a raised background. Moreover, stray light from nearby stars should also be somewhat different for the four spectrum orientations and should therefore be washed out to some degree when combined. We also mention that our results for the type 1 AGN are biassed to the bright blue AGN population, and given that we excluded BALs and radio-bright AGN from the main AGN sample, our analysis could be extended to include these interesting AGN types, especially the most extreme classes in these two categories (e.g. FeLoBALs, LoBALs). We could also perhaps fold in the AGN luminosity function to quantify the capability of NISP in retrieving the AGN classification from the NISP data.

The presented analysis further extends the campaign to assess the performance of the NISP spectrograph in the context of active galaxies. Finally, the expected high spectroscopic coverage of AGN at $z < 2$ will be of prime importance for studies of AGN demography, scaling relation, and AGN clustering from the epoch of the peak of AGN activity down to the present-day Universe.

**Acknowledgements.** We thank the anonymous reviewer for their thorough reading and for useful comments and suggestions that have improved the clarity of the paper. V. A. acknowledges support from INAF-PRIN 1.05.01.85.08. L. B. acknowledges financial support from grant PRIN2017. A. F. acknowledges the support from project "VLT-MOONS" CRAM 1.05.03.07, INAF Large Grant 2022 "The metal circle: a new sharp view of the baryon cycle up to Cosmic Dawn with the latest generation IFU facilities" and INAF Large Grant 2022 "Dual and binary SMBH in the multi-messenger era". F. S. acknowledges partial support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 860744. The Euclid Consortium acknowledges the European Space Agency and a number of agencies and institutes that have supported the development of *Euclid*, in particular the Academy of Finland, the Agenzia Spaziale Italiana, the Belgian Science Policy, the Canadian Euclid Consortium, the French Centre National d'Études Spatiales, the Deutsches Zentrum für Luft- und Raumfahrt, the Danish Space Research Institute, the Fundação para a Ciência e a Tecnologia, the Ministerio de Ciencia e Innovación, the National Aeronautics and Space Administration, the National Astronomical Observatory of Japan, the Netherlandse Onderzoekschool Voor Astronomie, the Norwegian Space Agency, the Romanian Space Agency, the State Secretariat for Education, Research and Innovation (SERI) at the Swiss Space Office (SSO), and the United Kingdom Space Agency. A complete and detailed list is available on the *Euclid* web site (http://www.euclid-ec.org).

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21 Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany

22 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

23 INAF-Instituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00100 Roma, Italy

24 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

25 School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK

26 Université Paris-Saclay, CNRS, Institut d'astrophysique spatiale, 91405 Orsay, France

27 ESAC/ESA, Camino Bajo del Castillo, s/n., Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada Madrid, Spain

28 Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK

29 INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20122 Milano, Italy

30 Aix-Marseille Université, CNRS/IN2P3, CPPM, Marseille, France

31 Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy

32 INFN-Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

33 Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany

34 INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, (TO), Italy

35 Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy

36 INFN-Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy

37 Department of Physics “E. Pancini”, University Federico II, Via Cinthia 6, 80126 Napoli, Italy

38 INFN section of Naples, Via Cinthia 6, 80126 Napoli, Italy

39 Dipartimento di Fisica, Università degli Studi di Torino, Via P. Giuria 1, 10125 Torino, Italy

40 INFN-Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy

41 Institut de Física d'Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, (Barcelona), Spain

42 Port d'Informació Científica, Campus UAB, C. Albareda s/n, 08193 Bellaterra, (Barcelona), Spain

43 Institute for Theoretical Particle Physics and Cosmology (TTK), RWTH Aachen University, 52056 Aachen, Germany

44 Dipartimento di Fisica e Astronomia “Augusto Righi” – Alma Mater Studiorum Università di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

45 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

46 Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK

47 European Space Agency/ESRIN, Largo Galileo Galilei 1, 00044 Frascati, Roma, Italy

48 University of Lyon, Univ. Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, UMR 5822, 69622 Villeurbanne, France

49 Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

50 UCB Lyon 1, CNRS/IN2P3, IUF, IP2I Lyon, 4 rue Enrico Fermi, 69622 Villeurbanne, France

51 Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Campo Grande 1749-016, Lisboa, Portugal

52 Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande 1749-016, Lisboa, Portugal

53 Department of Astronomy, University of Geneva, ch. d'Ecogia 16, 1290 Versoix, Switzerland

54 Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH, UK

1 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

2 Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy e-mail: elisabeta.lusso@unifi.it

3 School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

4 INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy

5 INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy

6 INAF-IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy

7 INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy

8 Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo 8, 35131 Padova, Italy

9 INAF-Osservatorio Astronomico di Padova, Via dell'Osservatorio 5, 35122 Padova, Italy

10 Department of Mathematics and Physics, Roma Tre University, Via della Vasca Navale 84, 00146 Rome, Italy

11 INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy

12 INFN-Padova, Via Marzolo 8, 35131 Padova, Italy

13 Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France

14 Dipartimento di Fisica e Astronomia “Augusto Righi” – Alma Mater Studiorum Università di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy

15 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

16 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

17 DTx – Digital Transformation CoLAB, Building 1, Azurém Campus, University of Minho, 4800-058 Guimarães, Portugal

18 Department of Physics, Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK

19 SISSA-International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, TS, Italy

20 INFN-Sezione di Trieste, Via Valerio 2, 34127 Trieste, TS, Italy
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21 Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany

22 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

23 INAF-Instituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00100 Roma, Italy

24 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

25 School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK

26 Université Paris-Saclay, CNRS, Institut d'astrophysique spatiale, 91405 Orsay, France

27 ESAC/ESA, Camino Bajo del Castillo, s/n., Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada Madrid, Spain

28 Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK

29 INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20122 Milano, Italy

30 Aix-Marseille Université, CNRS/IN2P3, CPPM, Marseille, France

31 Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy

32 INFN-Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

33 Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany

34 INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, (TO), Italy

35 Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy

36 INFN-Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy

37 Department of Physics “E. Pancini”, University Federico II, Via Cinthia 6, 80126 Napoli, Italy

38 INFN section of Naples, Via Cinthia 6, 80126 Napoli, Italy

39 Dipartimento di Fisica, Università degli Studi di Torino, Via P. Giuria 1, 10125 Torino, Italy

40 INFN-Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy

41 Institut de Física d'Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, (Barcelona), Spain

42 Port d'Informació Científica, Campus UAB, C. Albareda s/n, 08193 Bellaterra, (Barcelona), Spain

43 Institute for Theoretical Particle Physics and Cosmology (TTK), RWTH Aachen University, 52056 Aachen, Germany

44 Dipartimento di Fisica e Astronomia “Augusto Righi” – Alma Mater Studiorum Università di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy

45 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

46 Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK

47 European Space Agency/ESRIN, Largo Galileo Galilei 1, 00044 Frascati, Roma, Italy

48 University of Lyon, Univ. Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, UMR 5822, 69622 Villeurbanne, France

49 Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

50 UCB Lyon 1, CNRS/IN2P3, IUF, IP2I Lyon, 4 rue Enrico Fermi, 69622 Villeurbanne, France

51 Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Campo Grande 1749-016, Lisboa, Portugal

52 Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande 1749-016, Lisboa, Portugal

53 Department of Astronomy, University of Geneva, ch. d'Ecogia 16, 1290 Versoix, Switzerland

54 Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH, UK

1 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

2 Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy e-mail: elisabeta.lusso@unifi.it

3 School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

4 INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy

5 INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy

6 INAF-IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy

7 INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy

8 Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo 8, 35131 Padova, Italy

9 INAF-Osservatorio Astronomico di Padova, Via dell'Osservatorio 5, 35122 Padova, Italy

10 Department of Mathematics and Physics, Roma Tre University, Via della Vasca Navale 84, 00146 Rome, Italy

11 INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy

12 INFN-Padova, Via Marzolo 8, 35131 Padova, Italy

13 Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France

14 Dipartimento di Fisica e Astronomia “Augusto Righi” – Alma Mater Studiorum Università di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy

15 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

16 Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

17 DTx – Digital Transformation CoLAB, Building 1, Azurém Campus, University of Minho, 4800-058 Guimarães, Portugal

18 Department of Physics, Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK

19 SISSA-International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, TS, Italy

20 INFN-Sezione di Trieste, Via Valerio 2, 34127 Trieste, TS, Italy
55 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

56 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Via Irnerio 46, 40126 Bologna, Italy

57 University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany

58 Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway

59 Department of Physics, Lancaster University, Lancaster LA1 4YB, UK

60 von Hoerner & Sulger GmbH, SchloßPlatz 8, 68723 Schwetzingen, Germany

61 Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark

62 Cosmic Dawn Center (DAWN), Copenhagen, Denmark

63 Department of Physics and Helsinki Institute of Physics, Gustaf Hällströmin katu 2, 00014 University of Helsinki, Finland

64 Université de Genève, Département de Physique Théorique and Centre for Astroparticle Physics, 24 quai Ernest-Ansermet, 1211 Genève 4, Switzerland

65 Department of Physics, PO Box 64, 00014 University of Helsinki, Finland

66 Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, Helsinki, Finland

67 NOVA optical infrared instrumentation group at ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

68 Universität Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

69 Department of Physics, Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK

70 Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France

71 Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France

72 Institut d'Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France

73 European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

74 Department of Physics and Astronomy, University of Aarhus, Ny Munkegade 120, 8000 Aarhus C, Denmark

75 Centre for Astrophysics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

76 Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

77 Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada

78 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, Astrophysique, Instrumentation et Modélisation Paris-Saclay, 91191 Gif-sur-Yvette, France

79 Space Science Data Center, Italian Space Agency, Via del Politecnico snc, 00133 Roma, Italy

80 Centre National d'Études Spatiales – Centre spatial de Toulouse, 18 avenue Édouard Belin, 31401 Toulouse Cedex 9, France

81 Institute of Space Science, Str. Atomistilor, nr. 409 Măgurele, Ilfov 077125, Romania

82 Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38204 San Cristóbal de La Laguna, Tenerife, Spain

83 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain

84 Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile

85 Institut d'Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità 2-4, 08034 Barcelona, Spain

86 Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain

87 Satlantis, University Science Park, Sede Bld 48940, Leioa-Bilbao, Spain

88 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Avenida Complutense 40, 28040 Madrid, Spain

89 Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA

90 Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Tapada da Ajuda 1349-018, Lisboa, Portugal

91 Universidad Politécnica de Cartagena, Departamento de Electrónica y Tecnología de Computadoras, Plaza del Hospital 1, 30202 Cartagena, Spain

92 Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 14 Av. Édouard Belin, 31400 Toulouse, France

93 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700, AV Groningen, The Netherlands

94 INFN-Bologna, Via Irnerio 46, 40126 Bologna, Italy

95 IFPU-Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy

96 INAF-Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy

97 Centre de Calcul de l'IN2P3/CNRS, 21 avenue Pierre de Coubertin, 69627 Villeurbanne Cedex, France

98 Institut für Theoretische Physik, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany

99 Université St Joseph; Faculty of Sciences, Beirut, Lebanon

100 Junia, EPA department, 41 Bd Vauban, 59800 Lille, France

101 ICSC-Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing, Via Magnanelli 2, Bologna, Italy

102 Instituto de Física Teórica UAM-CSIC, Campus de Cantoblanco, 28049 Madrid, Spain

103 CERCA/ISO, Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA

104 Laboratoire Univers et Théorie, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, 92190 Meudon, France

105 Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy

106 Instituto Nazionale di Fisica Nucleare, Sezione di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy

107 Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

108 Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34131 Trieste, Italy

109 NASA Ames Research Center, Moffett Field, CA 94035, USA

110 Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA

111 Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA

112 Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St SE, Minneapolis, MN 55455, USA

113 Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Bd de l'Observatoire, CS 34229, 06304 Nice cedex 4, France

114 Institute Lorentz, Leiden University, PO Box 9506, Leiden 2300 RA, The Netherlands

115 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

116 Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA

117 Department of Astronomy & Physics and Institute for Computational Astrophysics, Saint Mary's University, 923 Robie Street, Halifax, Nova Scotia B3H 3C3, Canada

118 Departamento Física Aplicada, Universidad Politécnica de Cartagena, Campus Muralla del Mar, 30202 Cartagena, Murcia, Spain

119 Department of Computer Science, Aalto University, PO Box 15400, Espoo 00 076, Finland

120 Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing (GCCL), 44780 Bochum, Germany

121 Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53, Avenue des Martyrs, 38000 Grenoble, France

122 Department of Physics and Astronomy, Vesilinnantie 5, 20041 University of Turku, Finland
55 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France

56 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Via Irnerio 46, 40126 Bologna, Italy

57 University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany

58 Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway

59 Department of Physics, Lancaster University, Lancaster LA1 4YB, UK

60 von Hoerner & Sulger GmbH, SchloßPlatz 8, 68723 Schwetzingen, Germany

61 Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark

62 Cosmic Dawn Center (DAWN), Copenhagen, Denmark

63 Department of Physics and Helsinki Institute of Physics, Gustaf Hällströmin katu 2, 00014 University of Helsinki, Finland

64 Université de Genève, Département de Physique Théorique and Centre for Astroparticle Physics, 24 quai Ernest-Ansermet, 1211 Genève 4, Switzerland

65 Department of Physics, PO Box 64, 00014 University of Helsinki, Finland

66 Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, Helsinki, Finland

67 NOVA optical infrared instrumentation group at ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

68 Universität Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

69 Department of Physics, Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK

70 Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France

71 Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France

72 Institut d'Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France

73 European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

74 Department of Physics and Astronomy, University of Aarhus, Ny Munkegade 120, 8000 Aarhus C, Denmark

75 Centre for Astrophysics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

76 Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

77 Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada

78 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, Astrophysique, Instrumentation et Modélisation Paris-Saclay, 91191 Gif-sur-Yvette, France

79 Space Science Data Center, Italian Space Agency, Via del Politecnico snc, 00133 Roma, Italy

80 Centre National d'Études Spatiales – Centre spatial de Toulouse, 18 avenue Édouard Belin, 31401 Toulouse Cedex 9, France

81 Institute of Space Science, Str. Atomistilor, nr. 409 Măgurele, Ilfov 077125, Romania

82 Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38204 San Cristóbal de La Laguna, Tenerife, Spain

83 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain

84 Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile

85 Institut d'Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità 2-4, 08034 Barcelona, Spain

86 Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain

87 Satlantis, University Science Park, Sede Bld 48940, Leioa-Bilbao, Spain

88 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Avenida Complutense 40, 28040 Madrid, Spain

89 Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA

90 Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Tapada da Ajuda 1349-018, Lisboa, Portugal

91 Universidad Politécnica de Cartagena, Departamento de Electrónica y Tecnología de Computadoras, Plaza del Hospital 1, 30202 Cartagena, Spain

92 Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 14 Av. Édouard Belin, 31400 Toulouse, France

93 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700, AV Groningen, The Netherlands

94 INFN-Bologna, Via Irnerio 46, 40126 Bologna, Italy

95 IFPU-Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy

96 INAF-Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy

97 Centre de Calcul de l'IN2P3/CNRS, 21 avenue Pierre de Coubertin, 69627 Villeurbanne Cedex, France

98 Institut für Theoretische Physik, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany

99 Université St Joseph; Faculty of Sciences, Beirut, Lebanon

100 Junia, EPA department, 41 Bd Vauban, 59800 Lille, France

101 ICSC-Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing, Via Magnanelli 2, Bologna, Italy

102 Instituto de Física Teórica UAM-CSIC, Campus de Cantoblanco, 28049 Madrid, Spain

103 CERCA/ISO, Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA

104 Laboratoire Univers et Théorie, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, 92190 Meudon, France

105 Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy

106 Instituto Nazionale di Fisica Nucleare, Sezione di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy

107 Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

108 Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34131 Trieste, Italy

109 NASA Ames Research Center, Moffett Field, CA 94035, USA

110 Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA

111 Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA

112 Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St SE, Minneapolis, MN 55455, USA

113 Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Bd de l'Observatoire, CS 34229, 06304 Nice cedex 4, France

114 Institute Lorentz, Leiden University, PO Box 9506, Leiden 2300 RA, The Netherlands

115 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

116 Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA

117 Department of Astronomy & Physics and Institute for Computational Astrophysics, Saint Mary's University, 923 Robie Street, Halifax, Nova Scotia B3H 3C3, Canada

118 Departamento Física Aplicada, Universidad Politécnica de Cartagena, Campus Muralla del Mar, 30202 Cartagena, Murcia, Spain

119 Department of Computer Science, Aalto University, PO Box 15400, Espoo 00 076, Finland

120 Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing (GCCL), 44780 Bochum, Germany

121 Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3, 53, Avenue des Martyrs, 38000 Grenoble, France

122 Department of Physics and Astronomy, Vesilinnantie 5, 20041 University of Turku, Finland
<footnote n="123">123 Serco for European Space Agency (ESA), Camino bajo del Castillo, s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada 28692, Madrid, Spain</footnote>

<footnote n="124">124 ARC Centre of Excellence for Dark Matter Particle Physics, Melbourne, Australia</footnote>

<footnote n="125">125 Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Victoria 3122, Australia</footnote>

<footnote n="126">126 W.M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI, USA</footnote>

<footnote n="127">127 Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, Stockholm 106 91, Sweden</footnote>

<footnote n="128">128 Astrophysics Group, Blackett Laboratory, Imperial College London, London SW7 2AZ, UK</footnote>

<footnote n="129">129 Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy</footnote>

<footnote n="130">130 INFN-Sezione di Roma, Piazzale Aldo Moro, 2 – c/o Dipartimento di Fisica, Edificio G. Marconi, 00185 Roma, Italy</footnote>

<footnote n="131">131 Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal</footnote>

<footnote n="132">132 Zentrum für Astronomie, Universität Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany</footnote>

<footnote n="133">133 Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, Roma, Italy</footnote>

<footnote n="134">134 INFN-Sezione di Roma 2, Via della Ricerca Scientifica 1, Roma, Italy</footnote>

<footnote n="135">135 Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland</footnote>

<footnote n="136">136 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands</footnote>

<footnote n="137">137 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands</footnote>

<footnote n="138">138 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK</footnote>

<footnote n="139">139 Department of Physics and Astronomy, University of California, Davis, CA 95616, USA</footnote>

<footnote n="140">140 Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA</footnote>

<footnote n="141">141 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark</footnote>
123 Serco for European Space Agency (ESA), Camino bajo del Castillo, s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada 28692, Madrid, Spain

124 ARC Centre of Excellence for Dark Matter Particle Physics, Melbourne, Australia

125 Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Victoria 3122, Australia

126 W.M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI, USA

127 Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, Stockholm 106 91, Sweden

128 Astrophysics Group, Blackett Laboratory, Imperial College London, London SW7 2AZ, UK

129 Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy

130 INFN-Sezione di Roma, Piazzale Aldo Moro, 2 – c/o Dipartimento di Fisica, Edificio G. Marconi, 00185 Roma, Italy

131 Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

132 Zentrum für Astronomie, Universität Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany

133 Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, Roma, Italy

134 INFN-Sezione di Roma 2, Via della Ricerca Scientifica 1, Roma, Italy

135 Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland

136 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands

137 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands

138 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK

139 Department of Physics and Astronomy, University of California, Davis, CA 95616, USA

140 Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA

141 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
# Appendix A: Example of Euclid-like simulated spectra

We present in Figure A.1 and in Figure A.2 a few examples of Euclid-like simulated spectra for type 1 and type 2 AGN, respectively,

in the redshift interval $0.89 \le z \le 1.83$, thus where the $H\alpha+[N\II]$ complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels for the *good*, *false* and *lost*, redshift classes (see Sect. 5).

<figure><img src="image_17.png" /><figcaption>**Fig. A.1.** Example of Euclid-like simulated spectra for type 1 AGN in the redshift interval $0.89 \le z \le 1.83$, thus where the $H\alpha+[N\II]$ complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels (medium/high on the left and low on the right) for the *good* (top row), *false* (mid row), and *lost* (bottom row) redshift classes (see Sect. 5).</figcaption></figure>
**Appendix A: Example of Euclid-like simulated
spectra**

We present in Figure A.1 and in Figure A.2 a few examples of
*Euclid*-like simulated spectra for type 1 and type 2 AGN, respec-

tively, in the redshift interval 0.89 ≤ *z* ≤ 1.83, thus where the Hα+[NII] complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels for the *good*, *false* and *lost*, redshift classes (see Sect. 5).

<figure><img src="image_18.png" /><figcaption>**Fig. A.1.** Example of *Euclid*-like simulated spectra for type 1 AGN in the redshift interval 0.89 ≤ *z* ≤ 1.83, thus where the Hα+[NII] complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels (medium/high on the left and low on the right) for the *good* (top row), *false* (mid row), and *lost* (bottom row) redshift classes (see Sect. 5).</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. A.2.** Example of Euclid-like simulated spectra for type 2 AGN in the redshift interval $0.89 \le z \le 1.83$, thus where the H$\alpha$ complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels (medium/high on the left and low on the right) for the *good* (top row), *false* (mid row), and *lost* (bottom row) redshift classes (see Sect. 5).</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. A.2.** Example of Euclid-like simulated spectra for type 2 AGN in the redshift interval $0.89 \le z \le 1.83$, thus where the H$\alpha$ complex is within the spectral coverage of the red grism. Spectra are shown at different S/N and noise levels (medium/high on the left and low on the right) for the *good* (top row), *false* (mid row), and *lost* (bottom row) redshift classes (see Sect. 5).</figcaption></figure>
## Appendix B: Line profile modelling

To better understand whether we are capturing the entire line flux in the incident data, we test two spectral profiles (Lorentzian, and Voigt), in addition to the Gaussian one, to model known emission lines. Figure B.1 shows an example for the Hα complex fit in the type 2 incident spectra with each of the profile options. Gaussian profiles underestimate the peak of the emission lines, while effectively fitting the base and core of the lines. Lorentzian profiles overestimate the tips of the emission lines and underestimate the cores of the lines. Voigt profiles, which are a convolution of Gaussian and Lorentzian profiles, provide a middle ground where the majority of the line flux in the data is captured

by the model. Yet, the tips of the emission lines are still not fully modelled with a Voigt profile. We postulate that this is due to the type 2 incident spectra emission lines being constructed with an AGN and starburst component (i.e., two Gaussians) and so cannot be perfectly modelled using a single line profile. We find that Voigt profiles capture the most flux presented in the data and are hence the best trade-off to be used for type 2 incident spectra emission line modelling. To be consistent with the simulated spectra that are insensitive to the profile employed, we restrict the comparison to the results obtained with the Gaussian one only. Yet, we caveat that a (small) fraction of the flux may be already missed at the level of the spectral modelling of the incident data depending upon the profile assumed.

<figure><img src="image_19.png" /><figcaption>**Fig. B.1.** Example of a type 2 AGN incident spectrum Hα complex fitted using Gaussian (**left**), Lorentzian (**centre**) and Voigt (**right**) emission line profiles.</figcaption></figure>
## Appendix B: Line profile modelling

To better understand whether we are capturing the entire line flux in the incident data, we test two spectral profiles (Lorentzian, and Voigt), in addition to the Gaussian one, to model known emission lines. Figure B.1 shows an example for the Hα complex fit in the type 2 incident spectra with each of the profile options. Gaussian profiles underestimate the peak of the emission lines, while effectively fitting the base and core of the lines. Lorentzian profiles overestimate the tips of the emission lines and underestimate the cores of the lines. Voigt profiles, which are a convolution of Gaussian and Lorentzian profiles, provide a middle ground where the majority of the line flux in the data is captured

by the model. Yet, the tips of the emission lines are still not fully modelled with a Voigt profile. We postulate that this is due to the type 2 incident spectra emission lines being constructed with an AGN and starburst component (i.e., two Gaussians) and so cannot be perfectly modelled using a single line profile. We find that Voigt profiles capture the most flux presented in the data and are hence the best trade-off to be used for type 2 incident spectra emission line modelling. To be consistent with the simulated spectra that are insensitive to the profile employed, we restrict the comparison to the results obtained with the Gaussian one only. Yet, we caveat that a (small) fraction of the flux may be already missed at the level of the spectral modelling of the incident data depending upon the profile assumed.

<figure><img src="image_20.png" /><figcaption>**Fig. B.1.** Example of a type 2 AGN incident spectrum Hα complex fitted using Gaussian (**left**), Lorentzian (**centre**) and Voigt (**right**) emission line profiles.</figcaption></figure>
# Appendix C: Emission line flux from the OU-SPE automated spectral fitting

The Euclid collaboration is currently developing an automated pipeline with the main goal to compute spectral line measurements (e.g. emission lines, absorption lines, continuum breaks) in Euclid spectra. Briefly, this automatic routine computes line spectral parameters through Gaussian fitting with a non-linear least-squares minimisation. The Hα complex profile is modelled with three Gaussians, one for each feature (i.e., Hα and [N II]λλ6548, 6584), on top of a polynomial which accounts for the continuum. For this model, the [N II]λ6548 to [N II]λ6548 ratio is fixed to 3:1 and the FWHMs of the lines are assumed to be the same. In the case of a broad profile (i.e., FWHM > 2000 km s⁻¹), the spectral fit is performed with two Gaussians only, one for the broad component of the Hα and another for

the narrow (i.e., no spectral components are considered for the [N II] doublet). At the time of writing, the pipeline provides tested results for the integrated flux of the Hα emission line complex (flux uncertainties are currently not implemented). Spectral parameters and relative uncertainties for other emission lines are currently under analysis and will be delivered in the updated version of the pipeline.

Figure C.1 presents the comparison of the Hα complex line flux between the QSFIT and the OU-SPE measurements. Fluxes are in very good agreement, with a slightly more scattered dispersion around the one-to-one for the type 1 AGN, as expected. The relative flux difference between the QSFIT and OU-SPE value is defined as $\Delta F/F = (F_{H\alpha,\text{QSFIT}} - F_{H\alpha,\text{OU-SPE}})/F_{H\alpha,\text{OU-SPE}}$. The median $\Delta F/F$ are about -9% and 3% for the type 1 and type 2 AGN, respectively.

<figure><img src="image_20.png" /><figcaption>**Fig. C.1.** Comparison of the Hα+[N II] flux measurements of the simulated spectra between QSFIT and OU-SPE. The results for the type 1 and type 2 AGN are shown in the left and right panels, respectively. Sources with a spectroscopic redshift measured from the simulated spectra and flagged as good (see Sect. 5 for details) are marked with blue symbols. The bottom panel shows the percentage flux difference defined as $\Delta F/F = (F_{H\alpha,\text{QSFIT}} - F_{H\alpha,\text{OU-SPE}})/F_{H\alpha,\text{OU-SPE}}$. The one-to-one flux relation is shown with a red dashed line, which is set to zero in the bottom panel. The dot-dashed blue line in the bottom panel represents the 50th percentile of the $\Delta F/F$ distribution.</figcaption></figure>
# Appendix C: Emission line flux from the OU-SPE automated spectral fitting

The Euclid collaboration is currently developing an automated pipeline with the main goal to compute spectral line measurements (e.g. emission lines, absorption lines, continuum breaks) in Euclid spectra. Briefly, this automatic routine computes line spectral parameters through Gaussian fitting with a non-linear least-squares minimisation. The Hα complex profile is modelled with three Gaussians, one for each feature (i.e., Hα and [N II]λλ6548, 6584), on top of a polynomial which accounts for the continuum. For this model, the [N II]λ6584 to [N II]λ6548 ratio is fixed to 3:1 and the FWHMs of the lines are assumed to be the same. In the case of a broad profile (i.e., FWHM > 2000 km s⁻¹), the spectral fit is performed with two Gaussians only, one for the broad component of the Hα and another for

the narrow (i.e., no spectral components are considered for the [N II] doublet). At the time of writing, the pipeline provides tested results for the integrated flux of the Hα emission line complex (flux uncertainties are currently not implemented). Spectral parameters and relative uncertainties for other emission lines are currently under analysis and will be delivered in the updated version of the pipeline.

Figure C.1 presents the comparison of the Hα complex line flux between the QSFIT and the OU-SPE measurements. Fluxes are in very good agreement, with a slightly more scattered dispersion around the one-to-one for the type 1 AGN, as expected. The relative flux difference between the QSFIT and OU-SPE value is defined as $\Delta F/F = (F_{H\alpha,\text{QSFIT}} - F_{H\alpha,\text{OU-SPE}})/F_{H\alpha,\text{OU-SPE}}$. The median $\Delta F/F$ are about -9% and 3% for the type 1 and type 2 AGN, respectively.

<figure><img src="image_21.png" /><figcaption>**Fig. C.1.** Comparison of the Hα+[N II] flux measurements of the simulated spectra between QSFIT and OU-SPE. The results for the type 1 and type 2 AGN are shown in the left and right panels, respectively. Sources with a spectroscopic redshift measured from the simulated spectra and flagged as good (see Sect. 5 for details) are marked with blue symbols. The bottom panel shows the percentage flux difference defined as $\Delta F/F = (F_{H\alpha,\text{QSFIT}} - F_{H\alpha,\text{OU-SPE}})/F_{H\alpha,\text{OU-SPE}}$. The one-to-one flux relation is shown with a red dashed line, which is set to zero in the bottom panel. The dot-dashed blue line in the bottom panel represents the 50th percentile of the $\Delta F/F$ distribution.</figcaption></figure>
## Appendix D: Comparison of the near-infrared spectral region of the type 1 composite with the literature

The list of the sources used to build the NIR composite is given in Table D.1. We further investigated whether the NIR composite we included in the type 1 AGN main stack (Sect. 2) can be improved by including additional NIR spectra of quasars in the literature. For this purpose we collected all the cross-dispersed NIR spectra published by Landt et al. (2008, 2011, 2013). This data set consists on 29 well-known local type 1 AGN observed at the NASA infrared telescope facility (IRTF), a 3 m telescope, and the 8 m Gemini North Observatory, both located on Mauna Kea, Hawai'i (see Table 1 in Landt et al. 2014 for a summary of the sample properties). We note that a few sources in their sample that display a continuum dominated by the host galaxy emission were not considered here. The data were corrected for Galactic absorption, shifted to the rest-frame and stacked in the same way as done for the SDSS spectra (see Sect. 2.2). The resulting composite is shown in Fig. D.1 with the red line (available at the CDS). Our composite is flatter (redder), likely due to the contribution of the host galaxy emission, and it is more noisy around the Paα emission line, but there is good agreement overall. We defer a more in-depth analysis of the rest-frame NIR region to future work. A version of the composite shown in Fig. 4 corrected for host galaxy emission in the optical, for the IGM absorption in the UV, and that extends in the near-infrared through the spectral composite we build by considering the Landt et al. AGN sample, is also available online. The host galaxy correction at optical wavelengths (3500 < λ/Å < 7400) has been performed by assuming the intrinsic continuum of the type 1 template published by Saccheo et al. (2023).

<table><caption>**Table D.1.** List of AGN used to construct the NIR composite. The SDSS AGN in the table are drawn from Glikman et al. (2006), whilst the remaining are BAT AGN from Ricci et al. (2022).</caption><thead><tr><th>Name</th><th>Redshift</th></tr></thead><tbody><tr><td>SDSS J000943.1-090839.2</td><td>0.210</td></tr><tr><td>SDSS J001327.3+005231.9</td><td>0.362</td></tr><tr><td>SDSS J005709.9+144610.1</td><td>0.172</td></tr><tr><td>SDSS J005812.8+160201.3</td><td>0.211</td></tr><tr><td>SDSS J010226.3-003904.6</td><td>0.295</td></tr><tr><td>SDSS J011110.0-101631.8</td><td>0.179</td></tr><tr><td>SDSS J015530.0-085704.0</td><td>0.165</td></tr><tr><td>SDSS J015910.0+010514.5</td><td>0.217</td></tr><tr><td>SDSS J015950.2+002340.8</td><td>0.163</td></tr><tr><td>SDSS J021707.8-084743.4</td><td>0.292</td></tr><tr><td>SDSS J024250.8-075914.2</td><td>0.378</td></tr><tr><td>SDSS J031209.2-081013.8</td><td>0.265</td></tr><tr><td>SDSS J032213.8+005513.4</td><td>0.185</td></tr><tr><td>SDSS J034042.93-073125.5</td><td>0.217</td></tr><tr><td>SDSS J073623.12+392617.8</td><td>0.118</td></tr><tr><td>SDSS J125519.6+014412.2</td><td>0.343</td></tr><tr><td>SDSS J150610.5+021649.9</td><td>0.135</td></tr><tr><td>SDSS J170441.3+604430.5</td><td>0.372</td></tr><tr><td>SDSS J172711.8+632241.8</td><td>0.218</td></tr><tr><td>SDSS J211843.2-063618.0</td><td>0.328</td></tr><tr><td>SDSS J212619.6-065408.9</td><td>0.418</td></tr><tr><td>SDSS J232235.6-094438.1</td><td>0.372</td></tr><tr><td>SDSS J234932.7-003645.8</td><td>0.279</td></tr><tr><td>SDSS J235156.1-010913.3</td><td>0.174</td></tr><tr><td>419</td><td>0.034</td></tr><tr><td>432</td><td>0.060</td></tr><tr><td>441</td><td>0.039</td></tr><tr><td>447</td><td>0.057</td></tr><tr><td>488</td><td>0.057</td></tr><tr><td>689</td><td>0.084</td></tr><tr><td>1015</td><td>0.056</td></tr><tr><td>1079</td><td>0.144</td></tr><tr><td>1151</td><td>0.060</td></tr></tbody></table>
## Appendix D: Comparison of the near-infrared spectral region of the type 1 composite with the literature

The list of the sources used to build the NIR composite is given in Table D.1. We further investigated whether the NIR composite we included in the type 1 AGN main stack (Sect. 2) can be improved by including additional NIR spectra of quasars in the literature. For this purpose we collected all the cross-dispersed NIR spectra published by Landt et al. (2008, 2011, 2013). This data set consists on 29 well-known local type 1 AGN observed at the NASA infrared telescope facility (IRTF), a 3 m telescope, and the 8 m Gemini North Observatory, both located on Mauna Kea, Hawai'i (see Table 1 in Landt et al. 2014 for a summary of the sample properties). We note that a few sources in their sample that display a continuum dominated by the host galaxy emission were not considered here. The data were corrected for Galactic absorption, shifted to the rest-frame and stacked in the same way as done for the SDSS spectra (see Sect. 2.2). The resulting composite is shown in Fig. D.1 with the red line (available at the CDS). Our composite is flatter (redder), likely due to the contribution of the host galaxy emission, and it is more noisy around the Paα emission line, but there is good agreement overall. We defer a more in-depth analysis of the rest-frame NIR region to future work. A version of the composite shown in Fig. 4 corrected for host galaxy emission in the optical, for the IGM absorption in the UV, and that extends in the near-infrared through the spectral composite we build by considering the Landt et al. AGN sample, is also available online. The host galaxy correction at optical wavelengths (3500 < λ/Å < 7400) has been performed by assuming the intrinsic continuum of the type 1 template published by Saccheo et al. (2023).

<table><caption>**Table D.1.** List of AGN used to construct the NIR composite. The SDSS AGN in the table are drawn from Glikman et al. (2006), whilst the remaining are BAT AGN from Ricci et al. (2022).</caption><thead><tr><th>Name</th><th>Redshift</th></tr></thead><tbody><tr><td>SDSS J000943.1-090839.2</td><td>0.210</td></tr><tr><td>SDSS J001327.3+005231.9</td><td>0.362</td></tr><tr><td>SDSS J005709.9+144610.1</td><td>0.172</td></tr><tr><td>SDSS J005812.8+160201.3</td><td>0.211</td></tr><tr><td>SDSS J010226.3-003904.6</td><td>0.295</td></tr><tr><td>SDSS J011110.0-101631.8</td><td>0.179</td></tr><tr><td>SDSS J015530.0-085704.0</td><td>0.165</td></tr><tr><td>SDSS J015910.0+010514.5</td><td>0.217</td></tr><tr><td>SDSS J015950.2+002340.8</td><td>0.163</td></tr><tr><td>SDSS J021707.8-084743.4</td><td>0.292</td></tr><tr><td>SDSS J024250.8-075914.2</td><td>0.378</td></tr><tr><td>SDSS J031209.2-081013.8</td><td>0.265</td></tr><tr><td>SDSS J032213.8+005513.4</td><td>0.185</td></tr><tr><td>SDSS J034042.93-073125.5</td><td>0.217</td></tr><tr><td>SDSS J073623.12+392617.8</td><td>0.118</td></tr><tr><td>SDSS J125519.6+014412.2</td><td>0.343</td></tr><tr><td>SDSS J150610.5+021649.9</td><td>0.135</td></tr><tr><td>SDSS J170441.3+604430.5</td><td>0.372</td></tr><tr><td>SDSS J172711.8+632241.8</td><td>0.218</td></tr><tr><td>SDSS J211843.2-063618.0</td><td>0.328</td></tr><tr><td>SDSS J212619.6-065408.9</td><td>0.418</td></tr><tr><td>SDSS J232235.6-094438.1</td><td>0.372</td></tr><tr><td>SDSS J234932.7-003645.8</td><td>0.279</td></tr><tr><td>SDSS J235156.1-010913.3</td><td>0.174</td></tr><tr><td>419</td><td>0.034</td></tr><tr><td>432</td><td>0.060</td></tr><tr><td>441</td><td>0.039</td></tr><tr><td>447</td><td>0.057</td></tr><tr><td>488</td><td>0.057</td></tr><tr><td>689</td><td>0.084</td></tr><tr><td>1015</td><td>0.056</td></tr><tr><td>1079</td><td>0.144</td></tr><tr><td>1151</td><td>0.060</td></tr></tbody></table>
<figure><img src="image_21.png" /><figcaption>**Fig. D.1.** Comparison of the empirical composite incident spectrum of type 1 AGN utilised in this work (shown with the black solid line) with the one we constructed from the datasets published by Landt et al. (2008; 2011; 2013, in red). The Landt et al. composite is normalised at the 16 000 Å flux of our stack. Main emission lines are marked.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. D.1.** Comparison of the empirical composite incident spectrum of type 1 AGN utilised in this work (shown with the black solid line) with the one we constructed from the datasets published by Landt et al. (2008; 2011; 2013, in red). The Landt et al. composite is normalised at the 16 000 Å flux of our stack. Main emission lines are marked.</figcaption></figure>
# Appendix E: Comparison of the semi-empirical composite spectrum of type 2 AGN with the literature

The composite incident spectra utilised for the type 1 and the type 2 AGN have been computed in a very different way. The former have been derived from observed type 1 spectra of optically selected galaxies from the SDSS, while the latter from simulations (i.e. SPRITZ) with empirical composites of different kind of galaxies (e.g. classical obscured AGN, star-bursts and star forming galaxies with different AGN fractions, see Sect. 3 for details) considered as an input. The choice of using SPRITZ is justified by the fact that it is more difficult to define a clean sample of type 2 AGN. Indeed, even if type 2 AGN display strong narrow high-ionisation emission lines (with FWHM < 2000 km s⁻¹) and weak continua (Zakamska et al. 2003) in the rest-frame optical spectra, the literature works often refer to type 2 AGN as 'candidates', as their selection strongly depends on the diagnostic (and thus the available emission lines) considered (e.g. Hao et al. 2005; Reyes et al. 2008; Alexandroff et al. 2013; Mignoli et al. 2013).

We thus want to test whether our type 2 composite derived from a semi-empirical library is consistent with what is currently available in the literature. Figure E.1 shows the comparison of the semi-empirical template spectra of type 2 AGN utilised in this work (shown with the black solid line) with the empirical one published by Yuan et al. (2016). The blue line represents the composite spectrum of all type 2 quasars in the baryon oscillation spectroscopy survey (BOSS) analysed by Yuan et al. (2016).

The excess flux at $\lambda > 6500$ Å is due to the host galaxy emission, which also seems to be dominant at bluer wavelengths. Therefore, the difference between the composite by Yuan et al. (2016) and ours is mainly due to the different host galaxy. The red line is derived by normalising the composite spectrum by Yuan et al. (2016) to a continuum obtained by spline-interpolating between relatively line-free regions. This stack preserves the equivalent widths of the emission lines, although the intrinsic shape of the continuum is lost (see their Fig. 15 and their Sect. 3.7). Overall, there is good consistency between the two composites regarding the $H\alpha+[N\text{II}]$ complex and the spectral region around the $H\beta$ and [OIII]. There are instead some differences in the strength of the emission lines in the UV at $\lambda < 4000$ Å, such as [OII], [NeIII] and $H\delta$, with a few spectral features missing in our template (e.g. [NeV]).

To this comparison we also included the composite published by Mignoli et al. (2013, green line in Fig. E.1) obtained from a sample of 94 narrow-line AGN in the redshift range $0.65 < z < 1.20$. Their sample is selected from the 20k-Bright zCOSMOS galaxy sample by the detection of the high-ionisation [NeV]$\lambda$3426 line. This composite shows strong, high-ionisation narrow lines, such as [NeV], that is not present in our stack, and a very faint MgII emission line consistent with the emission seen in our composite. We stress that these differences do not affect any result in our analysis, as our comparison is mainly focused on the $H\alpha$ spectral region rather than the UV and the overall shape of the continuum. In the future, we will implement a more data-driven composite for the type 2 AGN to improve the incident template library for type 2 AGN in the UV.

<figure><img src="image_22.png" /></figure>

**Fig. E.1.** Comparison of the semi-empirical template incident spectra of type 2 AGN utilised in this work (shown with the black solid line) with the empirical one published by Yuan et al. (2016). The blue line represents the composite spectrum of all BOSS type 2 quasars in the sample analysed by Yuan et al. (2016), whilst the red line is derived by normalising the composite spectrum to a continuum obtained by spline-interpolating between relatively line-free regions. The green line represents the spectral stack of 94 narrow-line AGN published by Mignoli et al. (2013). For sake of comparison, the literature composites are normalised at the 5100 Å flux of our stack. Main emission lines are marked. The bottom panels are two zoom-ins around the $H\beta$ (left) and the $H\alpha$ (right) the spectral regions. Stellar absorption features are also included and visible in our type 2 AGN spectrum between $H\gamma$ and $H\beta$ (see also Fig. A1 by Vidal-García et al. 2017 and Figs. 9 e 10 by Bruzual & Charlot 2003), which will be blended to the continuum in any moderate (low) resolution spectrum.
## Appendix E: Comparison of the semi-empirical composite spectrum of type 2 AGN with the literature

The composite incident spectra utilised for the type 1 and the type 2 AGN have been computed in a very different way. The former have been derived from observed type 1 spectra of optically selected galaxies from the SDSS, while the latter from simulations (i.e. SPRITZ) with empirical composites of different kind of galaxies (e.g. classical obscured AGN, star-bursts and star forming galaxies with different AGN fractions, see Sect. 3 for details) considered as an input. The choice of using SPRITZ is justified by the fact that it is more difficult to define a clean sample of type 2 AGN. Indeed, even if type 2 AGN display strong narrow high-ionisation emission lines (with FWHM < 2000 km s⁻¹) and weak continua (Zakamska et al. 2003) in the rest-frame optical spectra, the literature works often refer to type 2 AGN as 'candidates', as their selection strongly depends on the diagnostic (and thus the available emission lines) considered (e.g. Hao et al. 2005; Reyes et al. 2008; Alexandroff et al. 2013; Mignoli et al. 2013).

We thus want to test whether our type 2 composite derived from a semi-empirical library is consistent with what is currently available in the literature. Figure E.1 shows the comparison of the semi-empirical template spectra of type 2 AGN utilised in this work (shown with the black solid line) with the empirical one published by Yuan et al. (2016). The blue line represents the composite spectrum of all type 2 quasars in the baryon oscillation spectroscopy survey (BOSS) analysed by Yuan et al. (2016).

The excess flux at $\lambda > 6500$ Å is due to the host galaxy emission, which also seems to be dominant at bluer wavelengths. Therefore, the difference between the composite by Yuan et al. (2016) and ours is mainly due to the different host galaxy. The red line is derived by normalising the composite spectrum by Yuan et al. (2016) to a continuum obtained by spline-interpolating between relatively line-free regions. This stack preserves the equivalent widths of the emission lines, although the intrinsic shape of the continuum is lost (see their Fig. 15 and their Sect. 3.7). Overall, there is good consistency between the two composites regarding the $H\alpha+[N\text{II}]$ complex and the spectral region around the $H\beta$ and [OIII]. There are instead some differences in the strength of the emission lines in the UV at $\lambda < 4000$ Å, such as [OII], [NeIII] and $H\delta$, with a few spectral features missing in our template (e.g. [NeV]).

To this comparison we also included the composite published by Mignoli et al. (2013, green line in Fig. E.1) obtained from a sample of 94 narrow-line AGN in the redshift range $0.65 < z < 1.20$. Their sample is selected from the 20k-Bright zCOSMOS galaxy sample by the detection of the high-ionisation [NeV]$\lambda$3426 line. This composite shows strong, high-ionisation narrow lines, such as [NeV], that is not present in our stack, and a very faint MgII emission line consistent with the emission seen in our composite. We stress that these differences do not affect any result in our analysis, as our comparison is mainly focused on the $H\alpha$ spectral region rather than the UV and the overall shape of the continuum. In the future, we will implement a more data-driven composite for the type 2 AGN to improve the incident template library for type 2 AGN in the UV.

<figure><img src="image_23.png" /></figure>

**Fig. E.1.** Comparison of the semi-empirical template incident spectra of type 2 AGN utilised in this work (shown with the black solid line) with the empirical one published by Yuan et al. (2016). The blue line represents the composite spectrum of all BOSS type 2 quasars in the sample analysed by Yuan et al. (2016), whilst the red line is derived by normalising the composite spectrum to a continuum obtained by spline-interpolating between relatively line-free regions. The green line represents the spectral stack of 94 narrow-line AGN published by Mignoli et al. (2013). For sake of comparison, the literature composites are normalised at the 5100 Å flux of our stack. Main emission lines are marked. The bottom panels are two zoom-ins around the $H\beta$ (left) and the $H\alpha$ (right) the spectral regions. Stellar absorption features are also included and visible in our type 2 AGN spectrum between $H\gamma$ and $H\beta$ (see also Fig. A1 by Vidal-García et al. 2017 and Figs. 9 e 10 by Bruzual & Charlot 2003), which will be blended to the continuum in any moderate (low) resolution spectrum.
## Appendix F: Analysis of the flux loss

Here we present some additional test we performed to investigate whether a fraction of the missing flux could be lost at the slitless spectra extraction level (see Sect. 5.1.3 in Euclid Collaboration 2023a, for details). We first compared the distribution of the ratios $r$ of the median observed flux values of the incident spectra $F_{\text{inc}}$ and the simulated spectra $F_{\text{sim}}$ (i.e., $r = \langle F_{\text{sim}} \rangle / \langle F_{\text{inc}} \rangle$, where the angle brackets $\langle \rangle$ represent medians); see Fig. F.1. The median of this distribution is about 12% for type 1 AGN and 2% for type 2 AGN, with a percentage error on the distributions of spectrum flux median values (defined as $1.253 \sigma / \sqrt{N}$, where $\sigma$ and $N$ represent the standard dispersion of the distribution and the number of objects, respectively) at the 2.5% level for type 1 and 1% for type 2. As an example, we present a comparison between the incident (black) and simulated (red) spectra for a representative type 1 (left) and a type 2 (right) AGN at the top row of Fig. F.1. The inset plot is a zoom-in of a continuum (emis-

sion line free) region. The overall flux of the simulated spectra are systematically lower for both the type 1 and 2 AGN. For completeness, we also carried out the same exercise of comparing median luminosity density values for the best-fit models of the incident and simulated spectra to their respective data [i.e. ratio = median(data) / median(model)], finding that the QSFIT best-fit model is close to the data, on average, with the biggest discrepancy for type 1 simulated spectra at ≈2% level. This analysis implies that flux loss could partly explain the remaining fraction of flux missing in the comparison between simulated and incident flux values, although most of the difference on the $H\alpha+[N\ II]$ emission between the simulated and incident data is due to the assumptions made in the spectral modelling of the emission lines. Nonetheless, the OU-SIR team is working on the spectral extraction pipeline and flux losses will be alleviated by the optimal extraction routine that is implemented in the upcoming version of the OU-SIR pipeline.

<figure><img src="image_23.png" /></figure>

**Fig. F.1.** *Top row:* Example of a comparison between the incident (black) and simulated (red) spectra for a type 1 (**left**) and a type 2 (**right**) AGN around the $H\alpha$ region. The inset plot is a zoom-in of a continuum (emission line free) region. *Bottom row:* distribution of the ratios of the median flux value (i.e., the median of the observed flux of the total spectrum) of the incident and simulated data for both type 1 (black) and type 2 (blue) AGN. The median values for the two distributions are also reported. The percentage error on the distributions of the spectrum flux median values are computed as $1.253 \sigma / \sqrt{N}$, where $\sigma$ and $N$ represent the standard dispersion of the distribution and the number of objects, respectively.
## Appendix F: Analysis of the flux loss

Here we present some additional test we performed to investigate whether a fraction of the missing flux could be lost at the slitless spectra extraction level (see Sect. 5.1.3 in Euclid Collaboration 2023a, for details). We first compared the distribution of the ratios $r$ of the median observed flux values of the incident spectra $F_{\text{inc}}$ and the simulated spectra $F_{\text{sim}}$ (i.e., $r = \langle F_{\text{sim}} \rangle / \langle F_{\text{inc}} \rangle$, where the angle brackets $\langle \rangle$ represent medians); see Fig. F.1. The median of this distribution is about 12% for type 1 AGN and 2% for type 2 AGN, with a percentage error on the distributions of spectrum flux median values (defined as $1.253 \sigma / \sqrt{N}$, where $\sigma$ and $N$ represent the standard dispersion of the distribution and the number of objects, respectively) at the 2.5% level for type 1 and 1% for type 2. As an example, we present a comparison between the incident (black) and simulated (red) spectra for a representative type 1 (left) and a type 2 (right) AGN at the top row of Fig. F.1. The inset plot is a zoom-in of a continuum (emis-

sion line free) region. The overall flux of the simulated spectra are systematically lower for both the type 1 and 2 AGN. For completeness, we also carried out the same exercise of comparing median luminosity density values for the best-fit models of the incident and simulated spectra to their respective data [i.e. ratio = median(data) / median(model)], finding that the QSFIT best-fit model is close to the data, on average, with the biggest discrepancy for type 1 simulated spectra at ≈2% level. This analysis implies that flux loss could partly explain the remaining fraction of flux missing in the comparison between simulated and incident flux values, although most of the difference on the $H\alpha + [N\ II]$ emission between the simulated and incident data is due to the assumptions made in the spectral modelling of the emission lines. Nonetheless, the OU-SIR team is working on the spectral extraction pipeline and flux losses will be alleviated by the optimal extraction routine that is implemented in the upcoming version of the OU-SIR pipeline.

<figure><img src="image_24.png" /><figcaption>**Fig. F.1.** Top row: Example of a comparison between the incident (black) and simulated (red) spectra for a type 1 (**left**) and a type 2 (**right**) AGN around the $H\alpha$ region. The inset plot is a zoom-in of a continuum (emission line free) region. Bottom row: distribution of the ratios of the median flux value (i.e., the median of the observed flux of the total spectrum) of the incident and simulated data for both type 1 (black) and type 2 (blue) AGN. The median values for the two distributions are also reported. The percentage error on the distributions of the spectrum flux median values are computed as $1.253 \sigma / \sqrt{N}$, where $\sigma$ and $N$ represent the standard dispersion of the distribution and the number of objects, respectively.</figcaption></figure>