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

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$\alpha$ 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.