RUBIES: A Spectroscopic Census of Little Red Dots; All V-Shaped Point Sources Have Broad Lines

TL;DR

This study uses the RUBIES spectroscopic survey to characterize Little Red Dots, revealing their spectral and morphological features, and highlighting the importance of spectroscopy over photometry for accurate identification.

Contribution

We present the largest spectroscopic sample of Little Red Dots, identifying their broad emission lines, v-shaped continua, and point-source features, and compare spectroscopic and photometric selection methods.

Findings

80 broad-line sources identified, including 28 at z > 6

Spectroscopic LRDs often missed by photometric searches, only 50-62% overlap

Spectroscopic campaigns are crucial for accurate LRD detection

Abstract

The physical nature of Little Red Dots (LRDs) - a population of compact, red galaxies revealed by JWST - remains unclear. Photometric samples are constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and prevalence of broad emission lines. We use 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 $\sim$1500 galaxies at $z > 3.1$. We identify 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 emerges 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 compare the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy, down to $\rm F444W<26.5$, only 50-62% of the RUBIES LRDs were previously identified. 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.