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Euclid: Early Release Observations -- The star cluster systems of the Local Group dwarf galaxies IC 10 and NGC 6822

J. M. Howell, A. M. N. Ferguson, S. S. Larsen, A. Lançon, F. Annibali, J. -C. Cuillandre, L. K. Hunt, D. Martínez-Delgado, D. Massari, T. Saifollahi, K. Voggel, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, S. Bardelli, A. Biviano, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, G. Cañas-Herrera, G. P. Candini, V. Capobianco, C. Carbone, J. Carretero, M. Castellano, G. Castignani, S. Cavuoti, 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, F. Dubath, C. A. J. Duncan, X. Dupac, S. Dusini, S. Escoffier, M. Farina, R. Farinelli, F. Faustini, S. Ferriol, F. Finelli, M. Frailis, E. Franceschi, M. Fumana, S. Galeotta, K. George, B. Gillis, C. Giocoli, 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, B. Kubik, M. Kümmel, M. Kunz, H. Kurki-Suonio, A. M. C. Le Brun, D. Le Mignant, S. Ligori, P. B. Lilje, V. Lindholm, I. Lloro, G. Mainetti, D. Maino, E. Maiorano, O. Mansutti, O. Marggraf, M. Martinelli, N. Martinet, F. Marulli, R. J. Massey, E. Medinaceli, S. Mei, M. Melchior, Y. Mellier, M. Meneghetti, E. Merlin, G. Meylan, A. Mora, M. Moresco, L. Moscardini, R. Nakajima, 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, F. Raison, A. Renzi, J. Rhodes, G. Riccio, E. Romelli, M. Roncarelli, R. Saglia, Z. Sakr, D. Sapone, B. Sartoris, M. Schirmer, P. Schneider, T. Schrabback, A. Secroun, G. Seidel, S. Serrano, P. Simon, C. Sirignano, G. Sirri, J. Skottfelt, L. Stanco, J. Steinwagner, P. Tallada-Crespí, A. N. Taylor, H. I. Teplitz, I. Tereno, S. Toft, R. Toledo-Moreo, F. Torradeflot, I. Tutusaus, L. Valenziano, J. Valiviita, T. Vassallo, Y. Wang, J. Weller, G. Zamorani, I. A. Zinchenko, J. Martín-Fleitas, V. Scottez

Abstract

Star clusters are valuable indicators of galaxy evolution, offering insights into the buildup of stellar populations across cosmic time. Understanding intrinsic star cluster populations of dwarf galaxies is particularly important given their role in the hierarchical growth of larger systems. Using Euclid Early Release Observation data, we study star clusters in two star-forming dwarf irregulars in the Local Group, NGC 6822 and IC 10 [$M_\star \sim$ (1--4) $\times10^8 M_\odot$]. With Euclid, clusters are resolved into individual stars across the main bodies and haloes of both galaxies. Visual inspection of $I_E$ images uncovers 30 new cluster candidates in NGC 6822 and 16 in IC 10, from compact to extended clusters. We re-evaluate literature candidates, producing combined catalogues of 52 (NGC 6822) and 71 (IC 10) clusters with confidence-based classifications. We present homogeneous photometry in $I_E$, $Y_E$, $J_E$, $H_E$, and archival UBVRI data, alongside size measurements and properties from BAGPIPES SED fitting. Synthetic cluster injection shows our sample is $\sim 50$% complete to $M \lesssim 10^3 M_\odot$ for ages $\lesssim 100$ Myr, and to $M \lesssim 3\times10^4 M_\odot$ for $\sim 10$ Gyr. IC 10 has more young clusters than NGC 6822, extending to higher masses, consistent with its starburst nature. Both dwarfs host several old massive ($\gtrsim 10^5 M_\odot$) clusters, including an exceptional $1.3 \times 10^6 M_\odot$ cluster in NGC 6822's outskirts. In NGC 6822, we identify a previously undetected, old, extended cluster ($R_h = 12.4 \pm 0.11$ pc). Using well-defined criteria, we identify 11 candidate GCs in NGC 6822 and eight in IC 10. Both galaxies have high specific frequencies ($S_N$) but remain consistent with known GC scaling relations at low luminosity [abridged].

Euclid: Early Release Observations -- The star cluster systems of the Local Group dwarf galaxies IC 10 and NGC 6822

Abstract

Star clusters are valuable indicators of galaxy evolution, offering insights into the buildup of stellar populations across cosmic time. Understanding intrinsic star cluster populations of dwarf galaxies is particularly important given their role in the hierarchical growth of larger systems. Using Euclid Early Release Observation data, we study star clusters in two star-forming dwarf irregulars in the Local Group, NGC 6822 and IC 10 [ (1--4) ]. With Euclid, clusters are resolved into individual stars across the main bodies and haloes of both galaxies. Visual inspection of images uncovers 30 new cluster candidates in NGC 6822 and 16 in IC 10, from compact to extended clusters. We re-evaluate literature candidates, producing combined catalogues of 52 (NGC 6822) and 71 (IC 10) clusters with confidence-based classifications. We present homogeneous photometry in , , , , and archival UBVRI data, alongside size measurements and properties from BAGPIPES SED fitting. Synthetic cluster injection shows our sample is % complete to for ages Myr, and to for Gyr. IC 10 has more young clusters than NGC 6822, extending to higher masses, consistent with its starburst nature. Both dwarfs host several old massive () clusters, including an exceptional cluster in NGC 6822's outskirts. In NGC 6822, we identify a previously undetected, old, extended cluster ( pc). Using well-defined criteria, we identify 11 candidate GCs in NGC 6822 and eight in IC 10. Both galaxies have high specific frequencies () but remain consistent with known GC scaling relations at low luminosity [abridged].

Paper Structure

This paper contains 27 sections, 21 figures, 9 tables.

Table of Contents

  1. Introduction
  2. Overview of the two galaxies
  3. NGC 6822
  4. IC 10
  5. Data
  6. Euclid Early Release Observations
  7. Local Group Galaxy Survey
  8. Cluster identification
  9. Blind cluster search
  10. Literature sample compilation
  11. Analysis
  12. Cluster photometry
  13. SED fitting
  14. Half-light radii
  15. Validation and completeness
  16. ...and 12 more sections

Figures (21)

  • Figure 1: Examples of old and young clusters across NGC 6822, both previously found and new from this study, showcased alongside an RGB image of the data displaying its full FoV. , , and are the blue, green, and red channels, respectively. Each cutout is ${\sim}44$ pc on the side in the band, displayed in log scale. The zoom-in region is roughly $16\farcm5 \times 13\farcm5$ ($2.5\,\text{kpc} \times 2 \,\text{kpc}$). The top row includes the GCs SC7 (ESCC-NGC6822-08) and SC5 (ESCC-NGC6822-06), the middle row shows the cluster Hubble-VI (ESCC-NGC6822-01), and the bottom row features a newly identified extended cluster candidate, ESCC-NGC6822-27.
  • Figure 2: Same as Fig. \ref{['fig:example_clusts']} for IC 10. Each cutout is 63 pc on the side in the band, displayed in asinh scale. The zoom-in region is roughly $15\farcm5 \times 12\farcm9$ ($3.25\,\text{kpc} \times 2.7 \,\text{kpc}$). The sample includes some clusters identified in the halo by Lim2015, specifically ESCC-IC10-49 and ESCC-IC10-50.
  • Figure 3: V-band aperture correction relationship for clusters in IC 10, derived using bright and isolated clusters.
  • Figure 4: Synthetic clusters displayed in asinh scale, placed within the outermost regions of the IC 10 image. Each cutout is 12 arcseconds on the side with apparent magnitude, age, and half-light radii displayed.
  • Figure 5: Cluster completeness derived from artificial cluster tests for inner and outer regions of each galaxy, as a function of magnitude and half-light radius.
  • ...and 16 more figures