Host Galaxy UV Emission and Dust Reddening in Little Red Dots

Emmanuel Durodola, Ryan C. Hickox, Jonathan Cohn, David M. Alexander

First listed 2026-09-15 | Last updated 2026-09-15

Abstract

Little Red Dots (LRDs) have been the subject of extensive analysis to place them within the context of galaxy and black hole evolution. Despite several proposed scenarios for the nature of the rest optical emission in LRDs, our understanding of the UV emission remains a matter of debate. Here, we aim to study the origin of the UV emission in LRDs. We conduct emission-line and continuum fitting analysis of thirteen LRDs using a combination of NIRSpec PRISM and medium-resolution G235M and G395M spectroscopy. We isolate and analyze the narrow-line emission arising from larger physical scales in LRDs. We observe that the narrow-line Balmer decrements are typically greater than those predicted for case B recombination and broadly correlated with both the UV slope and the break strength, providing clear evidence that the UV emission originates from galaxy scales and its slope is significantly affected by dust reddening. SED fitting analysis further shows that the UV continuum of LRDs is consistent with a dust attenuated stellar template. Finally, we show through a comparison of the UV slope and the Balmer break strength, that LRDs can be further divided based on break strength, such that the canonical LRD spectral shape (strong break strength ($>$ 3) and flat rest UV continuum) represents the most extreme sub-population. Our results imply that for most LRDs, the UV emission originates primarily from the young host galaxy, and that the characteristic optical emission may be due to a combination of dust reddening and intrinsically red accretion processes.

Short digest

Using JWST/NIRSpec PRISM plus medium-resolution G235M/G395M spectra for 13 LRDs, Durodola et al. separate narrow Balmer-line emission from the nuclear components to test the origin of the faint rest-UV continuum. Their narrow-line Balmer decrements are generally above case-B expectations and correlate with both UV slope and Balmer-break strength, while the rest-optical slope shows no comparable trend, pointing to dust-reddened UV light on host-galaxy scales. SED fits likewise favor dust-attenuated stellar UV continua, and the broader 89-object PRISM sample separates into break-strength classes in which the canonical strong-break, flat-UV LRDs are an extreme sub-population rather than the whole population. The result supports young, dusty host galaxies as the dominant UV source in most LRDs, alongside intrinsically red accretion-related optical emission.

Key figures to inspect

• Figure 3. This is the central empirical diagnostic: narrow-line Balmer decrement tracks both the observed UV slope and Balmer-break strength across the 13-object medium-resolution sample, but not the rest-optical slope. It directly motivates the interpretation of host-scale dust reddening for the UV component and physically distinct UV and optical SED origins.
• Figure 7. The SED-fitting synthesis shows that redder modeled UV slopes correspond tightly to higher stellar attenuation and that LRD hosts occupy a dustier, generally more massive regime than the comparison star-forming galaxies. It provides the stellar-continuum counterpart to the nebular attenuation evidence from the Balmer decrements.
• Figure 8. This population-level UV-slope versus Balmer-break diagram extends the interpretation from 13 medium-resolution targets to 89 PRISM-selected LRDs. Its dust-attenuation direction and break-strength split make clear that strong-break, flat-UV objects are the most extreme spectral subset.
• Figure 9. The break-strength-binned composite spectra provide the clearest visual summary of LRD spectral diversity, contrasting power-law, intermediate-break, and extreme-break populations on a common normalization. This figure supports the paper's conclusion that LRDs are not monolithic and that the canonical extreme-break shape is not representative of every object.