English

No [CII] or dust detection in two Little Red Dots at z$_{\rm spec}$ > 7

Astrophysics of Galaxies 2025-08-27 v2

Abstract

Little Red Dots (LRDs) are compact, point-like sources characterized by their red color and broad Balmer lines, which have been debated to be either dominated by active galactic nuclei (AGN) or dusty star-forming galaxies (DSFGs). Here we report two LRDs (ID9094 and ID2756) at zspec_{\rm spec}>7, recently discovered in the JWST FRESCO GOODS-North field. Both satisfy the "v-shape" colors and compactness criteria for LRDs and are identified as Type-I AGN candidates based on their broad Hβ\beta emission lines (full width at half maximum: 2280±\pm490 km/s for ID9094 and 1070±\pm240 km/s for ID2756) and narrow [OI] lines (\sim 300-400 km/s). To investigate their nature, we conduct deep NOEMA follow-up observations targeting the [CII] 158μm{\rm \mu m} emission line and the 1.3 mm dust continuum. We do not detect [CII] or 1.3 mm continuum emission for either source. Notably, in the scenario that the two LRDs were DSFGs, we would expect significant detections: >16σ>16\sigma for [CII] and >3σ>3\sigma for the 1.3 mm continuum of ID9094, and >5σ>5\sigma for [CII] of ID2756. Using the 3σ\sigma upper limits of [CII] and 1.3 mm, we perform two analyses: (1) UV-to-FIR spectral energy distribution (SED) fitting with and without AGN components, and (2) comparison of their properties with the L[CII]_{[CII]}-SFRtot_{tot} empirical relation. Both analyses are consistent with a scenario where AGN activity may contribute 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.

Keywords

Cite

@article{arxiv.2503.01945,
  title  = {No [CII] or dust detection in two Little Red Dots at z$_{\rm spec}$ > 7},
  author = {Mengyuan Xiao and Pascal A. Oesch and Longji Bing and David Elbaz and Jorryt Matthee and Yoshinobu Fudamoto and Seiji Fujimoto and Rui Marques-Chaves and Christina C. Williams and Miroslava Dessauges-Zavadsky and Francesco Valentino and Gabriel Brammer and Alba Covelo-Paz and Emanuele Daddi and Johan P. U. Fynbo and Steven Gillman and Michele Ginolfi and Emma Giovinazzo and Jenny E. Greene and Qiusheng Gu and Garth Illingworth and Kohei Inayoshi and Vasily Kokorev and Romain A. Meyer and Rohan P. Naidu and Naveen A. Reddy and Daniel Schaerer and Alice Shapley and Mauro Stefanon and Charles L. Steinhardt and David J. Setton and Marianne Vestergaard and Tao Wang},
  journal= {arXiv preprint arXiv:2503.01945},
  year   = {2025}
}

Comments

12 pages, 5 main figures, 1 extended figure, 3 tables; accepted by A&A