Origin of the weak broad permitted ultraviolet lines

Determine whether the weakness or absence of broad permitted ultraviolet lines in Little Red Dots is caused by absorption by dense gas or by an intrinsically softer ionizing spectrum.

Background

Broad permitted ultraviolet lines such as C IV, He II, and Mg II are expected from a conventional broad-line region, but previous observations had generally failed to detect them in Little Red Dots. The paper investigates whether dense gas surrounding the central source attenuates these lines or whether the ionizing source itself produces fewer high-energy photons.

The SPURS observations detect broad C IV in two objects but do not detect broad He II or Mg II at comparable strengths, so the physical explanation remains unresolved in general.

References

Existing, shallower spectra have not detected the broad permitted UV lines (C~{\small IV}, He~{\small II}, Mg~{\small II}) expected from a standard broad-line region \citep{Tang2025,Lambrides2026}, but whether this reflects absorption by dense gas or an intrinsically softer ionizing spectrum remains unclear.

— SPURS: An Ultra-deep View Inside the Compact, Nitrogen-Enriched Nuclei of Little Red Dots  (2609.11094 - Tang et al., 10 Sep 2026) in Section 1, Introduction