Origins of the UV continuum and Balmer emission lines in Little Red Dots: observational validation of dense gas envelope models enshrouding the AGN

TL;DR

This study uses JWST data to analyze the origins of UV and emission lines in Little Red Dots, supporting a model where dense gas envelopes enshroud the AGN, affecting observed emissions.

Contribution

It provides observational validation for dense gas envelope models around AGNs in high-redshift Little Red Dots, highlighting a transition phase in black hole growth.

Findings

Narrow and broad Hα correlate with UV continuum in LRDs.

UV to broad Hα ratios differ between LRDs and LBDs, indicating different dominant emission sources.

LRDs' emissions are consistent with young starburst galaxies, not direct AGN activity.

Abstract

We present a statistical study on the origins of the UV continuum and narrow/broad emission lines in little red dots (LRDs), presumably involving active galactic nuclei (AGNs). Leveraging all archived JWST/NIRSpec data, we build a sample of 27 spectroscopically-confirmed LRDs at $5<z_{\rm spec}<7.2$, by requiring broad H$\alpha$ emission, blue UV colors, V-shaped continua, and compact morphology. We define a control sample of 7 blue, compact, broad-line AGNs without red optical continua (hereafter little blue dots; LBDs), and examine correlations between rest UV and the narrow/broad H$\alpha$ luminosities in these populations. In LRDs, both narrow and broad H$\alpha$ components are tightly correlated with the UV continuum, and the luminosity ratios are consistent with those in young starburst galaxies. In contrast, the UV to broad H$\alpha$ ratios in LBDs closely match local unobscured AGNs and are statistically different from LRDs. The Ly$\alpha$ occurrence rates and strengths do not differ between LRDs and LBDs and are comparable to normal star-forming galaxies. These results are consistent with a scenario where the central BH in LRDs is enshrouded by a dense opaque gas envelope -- in this model, the UV continuum as well as narrow and even broad H$\alpha$ emissions are not powered by AGNs but predominantly by young massive stars surrounding the envelope, while the envelope radiates as a $\sim 5000$ K blackbody. As the envelope dissipates, direct AGN emission can emerge, potentially transforming LRDs into LBDs and marking the end of a short-lived phase of rapid black hole growth.