Bañados, ELe Brun, VBelladitta, SMomcheva, IStern, DWolf, JEzziati, MMortlock, D JHumphrey, ASmart, R LCasewell, S LPérez-Garrido, AGoldman, BMartín, E LMohandasan, AReylé, CDominguez-Tagle, CCopin, YLusso, EMatsuoka, YMcCarthy, KRicci, FRix, H-WRottgering, H J ASchindler, J-TWeaver, J RAllaoui, ABedrine, TCastellano, MChabaud, P-YDaste, GDufresne, FGracia-Carpio, JKümmel, MMoresco, MScodeggio, MSurace, CVibert, DBalestra, ABonnefoi, ACaillat, ACogato, FCostille, ADusini, SFerriol, SFranceschi, EGillard, WJahnke, KLe Mignant, DLigori, SMedinaceli, EMorgante, GPassalacqua, FPaterson, KPires, SSirignano, CAndika, I TAtek, HBarrado, DBisogni, SConselice, C JDannerbauer, HDecarli, RDole, HDupuy, TFeltre, AFotopoulou, SGillis, BLopez, X LopezOnoue, MRodighiero, GSedighi, NShankar, FSiudek, MSpinoglio, LVergani, DVietri, GWalter, FZamorani, GOsorio, M R ZapateroZhang, J-YBethermin, MAghanim, NAltieri, BAmara, AAndreon, SBaccigalupi, CBaldi, MBardelli, SBasset, ABattaglia, PBiviano, ABonchi, ABonino, DBranchini, EBrescia, MBrinchmann, JCamera, SCapobianco, VCarbone, CCarretero, JCasas, SCastignani, GCavuoti, SCimatti, AColodro-Conde, CCongedo, GConversi, LCourbin, FrédéricCourtois, H MCropper, MCuby, J-GDa Silva, ADegaudenzi, HDe Lucia, GGiorgio, A M DiDolding, CDubath, FDuncan, C A JDupac, XEalet, AFarina, MFaustini, FFourmanoit, NFrailis, MGaleotta, SGeorge, KGiocoli, CGranett, B RGrazian, AGrupp, FGuzzo, LHaugan, S V HHoar, JHoekstra, HHolmes, WHook, IHormuth, FHornstrup, AHudelot, PJhabvala, MJoachimi, BKeihänen, EKermiche, SKubik, BKuijken, KKunz, MKurki-Suonio, HLilje, P BLindholm, VLloro, IMainetti, GMaino, DMaiorano, EMansutti, OMarggraf, OMarkovic, KMartinelli, MMartinet, NMarulli, FMassey, RMei, SMellier, YMeneghetti, MMerlin, EMeylan, GMora, AMoscardini, LNeissner, CNiemi, S-MNightingale, J WPadilla, CPaltani, SPasian, FPedersen, KPercival, W JPettorino, VPolenta, GPoncet, MPopa, L APozzetti, LRaison, FRebolo, RRenzi, ARhodes, JRiccio, GRomelli, ERoncarelli, MRossetti, ESaglia, RSakr, ZSapone, DSartoris, BSchewtschenko, J ASchirmer, MSchneider, PSchrabback, TSecroun, ASefusatti, ESeidel, GSeiffert, MSerrano, SSirri, GStanco, LSteinwagner, JTallada-Crespí, PTaylor, A NTeplitz, H ITereno, IToft, SToledo-Moreo, RTorradeflot, FTutusaus, IValenziano, LValiviita, JVassallo, TKleijn, G VerdoesVeropalumbo, AWang, YWeller, JZerbi, F MZucca, EBolzonella, MBurigana, CCabanac, RGabarra, LScottez, VViel, MScott, D2025-08-202025-08-202025-08-192025-08-0410.1093/mnras/staf1274https://infoscience.epfl.ch/handle/20.500.14299/253109We demonstrate the potential of Euclid’s slitless spectroscopy to discover high-redshift (z > 5) quasars and their main photometric contaminant, ultracool dwarfs. Sensitive infrared spectroscopy from space is able to efficiently identify both populations, as demonstrated by Euclid Near-Infrared Spectrometer and Photometer Red Grism (NISP ${\rm RG}_{\scriptscriptstyle \rm E}$) spectra of the newly discovered z = 5.404 quasar EUCL J181530.01+652054.0, as well as several ultracool dwarfs in the Euclid Deep Field North and the Euclid Early Release Observation field Abell 2764. The ultracool dwarfs were identified by cross-correlating their spectra with templates. The quasar was identified by its strong and broad C iii and Mg ii emission lines in the NISP RGE 1206–1892 nm spectrum, and confirmed through optical spectroscopy from the Large Binocular Telescope. The NISP Blue Grism (NISP ${\rm BG}_{\scriptscriptstyle \rm E}$) 926–1366 nm spectrum confirms C iv and C iii emission. NISP ${\rm RG}_{\scriptscriptstyle \rm E}$ can find bright quasars at z ≈ 5.5 and z ≳ 7, redshift ranges that are challenging for photometric selection due to contamination from ultracool dwarfs. EUCL J181530.01+652054.0 is a high-excitation, broad absorption line quasar detected at 144 MHz by the LOw-Frequency Array (L144 = 4.0 × 1025 W Hz−1). The quasar has a bolometric luminosity of 3 × 1012 L⊙ and is powered by a 3.4 × 109 M⊙ black hole. The discovery of this bright quasar is noteworthy as fewer than one such object was expected in the ≈20 deg2 surveyed. This finding highlights the potential and effectiveness of NISP spectroscopy in identifying rare, luminous high-redshift quasars, previewing the census of these sources that Euclid’s slitless spectroscopy will deliver over about 14 000 deg2 of the sky.enGalaxies: quasars:individual EUCL J18153001+6520540 -Stars: brown dwarfs -Stars: individual:text::journal::journal article::research article