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 $\sim 3,500$ 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 ${\beta }_{\mathrm{UV}}$-${\beta }_{\mathrm{opt}}$ 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, $-1\lesssim {\beta }_{\mathrm{opt}}\lesssim 0$, hosts an intermediate population of $\sim 50$ 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 $\eta =M_{\mathrm{BH}}/M_{\ast }\approx 0.006-0.1$ at the LRD end. We propose $-1\lesssim {\beta }_{\mathrm{opt}}\lesssim 0$ 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.

20 pages, 17 figures