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

TL;DR

This study uses NOEMA FIR observations to probe two $z_{ m spec} > 7$ Little Red Dots (LRDs) identified by JWST in GOODS-North. Both sources show no detections in $[\mathrm{C\ II}]\,158\ \mu\mathrm{m}$ or 1.3 mm continuum, enabling $3\sigma$ upper limits that challenge a dusty star-forming galaxy interpretation. UV-to-FIR SED fitting with and without an AGN component, together with the $L_{\mathrm{[C\ II]}}$–SFR$_{\rm tot}$ relation, suggest an AGN contribution is plausible for ID9094, though a DSFG origin cannot be excluded given current data for ID2756. The results demonstrate the critical role of FIR data in distinguishing AGN-driven from star-formation-driven emission in high-$z$ LRDs and underscore the need for deeper FIR/submillimeter observations to fully characterize these extreme systems.

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 z$_{\rm spec}$>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 H$β$ emission lines (full width at half maximum: 2280$\pm$490 km/s for ID9094 and 1070$\pm$240 km/s for ID2756) and narrow [OI] lines ($\sim$ 300-400 km/s). To investigate their nature, we conduct deep NOEMA follow-up observations targeting the [CII] 158${\rm μm}$ 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: $>16σ$ for [CII] and $>3σ$ for the 1.3 mm continuum of ID9094, and $>5σ$ for [CII] of ID2756. Using the 3$σ$ 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$_{[CII]}$-SFR$_{tot}$ 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.

Figures (6)

• Figure 1: JWST images and spectra of the two sources. Top: 2$^{\prime\prime}$$\times$ 2$^{\prime\prime}$ stamps obtained in JWST/NIRCam filters (0.90$\mu$m,1.15$\mu$m,1.82$\mu$m, 2.10$\mu$m, 3.56$\mu$m, and 4.44$\mu$m). Bottom: 1D spectra (covering H$\beta$, [O iii] $\lambda\lambda4960,5008\AA$ emission lines) obtained from NIRCam/grism observations with the F410M filter, with RGB images embedded (F182M in blue, F210M in green, and F444W in red). The gray shaded areas show the associated 1$\sigma$ uncertainty. The best-fit Gaussian line model is shown in blue. Both sources have a broad H$\beta$ line, where the solid orange line shows the narrow component and the dashed red line shows the broad component.
• Figure 2: The locations of the two sources (red and blue stars) relative to the typical color and compactness selection criteria for LRDs. The red-shaded regions highlight LRDs exhibiting "v-shape" SED (left), red (middle), and compact morphology (right), defined by the criteria: $-0.5< \rm{F115W}-\rm{F200W} < 1.0$, $\rm{F277W}-\rm{F444W} > 1.0$, $\rm{F277W}-\rm{F356W} > 0.6$, and $f_{\rm F444W}(0\farcs4)/f_{\rm F444W}(0\farcs2) < 1.7$ (see Sect. \ref{['Sec: lrd']}). Error bars indicate 1$\sigma$ uncertainties. The blue-shaded region represents the distribution of sources observed in these filters from the prime JWST blank legacy fields in the DJA.
• Figure 3: No detection of the [C ii]$\,158\,{\rm \mu m}$ emission line and 1.3 mm dust continuum with NOEMA observations. Left: [C ii]$\,158\,{\rm \mu m}$ line moment-0 map (assuming line width of 250 km s$^{-1}$) and 1.3 mm dust contours overlaid on the JWST RGB image ($10^{\prime\prime}\times10^{\prime\prime}$). The contour levels start at 1$\sigma$ and increase in steps of $\pm1\sigma$, where positive and negative contours are solid and dashed, respectively. The beam sizes are displayed in the lower left corner. Right: NOEMA 1.3 mm spectra with 50 km s$^{-1}$ binning. The grey lines show the associated 1$\sigma$ uncertainty. The vertical dashed lines highlight the locations of [C ii]$\,158\,{\rm \mu m}$. The spectra are taken at the positions of the sources and match the apertures of the beam sizes, assuming the emission line is unresolved.
• Figure 4: Best-fit SEDs of ID9094 with fixed dust temperatures, providing the first hint of an AGN scenario in this work. Top panels: Best-fit SEDs without an AGN component; bottom panels: Best-fit SEDs with an AGN component. The 3$\sigma$ upper limit from NOEMA 1.3 mm, the deepest FIR photometric constraint, is highlighted, playing a key role in shaping the FIR SED and constraining the lower limit of $T_{\rm dust}$. The FIR photometry has been corrected for CMB effects. We show three representative $T_{\rm dust}$ during the fitting with and without AGN component, which results in the predicted 1.3 mm flux exceeding ($a$ and $d$), same as ($b$ and $e$), below ($c$ and $f$) the observed 3$\sigma$ upper limit. Panel $d$ shows the fit with AGN and $T_{\rm dust} = 40$ K, which slightly exceeds the 3$\sigma$ limit but is not ruled out given the model degeneracy and observational uncertainties (see Sect. \ref{['Sec: sed']}). Without an AGN, the SED fitting requires $T_{\rm dust} \,\gtrsim\, 110$ K. This value significantly exceeds the typical $T_{\rm dust}\sim40$ K observed in REBELS galaxies at similar redshifts Sommovigo2022, and may indicate additional dust heating mechanisms such as AGN activity. In contrast, including an AGN component (bottom panels) allows acceptable fits with $T_{\rm dust}$ as low as $\sim$40-60 K, similar to AGN and/or quasar host galaxies at $z>4$Walter2022Tsukui2023Tripodi2023Decarli2023, supporting the presence of an AGN in ID9094.
• Figure 5: [CII] luminosity as a function of SFR, providing the second hint of an AGN scenario in this work. ID9094 is shown with 3$\sigma$ upper limits on $L_{\rm [C\,\textsc{ii}]}$ ($L_{\rm [C\,\textsc{ii}]} < 1.7 \times 10^{8} L_\odot$) in red and blue arrows, representing cases with (fixed $T_{\rm dust} = 60$ K) and without (fixed $T_{\rm dust} = 110$ K) an AGN component, respectively. We note that results for $T_{\rm dust} = 40$ K with an AGN component are not shown here, as they yield similar or even better agreement with the relation (see Sect. \ref{['Sec: relation']}). The blue square indicates the expected $L_{\rm [C\,\textsc{ii}]}$ location of ID9094 if it is an SFG without an AGN. Grey and black solid lines, along with their shaded regions, represent the $L_{\rm [C\,\textsc{ii}]}$–SFR relations and $1\sigma$ uncertainties for low-redshift galaxies delooze2014 and high-redshift ALPINE galaxies schaerer2020, respectively. The orange dashed line shows the relation for low-$z$ composite galaxies and AGNs delooze2014. Previous observations of $z>6$ galaxies are plotted as grey points or downward triangles representing 3$\sigma$ upper limits harikane2020Schouws2023Fudamoto2024b.