Little Red Dots: One Photometric Tag Concealing Diverse Spectroscopic Flavors of Massive Star Formation and Black Hole Activity

Abstract

We compile JWST/NIRSpec prism and MIRI data for 249 Little Red Dots (LRDs) at 2.3<z<9.3, forming a representative spectroscopic subset of NIRCam-selected LRDs. We derive a median stacked spectrum covering rest-frame 0.09-1.2 $\mu$m, with MIRI photometry extending the spectral energy distribution to 4 $\mu$m. Four additional stacks for subsamples defined by optical-to-UV luminosity ratios show that LRDs form a heterogeneous population spanning diverse continuum slopes and line properties. Assuming LRDs host super-massive black holes (BHs) surrounded by dense gas clouds, and stars accompany this core, we infer masses of $M_{BH}\sim 10^{6.0-6.5}$ M${}_{\odot }$ and $M_{★}\sim 10^{8.3}$ M${}_{\odot }$, corresponding to BH-to-stellar mass ratios of 1-2%. The stacks show ubiquitous UV and optical FeII emission, indicating a direct view of the broad-line region and high (but sub-Eddington) accretion (${\lambda }_{Edd}=0.6\pm 0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim 80$% in the bluest systems. Possible Wolf-Rayet features (HeII$\lambda$4687, nitrogen lines) are identified, tracing a young (3-7 Myr) compact starburst event. We also detect strong Balmer breaks and atypical Balmer, Paschen, [OIII], and optical and near-infrared HeI line ratios, and an absorption at $\sim 4550$ Angstrom (probably linked to FeII), all consistent with radiative-transfer effects in high-density gas with warm temperatures (4000-7000 K). We find a diversity of LRD flavors modulated by the luminosity ratio between between a short ($\lesssim 20$ Myr) and intense phase of BH activity, the most extreme stage lasting $\sim 3-7$ Myr, characterized by near-Eddington-limit radiation, and a nuclear and compact starburst dominated by massive stars (even super-massive, ${\mathrm{M}}_{\mathrm{SMS}}\sim 10^5$ M${}_{\odot }$), all embedded in dense gas with modest dust content producing a variety of optical depths.

Accepted for publication in ApJ. Stacks available through https://doi.org/10.5281/zenodo.18679061