Euclid: Quick Data Release (Q1) ─ Secondary nuclei in early-type galaxies

June 2026 • 2026A&A...711A..39F

Authors • Fabricius, M. • Saglia, R. • Balzer, F. • Ecker, L. R. • Thomas, J. • Bender, R. • Gracia-Carpio, J. • Magliocchetti, M. • Marggraf, O. • Rawlings, A. • Sorce, J. G. • Voggel, K. • Wang, L. • van der Wel, A. • Altieri, B. • Amara, A. • Andreon, S. • Auricchio, N. • Baccigalupi, C. • Baldi, M. • Balestra, A. • Bardelli, S. • Biviano, A. • Branchini, E. • Brescia, M. • Brinchmann, J. • Camera, S. • Cañas-Herrera, G. • Capobianco, V. • Carbone, C. • Carretero, J. • Castellano, M. • Castignani, G. • Cavuoti, S. • Chambers, K. C. • Cimatti, A. • Colodro-Conde, C. • Congedo, G. • Conselice, C. J. • Conversi, L. • Copin, Y. • Courbin, F. • Courtois, H. M. • Cropper, M. • Degaudenzi, H. • De Lucia, G. • Dolding, C. • Dole, H. • Dubath, F. • Duncan, C. A. J. • Dupac, X. • Dusini, S. • Escoffier, S. • Farina, M. • Farinelli, R. • Ferriol, S. • Finelli, F. • Frailis, M. • Franceschi, E. • Fumana, M. • Galeotta, S. • George, K. • Gillis, B. • Giocoli, C. • Grazian, A. • Grupp, F. • Haugan, S. V. H. • Hoar, J. • Hoekstra, H. • Holmes, W. • Hook, I. M. • Hormuth, F. • Hornstrup, A. • Jahnke, K. • Jhabvala, M. • Joachimi, B. • Keihänen, E. • Kermiche, S. • Kiessling, A. • Kubik, B. • Kuijken, K. • Kümmel, M. • Kunz, M. • Kurki-Suonio, H. • Le Brun, A. M. C. • Ligori, S. • Lilje, P. B. • Lindholm, V. • Lloro, I. • Mainetti, G. • Maino, D. • Maiorano, E. • Mansutti, O. • Martinelli, M. • Martinet, N. • Marulli, F. • Massey, R. J. • Medinaceli, E. • Mei, S. • Mellier, Y. • Meneghetti, M. • Merlin, E. • Meylan, G. • Mora, A. • Moresco, M. • Moscardini, L. • Nakajima, R. • Neissner, C. • Niemi, S.-M. • Padilla, C. • Paltani, S. • Pasian, F. • Pedersen, K. • Percival, W. J. • Pettorino, V. • Pires, S. • Polenta, G. • Poncet, M. • Popa, L. A. • Pozzetti, L. • Raison, F. • Renzi, A. • Rhodes, J. • Riccio, G. • Romelli, E. • Roncarelli, M. • Rottgering, H. J. A. • Sakr, Z. • Sánchez, A. G. • Sapone, D. • Sartoris, B. • Schirmer, M. • Schneider, P. • Schrabback, T. • Secroun, A. • Seidel, G. • Serrano, S. • Simon, P. • Sirignano, C. • Sirri, G. • Skottfelt, J. • Stanco, L. • Starck, J.-L. • Steinwagner, J. • Tallada-Crespí, P. • Taylor, A. N. • Teplitz, H. I. • Tereno, I. • Tessore, N. • Toft, S. • Toledo-Moreo, R. • Torradeflot, F. • Tutusaus, I. • Valenziano, L. • Valiviita, J. • Vassallo, T. • Verdoes Kleijn, G. • Veropalumbo, A. • Wang, Y. • Weller, J. • Wetzstein, M. • Zacchei, A. • Zamorani, G. • Zinchenko, I. A. • Zucca, E. • Huertas-Company, M. • Scottez, V. • Scott, D. • Siudek, M.

Abstract • Massive early-type galaxies (ETGs; M > 10¹¹ M_⊙) are believed to form primarily through mergers of less massive progenitors, which leave behind numerous traces of violent formation histories, such as stellar streams and shells. A particularly striking signature of these mergers is the formation of supermassive black hole (SMBH) binaries, which can create depleted stellar cores through interactions with stars on radial orbits ─ a process known as core scouring. The secondary SMBH in such systems may still carry a dense stellar envelope and thereby remain observable for some time as a secondary nucleus while it sinks towards the shared gravitational potential of the merged galaxy. Direct observations of secondary nuclei on sub-kiloparsec scales remain rare, with only a few notable cases, such as NGC 5419. Investigating such features and building up statistics requires both high spatial resolution and wide-field coverage, a capability uniquely provided by Euclid. In this study, we leverage Euclid's Q1 Early Release data to systematically search for secondary nuclei in ETGs. We present a preliminary sample of 666 candidate systems distributed over 504 hosts (some of which contain multiple secondary nuclei). The vast majority of these fall at separations of 3 kpc to 15 kpc, indicative of normal mergers. However, 44 fall at projected separations of less than 2 kpc. We argue that this most interesting subset of secondary nucleus candidates ─ those at very close angular separations ─ are unlikely to be a consequence of chance alignments. We show that their stellar masses are mostly too large for them to be globular clusters and that a significant subset are unresolved even at Euclid's spatial resolution, rendering them too small to be dwarf galaxies. These objects may represent the highest-density nuclei of a previously merged galaxy currently sinking into the centre of the new common gravitational potential, and thus they likely host a secondary SMBH. We also demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the 30 times larger sky coverage of the future Euclid DR1. Finally, we argue that our method can detect the remnants of a recoil event from two merged SMBHs, as two of our secondary nuclei candidates are unresolved at the Euclid spatial resolution, appear at projected physical separations of less than 2 kpc, and appear in hosts of M > 10¹¹ M_⊙, which makes them viable candidates. ★ This paper is published on behalf of the Euclid Consortium.

Links

• POSTSCRIPT https://www.aanda.org/10.1051/0004-6361/202557217/postscript