Investigating Little Red Dots with UV Excess: Are They the High-Redshift Siblings of Blue Hot DOGs?

Lulu Bao, Chao-Wei Tsai, Jingwen Wu, Tao Wang, Guodong Li, Roberto J. Assef, Tanio Diaz-Santos, Peter R. M. Eisenhardt, Daniel Stern, Andrew W. Blain

First listed 2025-08-29 | Last updated 2025-08-29

Abstract

Little Red Dots (LRDs), newly identified compact and dusty galaxies with an unexpectedly high number density observed by JWST, have an unusual "V-shaped" rest-frame UV to near-infrared spectral energy distribution (SED). A group of hyper-luminous, obscured quasars with excess blue emission, called Blue-excess Hot Dust-Obscured Galaxies (BHDs), also exhibit qualitatively similar SEDs to those of LRDs. They represent a rare population of galaxies hosting supermassive black holes (SMBHs) accreting near the Eddington limit at redshifts z \sim 1--4. In this study, we compare their multi-wavelength SEDs to investigate whether LRDs, or a subset of them, could be high-redshift analogs of BHDs. Our analysis reveals that despite their similar "V-shape" SEDs, LRDs appear to be a different population than BHDs. The "V-shape" of BHDs appear at longer wavelengths compared to LRDs due to different selection strategies, suggesting LRDs have much less dust attenuation than typical BHDs. The bluer colors in the rest-frame infrared (continuum) emission of LRDs suggest the absence of hot dust heated by AGN accretion activities. We also argue that the blue excess in LRDs is unlikely from AGN scattered light. The compact morphologies and lower X-ray detection frequencies of LRDs suggest a distinct formation pathway from BHDs -- which are thought to be powered by super-Eddington accretion onto central SMBHs following major galaxy mergers.

Short digest

The authors stack NIRCam+MIRI photometry for 326 “V-shape” LRDs (from Akins24 and Kokorev24) and compare their UV–NIR SEDs to median-stacked Blue-excess Hot DOGs (BHDs) to test the analog hypothesis. Despite superficially similar “V-shapes,” LRDs turn over at shorter wavelengths, have much bluer rest-IR continua, show F1800W non-detections in stacks, and require far lower dust attenuation in SED fits—indicating little or no AGN-heated hot dust. They also argue the UV excess in LRDs is unlikely to be scattered AGN light, and note compact morphologies plus lower X-ray detection frequencies. Bottom line: LRDs are not simply high‑z siblings of BHDs, pointing to a distinct early SMBH growth pathway from the super‑Eddington, merger-driven Hot DOG phase.

Key figures to inspect

• Figure 1: Inspect where the LRD “V” turns relative to the Hot DOG envelope; note the very low vs high attenuation AGN templates (A≈0.02 and ≈2.5 mag) needed, and the stacked F1800W upper-limit triangle that underscores the lack of hot-dust emission.
• Figure 2: Compare the observed-frame LRD stacks to BHD templates shifted to z≈6.5 and scaled to typical LRD SMBH masses—predicted F770W–F1800W colors are much redder than the LRD data; check the five MIRI-detected LRD points and the overplotted ‘Virgil’ track to see how genuinely hot-dust systems would appear.
• Figure 3: Examine LRD bolometric luminosity functions versus pre-JWST quasars and the median BHD Lbol marker to assess number-density and luminosity mismatches that argue against a simple BHD analog picture.