Euclid: The potential of slitless infrared spectroscopy: A z=5.4 quasar and new ultracool dwarfs

E. Bañados, V. Le Brun, S. Belladitta, I. Momcheva, D. Stern, J. Wolf, M. Ezziati, D. J. Mortlock, A. Humphrey, R. L. Smart, S. L. Casewell, A. Pérez-Garrido, B. Goldman, E. L. Martín, A. Mohandasan, C. Reylé, C. Dominguez-Tagle, Y. Copin, E. Lusso, Y. Matsuoka, K. McCarthy, F. Ricci, H. -W. Rix, H. J. A. Rottgering, J. -T. Schindler, J. R. Weaver, A. Allaoui, T. Bedrine, M. Castellano, P. -Y. Chabaud, G. Daste, F. Dufresne, J. Gracia-Carpio, M. Kümmel, M. Moresco, M. Scodeggio, C. Surace, D. Vibert, A. Balestra, A. Bonnefoi, A. Caillat, F. Cogato, A. Costille, S. Dusini, S. Ferriol, E. Franceschi, W. Gillard, K. Jahnke, D. Le Mignant, S. Ligori, E. Medinaceli, G. Morgante, F. Passalacqua, K. Paterson, S. Pires, C. Sirignano, I. T. Andika, H. Atek, D. Barrado, S. Bisogni, C. J. Conselice, R. Decarli, H. Dole, T. Dupuy, A. Feltre, S. Fotopoulou, B. Gillis, X. Lopez Lopez, M. Onoue, G. Rodighiero, N. Sedighi, F. Shankar, M. Siudek, L. Spinoglio, D. Vergani, G. Vietri, F. Walter, G. Zamorani, M. R. Zapatero Osorio, J. -Y. Zhang, M. Bethermin, N. Aghanim, B. Altieri, A. Amara, S. Andreon, C. Baccigalupi, M. Baldi, S. Bardelli, A. Basset, P. Battaglia, A. Biviano, A. Bonchi, D. Bonino, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, V. Capobianco, C. Carbone, J. Carretero, S. Casas, G. Castignani, S. Cavuoti, A. Cimatti, C. Colodro-Conde, G. Congedo, L. Conversi, F. Courbin, H. M. Courtois, M. Cropper, J. -G. Cuby, A. Da Silva, H. Degaudenzi, G. De Lucia, A. M. Di Giorgio, C. Dolding, F. Dubath, C. A. J. Duncan, X. Dupac, A. Ealet, M. Farina, F. Faustini, N. Fourmanoit, M. Frailis, S. Galeotta, K. George, C. Giocoli, B. R. Granett, A. Grazian, F. Grupp, L. Guzzo, S. V. H. Haugan, J. Hoar, H. Hoekstra, W. Holmes, I. Hook, F. Hormuth, A. Hornstrup, P. Hudelot, M. Jhabvala, B. Joachimi, E. Keihänen, S. Kermiche, B. Kubik, K. Kuijken, M. Kunz, H. Kurki-Suonio, P. B. Lilje, V. Lindholm, I. Lloro, G. Mainetti, D. Maino, E. Maiorano, O. Mansutti, O. Marggraf, K. Markovic, M. Martinelli, N. Martinet, F. Marulli, R. Massey, S. Mei, Y. Mellier, M. Meneghetti, E. Merlin, G. Meylan, A. Mora, L. Moscardini, C. Neissner, S. -M. Niemi, J. W. Nightingale, C. Padilla, S. Paltani, F. Pasian, K. Pedersen, W. J. Percival, V. Pettorino, G. Polenta, M. Poncet, L. A. Popa, L. Pozzetti, F. Raison, R. Rebolo, A. Renzi, J. Rhodes, G. Riccio, E. Romelli, M. Roncarelli, E. Rossetti, R. Saglia, Z. Sakr, D. Sapone, B. Sartoris, J. A. Schewtschenko, M. Schirmer, P. Schneider, T. Schrabback, A. Secroun, E. Sefusatti, G. Seidel, M. Seiffert, S. Serrano, G. Sirri, 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, G. Verdoes Kleijn, A. Veropalumbo, Y. Wang, J. Weller, F. M. Zerbi, E. Zucca, M. Bolzonella, C. Burigana, R. Cabanac, L. Gabarra, V. Scottez, M. Viel, H. Dannerbauer, D. Scott

First listed 2025-06-16 | Last updated 2025-08-25

Abstract

We 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 RGE) 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 CIII] and MgII 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 BGE) 926-1366 nm spectrum confirms CIV and CIII] emission. NISP RGE 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=4e25 W/Hz). The quasar has a bolometric luminosity of 3e12 Lsun and is powered by a 3.4e9 Msun 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.

Short digest

Euclid NISP slitless spectra demonstrate blind, space-based identification of both high‑z quasars and their chief photometric contaminants, ultracool dwarfs. The team reports a bright z=5.404 quasar, EUCL J181530.01+652054.0, flagged by strong broad CIII] and MgII in RGE (1206–1892 nm) with CIV/CIII] also in BGE (926–1366 nm) and confirmed by LBT optical data. It is a high‑excitation BAL quasar detected by LOFAR at 144 MHz (L144≈4×10^25 W/Hz), with Lbol≈3×10^12 Lsun and MBH≈3.4×10^9 Msun—an unexpectedly luminous find within only ~20 deg2. Cross-correlation of NISP spectra likewise yields new ultracool dwarfs (e.g., T3 EUCL J002516.31+491618.5; M6 EUCL J174429.80+672728.1), underscoring RGE’s ability to uncover bright quasars at z~5.5 and z>7 where photometric selection struggles.

Key figures to inspect

• Figure 1: Use the line–redshift map to see where ≥2 strong lines fall in the NISP bandpasses—highlighting the z~5.5 and z>7 windows for reliable blind redshifts and the space-based advantage over telluric gaps.
• Figure 2: Inspect quasar vs M/L/T-dwarf colors and the marked objects (EUCL J181530.01+652054.0, EUCL J002516.31+491618.5, EUCL J174429.80+672728.1) to appreciate why photometry alone is degenerate and why spectroscopy is decisive.
• Figure 3: Check the T3 dwarf EUCL J002516.31+491618.5 postage stamps and RGE spectrum; the template match shows how cross-correlation cleanly identifies ultracool dwarfs in slitless data.
• Figure 4: Compare the M6 dwarf EUCL J174429.80+672728.1 NISP spectrum with the LBT/MODS optical confirmation and note the LOFAR upper limit—validating the Euclid spectral classification pipeline.