Between Little Red Dots and Star-Forming Galaxies: A Sequence in the Optical Continuum Slope
Abstract
The "Little Red Dots" (LRDs) uncovered by JWST are typically analyzed as a distinct class of objects; however, whether they are separated from ordinary star-forming galaxies by a genuine boundary has not been tested at the population level. Using a sample of ∼3,500 galaxies at 3 ≲ z ≲ 8 with NIRSpec/PRISM spectra from the DAWN JWST Archive, we modeled their rest-frame UV-to-optical continuum emission with power laws and located the sources in the βUV-βopt plane. Alongside a dense locus of star-forming galaxies and a diagonal sequence which can be explained by dust reddening and/or quiescence, we identify a vertical sequence extending from the locus of star-forming galaxies into the LRD region. This sequence is smoothly and continuously populated, with no clear boundary between the two populations. Its middle section, −1 ≲ βopt ≲ 0, hosts an intermediate population of ∼50 sources that falls outside the LRD selection boundary and has been largely overlooked; NIRCam imaging shows their rest-frame UV and optical median stacked images lie between point-like LRDs and extended star-forming galaxies. We modeled the vertical sequence as linear mixtures of stacked star-forming spectra and a black hole star (BH*) template derived from "The Cliff", one of the most compelling BH* candidates, and we infer black hole-to-stellar mass ratios η= MBH/M* ≈ 0.006−0.1 at the LRD end. We propose −1 ≲ βopt ≲ 0 as a practical selection window for possible transition candidates and argue that LRDs may be the compact, BH*-dominated extreme of a continuous population in which the relative contributions of the host and BH* vary smoothly.
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BibTeX
@misc{boonmee2026between,
title = {{Between Little Red Dots and Star-Forming Galaxies: A Sequence in the Optical Continuum Slope}},
author = {Kristiphong Boonmee and Akio K. Inoue and Masafusa Onoue and Xingyao Cai},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.07025},
eprint = {2609.07025},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.07025},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Between Little Red Dots and Star-Forming Galaxies: A Sequence in the Optical Continuum Slope
AU - Kristiphong Boonmee
AU - Akio K. Inoue
AU - Masafusa Onoue
AU - Xingyao Cai
PY - 2026
DA - 2026/09/07
PB - arXiv
DO - 10.48550/arxiv.2609.07025
UR - https://arxiv.org/abs/2609.07025
AB - The "Little Red Dots" (LRDs) uncovered by JWST are typically analyzed as a distinct class of objects; however, whether they are separated from ordinary star-forming galaxies by a genuine boundary has not been tested at the population level. Using a sample of ∼3,500 galaxies at 3 ≲ z ≲ 8 with NIRSpec/PRISM spectra from the DAWN JWST Archive, we modeled their rest-frame UV-to-optical continuum emission with power laws and located the sources in the β_UV-β_opt plane. Alongside a dense locus of star-forming galaxies and a diagonal sequence which can be explained by dust reddening and/or quiescence, we identify a vertical sequence extending from the locus of star-forming galaxies into the LRD region. This sequence is smoothly and continuously populated, with no clear boundary between the two populations. Its middle section, −1 ≲ β_opt ≲ 0, hosts an intermediate population of ∼50 sources that falls outside the LRD selection boundary and has been largely overlooked; NIRCam imaging shows their rest-frame UV and optical median stacked images lie between point-like LRDs and extended star-forming galaxies. We modeled the vertical sequence as linear mixtures of stacked star-forming spectra and a black hole star (BH*) template derived from "The Cliff", one of the most compelling BH* candidates, and we infer black hole-to-stellar mass ratios η= M_BH/M_* ≈ 0.006−0.1 at the LRD end. We propose −1 ≲ β_opt ≲ 0 as a practical selection window for possible transition candidates and argue that LRDs may be the compact, BH*-dominated extreme of a continuous population in which the relative contributions of the host and BH* vary smoothly.
ER -