J/A+A/711/A39ivo://CDS.VizieR/J/A+A/711/A39CDSFabricius 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.,Canas-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.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.Keihaenen E.Kermiche S.,Kiessling A.Kubik B.Kuijken K.Kuemmel 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.Sanchez 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-Crespi 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.Siudek M.2026-06-30T15:46:34Z2026-06-30T16:45:52ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frsurveysgalaxiesphotometryvisible-astronomyMassive early-type galaxies (ETGs; M>10^11^M_{sun}_) 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 NGC5419. 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, occur at projected physical separations of less than 2kpc, and occur in hosts of M>10^11^M_{sun}_, which makes them viable candidates.2026A&A...711A..39Fhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/711/A39CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toVII/259 : 6dF galaxy survey final redshift release (Jones+, 2009)V/161 : DESI DR1 redshift catalog (DESI Col.+ 2025)J/A+A/711/A10 : Euclid (Q1). Census of dwarf galaxies (Euclid Coll., 2026)J/A+A/711/A16 : X-ray point-like sources of Euclid (Q1) (Euclid Coll.+, 2026)J/A+A/711/A37 : Ultracool dwarfs in Euclid Deep Fields (Zerjal+, 2026)J/A+A/711/A38 : Euclid Q1. Photometric studies of known transientsJ/A+A/711/A39 : Euclid Q1. Secondary nuclei in ET galaxies (Fabricius+, 2026)J/A+A/711/A20 : Galaxies of Euclid (Q1) (Euclid Coll.+, 2026)J/A+A/711/A24 : Euclid Q1. Little red dots properties (Euclid Coll., 2026)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/711/A39https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/711/A39http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/711/A39https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/711/A39https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/711/A39http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/711/A39GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/711/A39/tablea1 (The catalog)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/711/A39/tablea1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/711/A39/tablea1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/711/A39/tablea1?GETtext/xml+votable180.050000true66.8554774700909-47.39938778614570.0055555555555555564/2145 5/3839 4010-4011 8506 8579 8592 8983 8989 6/10229-10231 10235 10237-10239 11601 11603-11604 11606-11607 11609 11612-11613 11615 15353-15355 16034-16035 16038 16056 33979 33982-33983 34028 34030 34307 34310-34311 34313 34315 34337 34340-34341 34352 34372 34374 34376-34377 35926-35927 35929 35931 36098 36104 36106 36128 36130https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/711/A39?format=asciiOpticaldefaultJ/A+A/711/A39/tablea1The catalogrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintObjectIdMER catalogue ID of the hostmeta.id;meta.tableRAJ2000Host right ascension (J2000)degpos.eq.ra;meta.mainDEJ2000Host declination (J2000)degpos.eq.dec;meta.mainz? Host redshiftsrc.redshiftdoublenullableztype[spec,phot] Type of redshift, photometricmeta.codecharr_zLiterature source of redshift (1)meta.ref;pos.framecharNucIdSecondary nucleus candidate identifiermeta.id;meta.mainintRANdegSecondary nucleus candidate right ascension (J2000)degpos.eq.raDENdegSecondary nucleus candidate declination (J2000)degpos.eq.rarkronNucKron radiusarcsecphys.angSizedoublePANucAngular separation from host centerarcsecpos.angDistancedoublemagNuc? Nuclear VIS magnitudemagphot.mag;em.optdoublenullable