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

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.

Figures (6)

• Figure 1: Top:JWST/NIRCam RGB cutouts of our targets obtained from publicly available imaging from the JWST/DAWN archive. Bottom: NIRSpec/PRISM spectra obtained from the BlackTHUNDER program (for JADES_954 and CEERS_00397) and from JADES DR3 for JADES_1146115. The spectra are normalized at the Balmer limit to emphasize the different $\beta_{opt}$ at redder wavelengths. JADES_1146115 is a LRD, JADES_954 has the V-shape but is slightly resolved, while CEERS_00397 is an extended blue Type I AGN.
• Figure 2: Top: median stack of the 1D spectra extracted at the position of the sources with beam-size apertures. The blue spectrum and the two blue horizontal dashed lines indicate the median stacked spectrum and rms at the native resolution of the cubes (2.5 km s$^{-1}$), while the orange lines refer to velocity channels of 25 km s$^{-1}$. Bottom: median stack map (50 pixels $\times$ 50 pixels) of the continuum maps of the three targets, revealing no detection in the center (the expected position of the targets).
• Figure 3: Position of our [C ii] $3\sigma$ upper limits (coloured hexagons and arrows) in the $L_{[CII]}-SFR$ plane. The black hexagon and arrow represent the results from the [C ii] stack. The black line and the gray shaded area represent the conservative relation from Schaerer20 and its scatter. The green dash-dotted lines represent correlations taken from HerreraCamus18 where the [C ii] is suppressed at a given SFR due to specific physical conditions, such as the presence of an AGN, galaxies in a starburst phase, or high radiation field intensities (i.e., high $\Sigma_{FIR}$). The horizontal dashed lines represent the range of SFR predicted by the SED-fitting decomposition presented in Sect. \ref{['sec:dustund']}. In particular, the higher value corresponds to the instantaneous SFR and the lower one to the SFR averaged over the last 10 Myr. We also show upper limits of other LRD and high-z JWST AGN undetected in [C ii] : Akins25_CI, Xiao25_noema. The gray hexagon shows a lensed LRD candidate Golubchik25, with a [C ii] detection reported by Knudsen16. The gray circle shows GS_3073 Vanzella06, a blue compact and X-ray weak $z\sim 5.5$ AGN with bolometric luminosity comparable to those of our sources, with SFR derived from narrow H$\alpha$ by Ubler23 and [C ii] luminosity by Bethermin20.
• Figure 4: SED fitting decomposition of JADES_954 (top), JADES_1146115 (middle), and CEERS_397 (bottom). JWST/PRISM data points are reported in blue, JWST/MIRI data in orange, and our new NOEMA upper limits in green. The positions of the upper limits (triangles) are reported at $2.5\sigma$, allowing the errors to go from zero to $5\sigma$. The blue line represents the stellar contribution, while the red solid line represents the AGN contribution (sum of obscured disk emission, torus emission, and polar dust emission). The red dashed line represents the contribution coming from the accretion disk emission, that for JADES_954, and JADES_1146115 is obscured in the polar direction. The brown line represents the host galaxy dust emission. The final model is reported in gray.
• Figure 5: $IRX$ versus $\beta_{UV}$ of our targets. The upper and lower limits of the three shaded areas are given by the FIR luminosities from the FIR modified blackbody fit and the SED-fitting (energy balance), respectively. The width of the shaded area in the horizontal direction is given by the uncertainties on $\beta_{UV}$. The expected relation for Calzetti-like dust is shown by the grey solid line, while the expected relation from an SMC extinction curve is shown by the dashed line Meurer99Reddy18a. These models assume an intrinsic $\beta_0$ for the stellar population of $-2.3$, consistent with recent JWST results at $z>6$Saxena24. Grey points show $6<z<8$ detections from the REBELS survey by Bowler24_IRX.