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

M. Fabricius, R. Saglia, F. Balzer, L. R. Ecker, J. Thomas, R. Bender, J. Gracia-Carpio, M. Magliocchetti, O. Marggraf, A. Rawlings, J. G. Sorce, K. Voggel, L. Wang, A. van der Wel, B. Altieri, A. Amara, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, A. Biviano, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, G. Cañas-Herrera, V. Capobianco, C. Carbone, J. Carretero, 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, H. Degaudenzi, G. De Lucia, C. Dolding, H. Dole, F. Dubath, C. A. J. Duncan, X. Dupac, S. Dusini, S. Escoffier, M. Farina, R. Farinelli, S. Ferriol, F. Finelli, M. Frailis, E. Franceschi, M. Fumana, S. Galeotta, B. Gillis, C. Giocoli, A. Grazian, F. Grupp, S. V. H. Haugan, J. Hoar, H. Hoekstra, W. Holmes, I. M. Hook, F. Hormuth, A. Hornstrup, K. Jahnke, M. Jhabvala, B. Joachimi, E. Keihänen, S. Kermiche, A. Kiessling, 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, G. Mainetti, D. Maino, E. Maiorano, O. Mansutti, M. Martinelli, N. Martinet, F. Marulli, R. J. Massey, E. Medinaceli, S. Mei, 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, L. Pozzetti, F. Raison, A. Renzi, J. Rhodes, G. Riccio, E. Romelli, M. Roncarelli, H. J. A. Rottgering, Z. Sakr, A. G. Sánchez, 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. -L. Starck, J. Steinwagner, P. Tallada-Crespí, A. N. Taylor, H. I. Teplitz, I. Tereno, N. Tessore, S. Toft, R. Toledo-Moreo, F. Torradeflot, I. Tutusaus, L. Valenziano, J. Valiviita, T. Vassallo, G. Verdoes Kleijn, A. Veropalumbo, Y. Wang, J. Weller, M. Wetzstein, A. Zacchei, G. Zamorani, I. A. Zinchenko, E. Zucca, M. Huertas-Company, V. Scottez, D. Scott, M. Siudek

TL;DR

This work leverages Euclid Q1 VIS data to systematically search for secondary nuclei in massive early-type galaxies as remnants of mergers and possible hosts of SMBH binaries. A two-stage light-profile modelling approach (MGE followed by Sérsic fitting) combined with visual classification identifies 666 secondary-nucleus candidates across 504 ETG hosts, including 44 at separations under 2 kpc, where core-scouring and SMBH interactions are most relevant. The study carefully assesses the likelihood of chance alignments and characterizes the properties (sizes, masses, redshifts) of the secondaries, finding many resemble UCDs or merger remnants and a subset as potential SMBH recoil candidates. Looking ahead to Euclid DR1, the authors demonstrate a CNN-based automated detection framework that could scale to ~30 times larger sky coverage, enabling robust statistical constraints on the incidence of multiple nuclei and the role of mergers in shaping ETGs.

Abstract

Massive early-type galaxies (ETGs) are believed to form primarily through mergers of less massive progenitors, leaving 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 is sinking towards the shared gravitational potential of the merged galaxy. 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. 44 fall at projected separations of less than 2 kpc. We argue those candidates 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 may represent the highest-density nuclei of a previously merged galaxy, currently sinking into the centre of the new, common gravitational potential and thus likely to host a secondary SMBH. We then demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the thirty-times larger sky coverage of the future Euclid DR1. Finally, we argue that our method could detect the remnants of a recoil event from two merged SMBHs.

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

TL;DR

This work leverages Euclid Q1 VIS data to systematically search for secondary nuclei in massive early-type galaxies as remnants of mergers and possible hosts of SMBH binaries. A two-stage light-profile modelling approach (MGE followed by Sérsic fitting) combined with visual classification identifies 666 secondary-nucleus candidates across 504 ETG hosts, including 44 at separations under 2 kpc, where core-scouring and SMBH interactions are most relevant. The study carefully assesses the likelihood of chance alignments and characterizes the properties (sizes, masses, redshifts) of the secondaries, finding many resemble UCDs or merger remnants and a subset as potential SMBH recoil candidates. Looking ahead to Euclid DR1, the authors demonstrate a CNN-based automated detection framework that could scale to ~30 times larger sky coverage, enabling robust statistical constraints on the incidence of multiple nuclei and the role of mergers in shaping ETGs.

Abstract

Massive early-type galaxies (ETGs) are believed to form primarily through mergers of less massive progenitors, leaving 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 is sinking towards the shared gravitational potential of the merged galaxy. 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. 44 fall at projected separations of less than 2 kpc. We argue those candidates 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 may represent the highest-density nuclei of a previously merged galaxy, currently sinking into the centre of the new, common gravitational potential and thus likely to host a secondary SMBH. We then demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the thirty-times larger sky coverage of the future Euclid DR1. Finally, we argue that our method could detect the remnants of a recoil event from two merged SMBHs.

Paper Structure

This paper contains 15 sections, 3 equations, 25 figures.

Table of Contents

  1. Introduction
  2. Data
  3. Methods
  4. Sample selection
  5. Fitting of analytic models of stellar light distribution
  6. Visual inspections
  7. Redshift estimates
  8. Results
  9. Discussion
  10. Chance alignments
  11. Nature of secondary nuclei
  12. Towards DR1 and automated detections
  13. Conclusions
  14. Effect of the selection criteria on sample characteristics
  15. Conversion of Kron radius to effective radius

Figures (25)

  • Figure 1: Modelling process. The object is the same as in Fig. \ref{['fig:central-segmentation']} (MER ID: $-571887374498383944$). To all the pixels that fall within our segmentation map (left), we fit a two-dimensional Sérsic light distribution using imfit (centre). We often observe central residuals after the Sérsic model subtraction, but spatially offset potential secondary nuclei become readily visible in the difference image on the right; there are two in this particular example.
  • Figure 2: Normalized density distributions of the Zoobot computed featured_or_disk parameter. The blue histogram shows all 15 172 objects that result from our initial sample selection. The red histogram shows those 4420 objects that we labelled 'early type'.
  • Figure 3: Comparison of photometric redshift estimates by PHZ to spectroscopic measurements. We show all objects in our early-type subsample where spectroscopic data are available. The limited precision in the various space- and ground-based imaging channels still limits the accuracy of photometric redshifts in Q1. Any quantity that is derived from the photometric redshift thus needs to be taken with care.
  • Figure 4: Same as Fig. \ref{['fig:specz_photz']} but showing the comparison to the redshifts by ENIA25.
  • Figure 5: Distribution of absolute magnitudes, corrected for Galactic extinction. The blue histogram shows all early-type systems in our sample, the red histogram shows those that have spectroscopic redshifts. We truncate the plot at $M(\IE) = -26$ as such luminosities are probably a consequence of failures in the photometric redshift estimates.
  • ...and 20 more figures