The physical nature of little red dots (LRDs), a population of compact red galaxies revealed by JWST, remains unclear. Photometric samples were constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and the prevalence of broad emission lines. We used the RUBIES survey, a large spectroscopic program with wide color-morphology coverage and homogeneous data quality, to systematically analyze the emission-line kinematics, spectral shapes, and morphologies of ∼1500 galaxies at z > 3.1. We identified broad Balmer lines via a novel fitting approach that simultaneously models NIRSpec/PRISM and G395M spectra, yielding 80 broad-line sources with 28 (35%) at z > 6. A large subpopulation naturally emerged from the broad Balmer line sources, with 36 exhibiting v-shaped UV-to-optical continua and a dominant point source component in the rest-optical; we define these as spectroscopic LRDs, constituting the largest such sample to date. Strikingly, the spectroscopic LRD population is largely recovered when either a broad line or rest-optical point source is required in combination with a v-shaped continuum, suggesting an inherent link between these three defining characteristics. We compared the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy down to rm F444W<26.5, only of the RUBIES LRDs were photometrically identified depending on the selection criteria used. The remainder were missed due to a mixture of faint rest-UV photometry, comparatively blue rest-optical colors, or highly uncertain photometric redshifts. Our findings highlight that well-selected spectroscopic campaigns are essential for robust LRD identification, while photometric criteria require refinement to capture the full population.
Hviding et al. (2025) studied this question.