, V. Lindholm, I. Lloro, D. Maino, E. Maiorano, O. Mansutti, O. Marggraf, K. Markovic, M. Martinelli, N. Martinet, F. Marulli, R. Massey, E. Medinaceli, S. Mei, Y. Mellier, M. Meneghetti, G. Meylan, A. Mora, M. Moresco, L. Moscardini, C. Neissner, S. -M. Niemi, C. Padilla, S. Paltani, F. Pasian, K. Pedersen, W. J. Percival, V. Pettorino, S. Pires, G. Polenta, M. Poncet, L. A. Popa, L. Pozzetti, F. Raison, A. Renzi, J. Rhodes, G. Riccio, E. Romelli, M. Roncarelli, R. Saglia, Z. Sakr, D. Sapone, B. Sartoris, J. A. Schewtschenko, M. Schirmer, P. Schneider, A. Secroun, G. Seidel, M. Seiffert, S. Serrano, P. Simon, C. Sirignano, G. Sirri, J. Skottfelt, L. Stanco, J. Steinwagner, P. Tallada-Crespí, A. N. Taylor, I. Tereno, R. Toledo-Moreo, F. Torradeflot, A. Tsyganov, I. Tutusaus, L. Valenziano, J. Valiviita, T. Vassallo, G. Verdoes Kleijn, A. Veropalumbo, Y. Wang, J. Weller, G. Zamorani, F. M. Zerbi, E. Zucca, J. Martín-Fleitas, V. Scottez

TL;DR

This study leverages Euclid’s high-resolution, wide-field imaging from the Early Release Observations to census star clusters in IC 342, NGC 2403, and Holmberg II, with a focus on old globular clusters (GCs) and their implications for hierarchical assembly. It combines Euclid VIS/NISP data with ground-based MegaCam/LBT imaging to identify GC candidates via size and colour criteria, quantify completeness with artificial clusters, and characterize photometry and sizes across the three galaxies. The results reveal bimodal GC colour distributions in IC 342 and NGC 2403, an overall GCLF that shows an excess of faint clusters in these spirals, and relatively low GC specific frequencies in IC 342, while NGC 2403 and Holmberg II show more typical values; the LF of young clusters follows a power law with slope around -2.3 across the sample. Overall, the work demonstrates Euclid’s power to map outer GC systems and young clusters in nearby galaxies, enabling detailed comparisons with Local Group analogues and informing models of galaxy assembly and star-cluster formation.

Abstract

We examine the star cluster populations in the three nearby galaxies IC 342, NGC 2403, and Holmberg II, observed as part of the Euclid Early Release Observations programme. Our main focus is on old globular clusters (GCs), for which the wide field-of-view and excellent image quality of Euclid offer substantial advantages over previous work. For IC 342 this is the first study of stellar clusters other than its nuclear cluster. After selection based on size and magnitude criteria, followed by visual inspection, we identify 111 old (> 1 Gyr) GC candidates in IC 342, 50 in NGC 2403 (of which 15 were previously known), and 7 in Holmberg II. In addition, a number of younger and/or intermediate-age candidates are identified. The colour distributions of GC candidates in the two larger galaxies show hints of bimodality with peaks at IE-HE = 0.36 and 0.79 (IC 342) and IE-HE = 0.36 and 0.80 (NGC 2403), corresponding to metallicities of [Fe/H]=-1.5 and [Fe/H]=-0.5, similar to those of 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). The excess objects may be similar to those previously identified in other galaxies. The specific frequency of classical old GCs in IC 342, as determined based on 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, dN/dL ~ L^(-2.3+/-0.1) and 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, that is, M33 and the Small Magellanic Cloud, respectively.

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

TL;DR

This study leverages Euclid’s high-resolution, wide-field imaging from the Early Release Observations to census star clusters in IC 342, NGC 2403, and Holmberg II, with a focus on old globular clusters (GCs) and their implications for hierarchical assembly. It combines Euclid VIS/NISP data with ground-based MegaCam/LBT imaging to identify GC candidates via size and colour criteria, quantify completeness with artificial clusters, and characterize photometry and sizes across the three galaxies. The results reveal bimodal GC colour distributions in IC 342 and NGC 2403, an overall GCLF that shows an excess of faint clusters in these spirals, and relatively low GC specific frequencies in IC 342, while NGC 2403 and Holmberg II show more typical values; the LF of young clusters follows a power law with slope around -2.3 across the sample. Overall, the work demonstrates Euclid’s power to map outer GC systems and young clusters in nearby galaxies, enabling detailed comparisons with Local Group analogues and informing models of galaxy assembly and star-cluster formation.

Abstract

We examine the star cluster populations in the three nearby galaxies IC 342, NGC 2403, and Holmberg II, observed as part of the Euclid Early Release Observations programme. Our main focus is on old globular clusters (GCs), for which the wide field-of-view and excellent image quality of Euclid offer substantial advantages over previous work. For IC 342 this is the first study of stellar clusters other than its nuclear cluster. After selection based on size and magnitude criteria, followed by visual inspection, we identify 111 old (> 1 Gyr) GC candidates in IC 342, 50 in NGC 2403 (of which 15 were previously known), and 7 in Holmberg II. In addition, a number of younger and/or intermediate-age candidates are identified. The colour distributions of GC candidates in the two larger galaxies show hints of bimodality with peaks at IE-HE = 0.36 and 0.79 (IC 342) and IE-HE = 0.36 and 0.80 (NGC 2403), corresponding to metallicities of [Fe/H]=-1.5 and [Fe/H]=-0.5, similar to those of 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). The excess objects may be similar to those previously identified in other galaxies. The specific frequency of classical old GCs in IC 342, as determined based on 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, dN/dL ~ L^(-2.3+/-0.1) and 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, that is, M33 and the Small Magellanic Cloud, respectively.

Paper Structure

This paper contains 23 sections, 29 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Overview of the three galaxies
  4. Analysis
  5. Identification of cluster candidates
  6. Artificial cluster tests: expected appearance of GCs
  7. Artificial cluster tests: completeness and photometric uncertainties
  8. Visual inspection of the candidates
  9. Photometry of the cluster candidates
  10. Simple stellar population model colours
  11. The effect of line emission
  12. Globular cluster candidate selection: MegaCam and LBT photometry
  13. Spatial distributions
  14. Comparison with HST imaging
  15. Sizes of the cluster candidates
  16. ...and 8 more sections

Figures (29)

  • Figure 1: PSF-corrected FWHM size as a function of magnitude for sources in the NGC 2403 field. Previously known GCs are shown with red filled circles while the new candidates identified in ERONearbyGals are shown with open circles. Also included are data for Milky Way GCs, scaled to the distance of NGC 2403 (black filled circles). The red line indicates our size cut for selection of cluster candidates.
  • Figure 2: Simulated images of GCs added to the VIS image of NGC 2403, next to the clusters D6 (top), JC15 (centre), and a new candidate closer to the centre of NGC 2403 (bottom). In each panel, simulated GCs are shown for masses of $10^5 M_\odot$, $3\times10^5 M_\odot$, and $10^6 M_\odot$ (left to right) a half-light radius of 3 pc, and an assumed age of 10 Gyr. Each panel measures $295\times70$ VIS pixels ($29\farcs5\times7\farcs0$)
  • Figure 3: Top and centre panels: as Fig. \ref{['fig:simGCsN2403']}, but for IC 342. Bottom panel: simulated clusters with ages of 20 Myr.
  • Figure 4: Luminosity functions for simulated Milky Way GCs added to the images of IC 342. The distributions of input magnitudes are shown for all clusters added within the footprint (blue), those recovered in the VIS images (orange), and those that also have photometry in the NISP (black hatched) and MegaCam images (green circular-hatched).
  • Figure 5: Input- and measured colour-magnitude diagrams for artificial Milky Way GCs added to the IC 342 images. The horizontal dashed lines indicate our magnitude limit for selection of cluster candidates.
  • ...and 24 more figures