No [C II] or dust detection in two Little Red Dots at <i>z</i><sub>spec</sub>> 7
Mengyuan Xiao, Pascal A. Oesch, L. Bing, D. Elbaz, Jorryt Matthee, Yoshinobu Fudamoto, Seiji Fujimoto, R. Marques-Chaves, Christina C. Williams, M. Dessauges‐Zavadsky, Francesco Valentino, Gabriel Brammer, Alba Covelo-Paz, Emanuele Daddi, J. P. U. Fynbo, Steven Gillman, M. Ginolfi, Emma Giovinazzo, Jenny Greene, Qiusheng Gu, G. D. Illingworth, Kohei Inayoshi, Vasily Kokorev, R. A. Meyer, Rohan P. Naidu, Naveen A. Reddy, D. Schaerer, Alice E. Shapley, Mauro Stefanon, Charles L. Steinhardt, David J. Setton, M. Vestergaard, Tao Wang
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 H β 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: > 16 σ for [C II ] and > 3 σ for the 1.3 mm continuum of ID9094, and > 5 σ for the [C II ] of ID2756. Using the 3 σ 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.