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arXiv · 2506.13945

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·H. Dannerbauer·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
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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.

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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, H. Dannerbauer, 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. 2025-06-16. Euclid: The potential of slitless infrared spectroscopy: A z=5.4 quasar and new ultracool dwarfs. https://doi.org/10.1093/mnras%2Fstaf1274

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