No [C II] or dust detection in two Little Red Dots at z_spec> 7

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Cosmic Dawn Center (DAWN), Copenhagen, Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København N, DK-220, Denmark
⁴ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
⁷ Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
⁸ Steward Observatory, University of Arizona, 933 N Cherry Avenue, Tucson, AZ 85721, USA
⁹ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
¹⁰ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹¹ NSF National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹² DTU-Space, Elektrovej, Building 327, 2800 Kgs. Lyngby, Denmark
¹³ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, I-50019 Sesto F.no (Firenze), Italy
¹⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Florence, Italy
¹⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton NJ 08544, USA
¹⁶ School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu 210093, China
¹⁷ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, China
¹⁸ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
²⁰ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
²¹ Department of Physics and Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA
²² Department of Physics & Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles CA 90095, USA
²³ Departament d’Astronomia i Astrofìsica, Universitat de València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain
²⁴ Unidad Asociada CSIC “Grupo de Astrofísica Extragaláctica y Cosmología” (Instituto de Física de Cantabria – Universitat de València), Spain
²⁵ Department of Physics and Astronomy, University of Missouri, 701 S. College Ave., Columbia, MO 65201, USA
²⁶ DARK, The Niels Bohr Institute, Jagtvej 155, 2200 Copenhagen N, Denmark

^⋆ Corresponding author: mengyuan.xiao@unige.ch

Received: 3 March 2025
Accepted: 1 July 2025

Abstract

Little Red Dots (LRDs) are compact, point-like sources characterized by their red color and broad Balmer lines; it is a matter of debate whether they are dominated by active galactic nuclei (AGNs) or dusty star-forming galaxies (DSFGs). Here we report two LRDs (ID9094 and ID2756) at z_spec > 7 recently discovered in the JWST FRESCO GOODS-North field. Both satisfy the “v-shaped” color and compactness criteria for LRDs and are identified as Type-I AGN candidates based on their broad Hβ emission lines (full width at half maximum: 2280 ± 490 km s⁻¹ for ID9094 and 1070 ± 240 km s⁻¹ for ID2756) and narrow [O III] lines (≃300 − 400 km s⁻¹). To investigate their nature, we conducted deep NOEMA follow-up observations targeting the [C II] 158 μm emission line and the 1.3 mm dust continuum. We do not detect [C II] or 1.3 mm continuum emission for either source. If the two LRDs were DSFGs, we would expect significant detections: > 16σ for [C II] and > 3σ for the 1.3 mm continuum of ID9094, and > 5σ for the [C II] of ID2756. Using the 3σ upper limits of [C II] and 1.3 mm, we performed two analyses: (1) UV-to-far-infrared spectral energy distribution fitting with and without AGN components, and (2) comparison of their properties with the L_[C II]–SFR_tot empirical relation. Both analyses are consistent with a scenario in which AGN activity contributes to the observed properties, though a dusty star-forming origin cannot be fully ruled out. Our results highlight the importance of far-infrared observations for studying LRDs, a regime that remains largely unexplored.