The UV Side of Little Red Dots: Red, Compact, and Iron-Enhanced Rest-UV Emission with a Strong Downturn around Ly$α$

Makoto Ando, Yuichi Harikane, Harley Katz, Kohei Inayoshi, Takumi S. Tanaka

First listed 2026-06-03 | Last updated 2026-06-02

Abstract

Little Red Dots (LRDs) are candidates for growing supermassive black holes newly discovered by the James Webb Space Telescope (JWST), characterized by compact rest-optical morphology, V-shaped spectra, and broad Hydrogen Balmer lines. While recently proposed BH-star/envelope models have made progress in explaining their optical features, their rest-UV emission, which is considered to originate from host galaxies, remains poorly investigated. In this paper, we present a comprehensive analysis of the UV emission, including continuum shapes, emission line strengths, and morphology, using $\sim100$ LRDs selected from the JWST spectral archive. Compared to star-forming galaxies at the same redshifts and UV magnitudes, LRDs show systematically redder UV slopes and more compact UV sizes, indicating that their UV emission cannot be explained solely by normal star-forming galaxies and requires a significant contribution from central red and compact emission. From stacked spectra, we find that the Balmer break strength, UV slope, downturn depth around Ly$α$, and $\mathrm{Fe\, II}$ equivalent width are positively correlated, while the UV size is anticorrelated with the Balmer break strength, suggesting that diversity in the UV continuum shape reflects the varying dominance of the central emission relative to its host. We also measure $\mathrm{Fe\, II//Mg\, II}\sim8-10$, higher than in quasars at similar redshifts, further supporting a substantial contribution from the central component. Spectral modeling suggests that the observed red UV continuum cannot be reproduced by host galaxy emission alone, but requires an additional very red continuum source ($β_\mathrm{UV}\sim0$), possibly nebular continuum emission leaking from dense ionized gas through a clumpy or porous neutral gas envelope.

Short digest

Using a spectroscopic sample of about 100 JWST-selected little red dots, this paper shows that their rest-UV light is not just ordinary host-galaxy emission: at fixed redshift and UV magnitude, LRDs are systematically redder and several times more compact than normal star-forming galaxies. Stacked spectra tie that UV diversity to the rest-optical phenomenology, with stronger Balmer breaks accompanying redder UV slopes, deeper downturns around Lyα, stronger Fe II, and smaller UV sizes. The measured Fe II/Mg II ratio of about 8-10, above comparable high-redshift quasar values, further argues for a substantial central component in the UV. Spectral modeling then requires an extra very red continuum with β_UV around 0, consistent with nebular continuum leaking from dense ionized gas through a clumpy or porous neutral envelope.

Key figures to inspect

• Figure 2. This is the cleanest population-level demonstration that LRDs have redder rest-UV continua than normal star-forming galaxies at the same redshift and UV luminosity. It establishes one of the paper’s core observational facts directly from the sample, rather than from model interpretation, and should be shown early because the anomalous UV slope is central to the paper’s argument.
• Figure 4. This figure makes the equally important morphological point that LRDs are much more compact in the rest-UV than matched star-forming galaxies, with many sources unresolved or near the PSF limit. It is the strongest direct evidence that the UV emission includes a central compact component rather than being dominated by ordinary extended host-galaxy light.
• Figure 9. This is the key synthesis figure linking the paper’s separate diagnostics into one physical picture: stronger Balmer breaks come with redder UV slopes, deeper downturns around Lyα, and smaller UV sizes. It is especially valuable because it turns the sample diversity into an interpretable sequence in central-source dominance versus host contribution.
• Figure 12. This figure captures the paper’s main line-diagnostic result by showing elevated Fe II/Mg II ratios for LRD stacks relative to quasar measurements and the increase of Fe II equivalent width with redder UV slope. It matters because the iron enhancement is one of the strongest spectral arguments that the UV emission includes a non-stellar central component.
• Figure 13. This is the conclusion-driving modeling figure: it compares physically motivated UV-continuum models and shows that a normal blue host alone cannot reproduce the observed red UV spectra. The figure is important because it connects the empirical trends to the paper’s preferred interpretation, namely an additional very red continuum likely associated with dense ionized gas and leakage through a clumpy or porous envelope.