MEGA: Spectrophotometric SED Fitting of Little Red Dots Detected in JWST MIRI

Kaila Ronayne, Casey Papovich, Allison Kirkpatrick, Bren E. Backhaus, Fergus Cullen, Lu Shen, Micaela B. Bagley, Guillermo Barro, Steven L. Finkelstein, Kurt Hamblin, Jeyhan S. Kartaltepe, Dale D. Kocevski, Anton M. Koekemoer, Erini Lambrides, Fabio Pacucci, Guang Yang

First listed 2025-08-27 | Last updated 2025-09-02

Abstract

We analyze eight spectroscopically confirmed Little Red Dots (LRDs) at redshifts $z = 5.1-8.7$ with JWST/NIRCam, NIRSpec, and MIRI data. The LRDs have red NIRCam colors, F150W-F444W $>$ 1, but flat NIRCam-MIRI colors, $-0.5 < \mathrm{F444W - F770W} < 0.5$, suggesting weak warm/hot dust components. The LRDs have $-1.0 < {F1000W - F1500W} < 1.1$, suggestive of non-uniform rest near-IR properties within the sample. We model the spectral energy distributions (SEDs) of the LRDs using the CIGALE and Prospector codes to assess how the differing templates impact the interpretation for LRDs for cases of: (1) models with star-forming stellar populations only; (2) active galactic nuclei (AGN) dominated models; and (3) composite AGN and star-forming models. Using the Bayesian information criterion, we find that six of the eight LRDs favor AGN models compared to star-forming models, though no model reproduces all of the observed properties. Two LRDs with pronounced Balmer-breaks and broad H$α$ have SEDs that are reproduced with hot, dense-gas ($\log T/\mathrm{K}=5-5.7$, $\log n/\mathrm{cm^{-3}} = 9-11$) models with low dust attenuation ($A(V)\simeq 0.5$ mag). However, these models require an additional thermal component (800-1400 K) to account for the MIRI data, and fail to reproduce the rest-UV and narrow [OIII] emission. The total bolometric emission from the dense-gas models, and possibly CIGALE AGN models, appear consistent with literature constraints in the far-IR and radio, and require $\log L_{bol}/L_\odot<12$. These results suggest that our LRDs cannot be modeled entirely with standard templates, but instead require a novel treatment of gas conditions, AGN and star-formation.

Short digest

MEGA fits JWST/NIRCam+NIRSpec+MIRI SEDs for eight spectroscopically confirmed Little Red Dots at z=5.1–8.7 using CIGALE and Prospector across star-forming, AGN-dominated, and composite scenarios. By BIC, six of eight prefer AGN-dominated solutions, consistent with red F150W–F444W > 1 but flat F444W–F770W and −1.0 < F1000W–F1500W < 1.1 colors that imply weak hot dust. Two Balmer-break LRDs with broad Hα are matched by hot, dense-gas (log T=5–5.7; log n=9–11) plus an added 800–1400 K thermal component, with total Lbol/L_sun < 10^12 and consistency with far-IR/radio limits. However, no single template reproduces all observables—the dense-gas fits miss the rest-UV and narrow [O III]—arguing for non-standard gas conditions and mixed AGN–star-formation.

Key figures to inspect

• Figure 1: Inspect NIRCam vs MIRI cutouts and the NIRSpec G395M spectrum of ID 10444 to verify point-like morphology, slit coverage, and the identification/width of Balmer and [O III] lines that motivate AGN-like emission.
• Figure 2: Read the F1000W–F1500W vs F444W–F770W and F150W–F444W color–color planes to see that LRDs cluster at flat NIRCam–MIRI colors (weak hot dust), and compare their loci against redshifted SWIRE tracks (Type 2 QSO/torus/SF) to gauge which templates are plausible.
• Figure 3: Compare star-forming–only fits (CIGALE vs Prospector) for representative LRDs; look for failures around the Balmer break and mid-IR (F770W–F1500W) where pure stellar models underpredict or mis-shape the SED.
• Figure 4: Examine AGN-dominated fits and the decomposition (disk vs torus) to see how flat mid-IR colors constrain the torus and where AGN models better match the SED than SF-only, yet still leave UV/[O III] mismatches.