Between Little Red Dots and Star-Forming Galaxies: A Sequence in the Optical Continuum Slope

Kristiphong Boonmee, Akio K. Inoue, Masafusa Onoue, Xingyao Cai

First listed 2026-09-08 | Last updated 2026-09-07

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 $\sim3,500$ galaxies at $3 \lesssim z \lesssim 8$ 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 $β_{\rm{UV}}$-$β_{\rm{opt}}$ 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, $-1 \lesssim β_{\rm{opt}} \lesssim 0$, hosts an intermediate population of $\sim50$ 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 $η= M_{\rm{BH}}/M_* \approx 0.006-0.1$ at the LRD end. We propose $-1 \lesssim β_{\rm{opt}} \lesssim 0$ 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.

Short digest

Using 3,566 DAWN JWST Archive NIRSpec/PRISM spectra at 3≲z≲8, Boonmee et al. fit rest-frame UV and optical power-law slopes and map galaxies in the βUV–βopt plane without preselecting LRDs. They find a smoothly populated, near-vertical sequence from ordinary star-forming galaxies into the LRD region, including an overlooked ≈50-object transition population at −1≲βopt≲0 whose stacked NIRCam morphologies are intermediate between extended galaxies and point-like LRDs. Linear mixtures of star-forming stacks and The Cliff BH* template reproduce the sequence and imply MBH/M*≈0.006–0.1 at its LRD end, supporting a picture in which LRDs are the compact, BH*-dominated extreme of a continuous host-plus-central-source population rather than a sharply separate class.

Key figures to inspect

• Figure 3. This is the central observational map: it shows the full βUV–βopt distribution, conventional V-shaped and LRD selection boundaries, and the vertical population that extends continuously from non-LRD star-forming galaxies into the red LRD region.
• Figure 7. The BH* plus host mixing tracks provide the paper’s key physical interpretation, demonstrating how varying the relative BH* contribution can move stacked star-forming spectra along the observed vertical sequence toward The Cliff and the LRD population.
• Figure 8. This figure operationalizes the proposed transition selection by dividing the vertical sequence into four βopt bins after isolating the trend, making the intermediate population and its progression toward the LRD end explicit.
• Figure 10. The rest-frame UV stacked morphologies and extendedness ratios test whether structural properties evolve coherently along the continuum-slope sequence, linking the spectral transition to changing compactness.
• Figure 11. The corresponding rest-frame optical stacks are especially important for the BH*-dominated interpretation, because the optical component is where the sequence becomes redder and increasingly compact relative to star-forming hosts.