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

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 zspec > 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]–SFRtot 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.