(Euclid Collaboration: Y. Fu, Publication date ?) ================================================================================ Euclid Quick Data Release (Q1). Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars Euclid Collaboration: Y. Fu, R. Bouwens, K. I. Caputi, D. Vergani, M. Scialpi, B. Margalef-Bentabol, L. Wang, M. Bolzonella, M. Banerji, E. Bañados, A. Feltre, Y. Toba, J. Calhau, F. Tarsitano, P. A. C. Cunha, A. Humphrey, G. Vietri, F. Mannucci, S. Bisogni, F. Ricci, H. Landt, L. Spinoglio, T. Matamoro Zatarain, D. Stern, M. J. Page, D. M. Alexander, G. Zamorani, W. Roster, M. Salvato, Y. Copin, J. G. Sorce, D. Scott, Y.-H. Zhang, E. Lusso, J. Wolf, D. Yang, H. J. A. Rottgering, B. Laloux, M. Siudek, S. Belladitta, Q. Liu, V. Allevato, K. Kuijken, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, E. Branchini, M. Brescia, J. Brinchmann, S. Camera, G. Cañas-Herrera, V. Capobianco, C. Carbone, J. Carretero, S. Casas, M. Castellano, G. Castignani, S. Cavuoti, K. C. Chambers, A. Cimatti, C. Colodro-Conde, G. Congedo, C. J. Conselice, L. Conversi, A. Costille, F. Courbin, H. M. Courtois, M. Cropper, A. Da Silva, H. Degaudenzi, G. De Lucia, C. Dolding, H. Dole, F. Dubath, C. A. J. Duncan, X. Dupac, S. Dusini, S. Escoffier, M. Fabricius, M. Farina, R. Farinelli, S. Ferriol, F. Finelli, P. Fosalba, N. Fourmanoit, M. Frailis, E. Franceschi, P. Franzetti, M. Fumana, S. Galeotta, K. George, W. Gillard, B. Gillis, C. Giocoli, J. Gracia-Carpio, A. Grazian, F. Grupp, L. Guzzo, S. V. H. Haugan, H. Hoekstra, W. Holmes, I. M. Hook, F. Hormuth, A. Hornstrup, K. Jahnke, M. Jhabvala, B. Joachimi, E. Keihänen, S. Kermiche, A. Kiessling, B. Kubik, M. Kümmel, M. Kunz, H. Kurki-Suonio, R. Laureijs, A. M. C. Le Brun, S. Ligori, P. B. Lilje, V. Lindholm, I. Lloro, G. Mainetti, D. Maino, E. Maiorano, O. Mansutti, S. Marcin, O. Marggraf, K. Markovic, M. Martinelli, N. Martinet, F. Marulli, R. J. Massey, E. Medinaceli, S. Mei, M. Melchior, Y. Mellier, M. Meneghetti, E. Merlin, G. Meylan, A. Mora, M. Moresco, L. Moscardini, R. Nakajima, C. Neissner, R. C. Nichol, S.-M. Niemi, C. Padilla, S. Paltani, F. Pasian, K. Pedersen, W. J. Percival, V. Pettorino, S. Pires, 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, M. Schirmer, P. Schneider, T. Schrabback, M. Scodeggio, A. Secroun, E. Sefusatti, G. Seidel, S. Serrano, P. Simon, C. Sirignano, G. Sirri, L. Stanco, J.-L. Starck, J. Steinwagner, C. Surace, P. Tallada-Crespí, D. Tavagnacco, A. N. Taylor, H. I. Teplitz, I. Tereno, N. Tessore, S. Toft, R. Toledo-Moreo, F. Torradeflot, I. Tutusaus, L. Valenziano, J. Valiviita, T. Vassallo, A. Veropalumbo, D. Vibert, Y. Wang, J. Weller, A. Zacchei, E. Zucca, M. Ballardini, E. Bozzo, C. Burigana, R. Cabanac, M. Calabrese, A. Cappi, D. Di Ferdinando, J. A. Escartin Vigo, L. Gabarra, W. G. Hartley, M. Huertas-Company, J. Martín-Fleitas, S. Matthew, N. Mauri, R. B. Metcalf, A. A. Nucita, A. Pezzotta, M. Pöntinen, C. Porciani, I. Risso, V. Scottez, M. Sereno, M. Tenti, M. Viel, M. Wiesmann, Y. Akrami, S. Alvi, I. T. Andika, S. Anselmi, M. Archidiacono, F. Atrio-Barandela, E. Aubourg, D. Bertacca, M. Bethermin, L. Bisigello, A. Blanchard, L. Blot, M. Bonici, S. Borgani, M. L. Brown, S. Bruton, A. Calabro, B. Camacho Quevedo, F. Caro, C. S. Carvalho, T. Castro, F. Cogato, S. Conseil, A. R. Cooray, O. Cucciati, G. Daste, F. De Paolis, G. Desprez, A. Díaz-Sánchez, J. J. Diaz, S. Di Domizio, J. M. Diego, P. Dimauro, P.-A. Duc, M. Y. Elkhashab, A. Enia, Y. Fang, A. G. Ferrari, A. Finoguenov, F. Fontanot, A. Franco, K. Ganga, J. García-Bellido, T. Gasparetto, V. Gautard, E. Gaztanaga, F. Giacomini, F. Gianotti, G. Gozaliasl, M. Gray, M. Guidi, C. M. Gutierrez, A. Hall, C. Hernández-Monteagudo, H. Hildebrandt, J. Hjorth, J. J. E. Kajava, Y. Kang, V. Kansal, D. Karagiannis, K. Kiiveri, J. Kim, C. C. Kirkpatrick, S. Kruk, V. Le Brun, J. Le Graet, L. Legrand, M. Lembo, F. Lepori, G. Leroy, G. F. Lesci, J. Lesgourgues, T. I. Liaudat, A. Loureiro, J. Macias-Perez, M. Magliocchetti, C. Mancini, R. Maoli, C. J. A. P. Martins, L. Maurin, M. Miluzio, P. Monaco, C. Moretti, G. Morgante, S. Nadathur, K. Naidoo, P. Natoli, A. Navarro-Alsina, S. Nesseris, D. Paoletti, F. Passalacqua, K. Paterson, L. Patrizii, A. Pisani, D. Potter, S. Quai, M. Radovich, P.-F. Rocci, G. Rodighiero, S. Sacquegna, M. Sahlén, D. B. Sanders, E. Sarpa, C. Scarlata, A. Schneider, D. Sciotti, E. Sellentin, F. Shankar, L. C. Smith, E. Soubrie, K. Tanidis, C. Tao, G. Testera, R. Teyssier, S. Tosi, A. Troja, M. Tucci, C. Valieri, A. Venhola, G. Verza, P. Vielzeuf, A. Viitanen, N. A. Walton, J. R. Weaver =aa58490-25 ================================================================================ Keywords: Galaxies: quasars: general, Infrared: galaxies, Galaxies: active, Tech niques: spectroscopic, Galaxies: distances and redshifts Abstract: The slitless spectroscopy mode of the Near-Infrared Spectrometer and Photometer (NISP) on board the Euclid telescope has enabled efficient spectroscopy of objects within a large field of view. Nevertheless, the relatively low spectral resolution, overlapping spectra, and contamination pose challenges to source classification and redshift determination using the NISP spectra alone. In this work, we present a large and homogeneous sample of bright quasars identified from the Euclid Quick Data Release (Q1), constructed by combining high-purity candidate selections from Gaia and WISE with the new spectroscopic capabilities of Euclid. Through visual inspection of the Euclid spectra of these quasar candidates, we identify approximately 3500 quasars and determine reliable redshifts in the range of 0 < z 4.8. Of these, 2686 are new spectroscopic identifications relative to existing public compilations. We generated the first Euclid composite spectrum of quasars covering rest-frame near- ultraviolet (NUV) to near-infrared (NIR) wavelengths without telluric lines, which will be pivotal to NIR quasar spectral analysis. We obtained an empirical spectroscopic depth of JE 21.5 and HE 21.3 at the sensitivity of the Wide Field Survey, beyond which the number of securely identified quasars declines sharply. Accordingly, the sample presented in this paper comprises spectroscopically confirmed quasars brighter than these limits. We analysed morphological parameters from the Visible Camera (VIS) using Sérsic and model- independent (CAS) metrics, and a deep-learning point spread function fraction to track nuclear dominance. The VIS morphologies show a clear redshift dependence: at low redshift (z < 0.5), obvious host structures are common and a single Sérsic model fits about half of the sources; at intermediate redshift (0.5 < z < 2), the nuclear component dominates, with 90% of the Sérsic fits saturating at the upper index limit. In this intermediate redshift regime, fPSF is available, and we use it as a more reliable compactness measure than the single-Sérsic and CAS parameters to quantify nuclear versus host emission. We also explore the novel Euclid NIR colour space and discuss the role of these quasars in refining active galactic nucleus selection techniques for future Euclid data releases. Description: The composite spectra are built using 2868 sources of the golden sample, covering a restframe wavelength range of 240 nm to 1700 nm, with a constant wavelength bin size of 0.4 nm. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table2.dat 91 3651 Mean, geometric mean, and median Euclid Q1 quasar composite spectra, along with RMS, S/N and the number of spectra in each wavelength bin. tableb1.dat 294 3468 Catalogue of spectroscopically identified bright quasars from Euclid Q1. -------------------------------------------------------------------------------- Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 F8.1 nm Wavelength [240.0/1700.0] Wavelength bin center (nm) 10-19 F10.3 --- FluxMean [0.77/16.58] Arithmetic-mean flux density (arbitrary units) 21-30 F10.3 --- e_FluxMean [0.0/1.23] Uncertainty on arithmetic-mean flux density 32-41 F10.3 --- FluxGeo [0.72/15.98] Geometric-mean flux density (arbitrary units) 43-52 F10.3 --- e_FluxGeo [0.0/12.31] Uncertainty on geometric-mean flux density 54-63 F10.3 --- FluxMed [0.79/17.12] Median flux density (arbitrary units) 65-74 F10.3 --- FluxRMS [0.04/5.09] RMS flux density in bin (arbitrary units) 76-85 F10.3 --- snr [9.68/616.47] Signal-to-noise ratio in bin 87-91 I5 --- nSpectra [5/1761] Number of spectra contributing to the bin -------------------------------------------------------------------------------- Byte-by-byte Description of file: tableb1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 19 I19 --- object_id ? Euclid Q1 unique source identifier 21- 44 A24 --- name IAU-formatted source name (EUCL JHHMMSS.ss+DDMMSS.s) 46- 57 F12.6 deg ra [50.68/276.57] Source barycenter right ascension coordinate in decimal degrees 59- 70 F12.6 deg dec [-51.4/68.8] Source barycenter declination coordinate in decimal degrees 72- 74 A3 --- class_vi Visual classification of the source 76- 87 F12.4 --- z_vi [0.0/4.77] Visual redshift of the source 89- 98 I10 --- sum_mask [225/3579] Sum of the mask values of the spectrum within [12047, 18734] Å 100-109 I10 --- n_invalid Number of invalid pixels in the spectrum within [12047, 18734] Å 111-122 F12.4 --- med_snr [-11.22/204.06] Median signal-to-noise ratio of the spectrum within [12047, 18734] Å 124-135 F12.4 mag.arcsec-2 mumax_minus_mag [-3.64/4.31] The difference between mu_max and mag_stargal_sep, valid even for NIR-only sources 137-148 F12.4 pix kron_radius [10.45/220.16] Major semi-axis (in pixels) of the elliptical aperture used for total (Kron) aperture photometry on the detection image 150-168 I19 --- gaia_id ? The associated Gaia source id 170-190 F21.18 --- f_psf [0.01/1.0]? PSF fraction of the source from VIS image, derived from the model of Euclid Collaboration: Margalef- Bentabol et al. (2025) 192-203 F12.4 mag mag_vis_psf [16.1/27.24]? VIS IE band PSF-fitting AB magnitude (from flux_vis_psf) 205-216 F12.4 mag magerr_vis_psf [0.0/0.18]? Error on VIS IE band PSF- fitting AB magnitude 218-229 F12.4 mag mag_y_templfit [14.78/24.05]? YE band template-fit AB magnitude (from flux_y_templfit) 231-242 F12.4 mag magerr_y_templfit [0.0/0.26]? Error on YE band template- fit AB magnitude 244-255 F12.4 mag mag_j_templfit [14.56/23.07]? JE band template-fit AB magnitude (from flux_j_templfit) 257-268 F12.4 mag magerr_j_templfit [0.0/0.1]? Error on JE band template- fit AB magnitude 270-281 F12.4 mag mag_h_templfit [14.76/22.4]? HE band template-fit AB magnitude (from flux_h_templfit) 283-294 F12.4 mag magerr_h_templfit [0.0/0.08]? Error on HE band template- fit AB magnitude -------------------------------------------------------------------------------- See also: None Acknowledgements: References: ================================================================================ (prepared by author / pyreadme )