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

Pablo G. Pérez-González, Guillermo Barro, Stefano Carniani, Francesco D'Eugenio, George H. Rieke, Roberta Tripodi, Andrew J. Bunker, Xihan Ji, Rui Marques-Chaves, Daniel Schaerer, Giacomo Venturi, Flor Arévalo-González, Santiago Arribas, Pierluigi Rinaldi, Bruno Rodríguez Del Pino, Joris Witstok, Rachana Bhatawdekar, Leindert A. Boogaard, Stephane Charlot, Jacopo Chevallard, Luca Costantin, Mirko Curti, Emma Curtis-Lake, Emanuele Daddi, Kelcey Davis, Mark Dickinson, Callum T. Donnan, Fergus R. Donnan, James S. Dunlop, Daniel J. Eisenstein, Henry C. Ferguson, Román Fernández Aranda, Steven L. Finkelstein, Seiji Fujimoto, Giovanni Gandolfi, Mauro Giavalisco, Norman A. Grogin, Mahmoud Hamed, Michaela Hirschmann, Jeyhan S. Kartaltepe, Dale D. Kocevski, Anton M. Koekemoer, Gene C. K. Leung, Cristina M. Lofaro, Ray A. Lucas, Derek J. McLeod, Jens Melinder, Goran Östlin, Casey Papovich, Laura Pentericci, Borja Pérez-Díaz, Marcia Rieke, Jan Scholtz, Rachel S. Somerville, Thomas M. Stanton, Struan D. Stevenson, Irene Shivaei, Sandro Tacchella, Jonathan R. Trump, Hannah Übler, Xin Wang, Christina C. Williams, Christopher N. A. Willmer, L. Y. Aaron Yung, Yongda Zhu

First listed 2026-02-23 | Last updated 2026-02-27

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 $μ$m, with MIRI photometry extending the spectral energy distribution to 4 $μ$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 $M_{BH}\sim10^{6.0-6.5}$ M$_\odot$ and $M_\bigstar\sim10^{8.3}$ 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 ($λ_{Edd}=0.6\pm0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim80$% in the bluest systems. Possible Wolf-Rayet features (HeII$λ$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 $\sim4550$ 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 ($\lesssim20$ Myr) and intense phase of BH activity, the most extreme stage lasting $\sim3-7$ Myr, characterized by near-Eddington-limit radiation, and a nuclear and compact starburst dominated by massive stars (even super-massive, $\mathrm{M}_\mathrm{SMS}\sim10^{5}$ M$_\odot$), all embedded in dense gas with modest dust content producing a variety of optical depths.

Short digest

JWST/NIRSpec+MIRI spectroscopy for 249 Little Red Dots (2.3<z<9.3) yields a median 0.09–1.2 μm stack (SED to 4 μm) and subtype stacks showing wide diversity in continua and lines. The stacks point to compact systems hosting 10^6.0–10^6.5 M⊙ black holes embedded in dense, warm gas, with ubiquitous UV/optical Fe II indicating BLR sightlines and high, sub-Eddington accretion (λ_Edd≈0.6). Far-UV light can be ≈80% stellar in the bluest LRDs, with possible WR features (He II 4687, N lines) implying a 3–7 Myr nuclear starburst alongside strong Balmer breaks and atypical Balmer/Paschen/[O III]/He I ratios plus a ~4550 Å absorption. The authors interpret LRD “flavors” as the luminosity-weighted mix of a short (≲20 Myr; extreme 3–7 Myr) near-Eddington BH phase and a coeval compact starburst, giving BH-to-stellar mass ratios of 1–2% and a framework for early BH growth.

Key figures to inspect

• Figure 1: Inspect where the spectroscopic LRDs fall in the Barro et al. color–color and color–magnitude cuts and compare histograms (F444W, F115W–F200W, F200W–F444W, F277W–F444W) to judge how representative the prism subsample is relative to the full NIRCam-selected LRD set.
• Figure 2: Compare redshift distributions of the spectroscopic versus photometric LRD samples and across optical-to-UV subtypes; check median z shifts with subtype to see whether the “bluest” versus “reddest” LRDs occupy different epochs.
• Figure 3: Use the full-stack spectrum (with MIRI photometry) and zoom-ins to verify ubiquitous Fe II complexes, Balmer break strength, and the unusual Balmer/Paschen/[O III]/He I ratios; check the ~4550 Å absorption and how LRD stacks compare to QSO and LAE templates across Mg II, [O III]+Hβ, Hα, and NIR windows.
• Figure 4: Read the placement of the median and subtype stacks in the C III] EW versus C III]/He II plane against AGN- and SF-dominated model loci to see which LRD flavors sit in the AGN, hybrid, or SF regimes and how optical-to-UV ratio tracks this shift.