Constraints on the host galaxy and AGN properties of three z > 6 JWST AGN from NOEMA observations

Giovanni Mazzolari, Hannah Übler, Rodrigo Herrera Camus, Ric Davies, Linda Tacconi, Dieter Lutz, Natascha Förster Schreiber, Francesco D'Eugenio, Minju Lee, Capucine Barfety, Elena Bertola, Andrew Bunker, Andreas Burkert, Jianhang Chen, Giovanni Cresci, Frank Eisenhauer, Juan Manuel Espejo Salcedo, Simon Flesch, Reinhard Genzel, Xihan Ji, Lilian Lee, Daizhong Liu, Cosimo Marconcini, Roberto Maiolino, Thorsten Naab, Amit Nestor Shachar, Meghana Pannikkote, Eleonora Parlanti, Stavros Pastras, Michele Perna, Claudia Pulsoni, Bruno Rodriguez del Pino, Eckhard Sturm, Taro Shimizu, Giulia Tozzi

First listed 2026-03-31 | Last updated 2026-03-31

Abstract

We targeted with deep NOEMA observations the [CII]158$μ$m emission of three JWST-discovered AGN at z>6. Two of them have the typical features of Little Red Dots (LRDs), while the third one is a blue, extended, Type I AGN. We do not significantly detect [CII] emission or dust continuum in any of the targets, even after stacking. The resulting [CII] luminosity upper limits, $\log (L_{[CII]}/L_{\odot})<7.77-8.1$, lie $\sim2σ$ below the values expected from the [CII]-SFR relation, and we explore different scenarios to explain the lack of [CII]. We obtained upper limits on the gas masses of $\log (M_{gas}/M_{\odot})<9.26-9.59$ corresponding to $\log( M_{dust}/M_{\odot})<5.68-6.55$ assuming a metallicity dependent dust to gas ratio. Using the continuum non-detections (rms $\sim 16-25 ~μJy$) together with JWST/MIRI constraints, we performed a revised SED-fitting decomposition, resulting in stellar masses up to $\sim 2$ dex lower than previously reported, and implying $0.03\lesssim M_{BH}/M_{*}\lesssim0.7$. For the two LRDs, the SED is well reproduced by stellar emission in the rest-frame UV, while the rising rest-frame optical slope, flattening toward the near-infrared, is consistent with emission from a Type I AGN partially obscured along the polar direction with $E(B-V)_{\rm polar}\simeq 1$, in agreement with attenuation derived from the broad lines Balmer decrement. This decomposition demonstrates that a relatively standard AGN configuration can reproduce the SEDs of the two LRDs, without invoking more exotic scenarios. Finally, we investigate the positions of the three sources in the $IRX-β_{UV}$ plane, finding that they lie in a parameter space where galaxies are typically characterized by patchy dust distributions. Our analysis highlights the importance of millimeter constraints to characterize the different physical properties of high-z AGN.

Short digest

This paper uses NOEMA observations to constrain the host-galaxy and AGN properties of three z > 6 JWST AGN, including LRD-like systems. The main result is that the non-detections in [C II] and continuum still push the sources into a suppressed-[C II] regime and sharpen the allowed SED decompositions once the millimeter limits are included. The paper matters because it shows how non-detections can still materially constrain the physical picture of early AGN hosts.

Key figures to inspect

• Figure 3 is the must-see plot: it places the [C II] upper limits on the L[C II]-SFR plane and shows that these systems sit in the suppressed-[C II] regime usually associated with AGN-like or intense radiation-field conditions.
• Figure 4 is where to inspect the SED decomposition itself, including how the stellar, AGN, and host-dust components combine once the NOEMA limits are folded in.
• Figure 1 is the orientation figure: it shows the imaging and PRISM spectra of the three sources side by side, making clear how the LRD-like and AGN-like objects differ morphologically and spectroscopically.
• Figure 2 is the clean null-result figure: it shows the stacked NOEMA spectra and continuum maps, emphasizing that neither [C II] nor continuum is detected even after stacking.