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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  RG_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  CIII and  MgII emission lines in the NISP  RG_E 1206─1892 nm spectrum, and confirmed through optical spectroscopy from the Large Binocular Telescope. The NISP Blue Grism (NISP  BG_E) 926─1366 nm spectrum confirms  CIV and  CIII   emission. NISP  RG_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=4.0×10²⁵W Hz ⁻¹). The quasar has a bolometric luminosity of  3×1012L_⊙ and is powered by a  3.4×109M_⊙ black hole. The discovery of this bright quasar is noteworthy as fewer than one such object was expected in the  ≈ 20 deg ² 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  14000 deg² of the sky.

Keywords
stars low mass brown dwarfs / galaxies quasars

Monthly Notices of the Royal Astronomical Society
Volume 542, Page 15
2025 September

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