Exploring the Nature of Little Red Dots: Constraints on AGN and Stellar Contributions from PRIMER MIRI Imaging

TL;DR

This study probes the nature of Little Red Dots (LRDs) at z>4 by performing extensive rest-frame UV-to-near-IR SED modeling on 95 LRDs using JWST PRIMER data that span 1–18 μm. By constructing extreme AGN-dominated, galaxy-dominated, and hybrid SED scenarios, the authors derive implied physical properties and assess the viability of each scenario with long-wavelength MIRI constraints. They find that galaxy-only fits allow massive, dusty stellar populations but imply extreme mass densities, while AGN-only fits require dusty, luminous AGNs with low hot-dust fractions; the hybrid model suggests partial AGN contributions and possible overmassive black holes, indicating mixed emission mechanisms. The results underscore the need for caution in interpreting LRD stellar masses and favor a mixed or novel interpretation for their red continua, aided by the inclusion of MIRI data and spectroscopy from RUBIES.

Abstract

JWST has revealed a large population of compact, red galaxies at $z>4$ known as Little Red Dots (LRDs). We analyze the spectral energy distributions (SEDs) of 95 LRDs from the JWST PRIMER survey with complete photometric coverage from $1-18\ μ$m using NIRCam and MIRI imaging, representing the most extensive SED analysis on a large LRD sample with long-wavelength MIRI data. We examine SED models in which either galaxy or active galactic nucleus (AGN) emission dominates the rest-frame UV or optical continuum, extracting physical properties to explore each scenario's implications. In the galaxy-only model, we find massive, dusty stellar populations alongside unobscured, low-mass components, hinting at inhomogeneous obscuration. The AGN-only model indicates dusty, luminous AGNs with low hot dust fractions compared to typical quasars. A hybrid AGN and galaxy model suggests low-mass, unobscured galaxies in the UV, with stellar mass estimates spanning $\sim$2 dex across the different models, underscoring the need for caution in interpreting LRD stellar masses. With MIRI photometry, the galaxy-only model produces stellar masses within cosmological limits, but extremely high stellar mass densities are inferred. The hybrid model infers highly overmassive black holes exceeding those in recently reported high-redshift AGNs, hinting at a partial AGN contribution to the rest-optical continuum or widespread super-Eddington accretion. Our findings highlight the extreme conditions required for both AGN or galaxy dominated scenarios in LRDs, supporting a mixed contribution to the red continuum, or novel scenarios to explain the observed emission.

Figures (1)

• Figure 1: Left: The F444W magnitude versus photometric redshift for the LRD sample in this paper. The six LRDs with a significant detection in F1800W at S/N$>5$ are shown in the orange points. The horizontal error bars shows the 68% confidence intervals of the photometric redshift. The LRDs detected in F1800W are generally bright, with F444W magnitude of $<25$ AB mag. Right: The F277W$-$F444W versus F444W$-$F1800W color-color diagram. The LRDs are red in F277W$-$F444W by selection. The LRDs detected in F1800W span a wide range of F444W$-$F1800W colors. For the rest of the sample, the $5\sigma$ upper limit on the F444W$-$F1800W are shown.