Interpretation of the broad N III] emission component

Determine whether the broad N III] emission profiles observed in SPURS-GN-2 and SPURS-GN-2004 arise from genuinely broad nitrogen emission or from contamination by Fe II λ1743 emission.

Background

Broad N III] components are preferred by profile fits for GN-2 and GN-2004. However, Fe II λ1743 lies near the N III] quintuplet and could reproduce part of the apparent broad excess if its emission were unusually strong.

The authors find the required Fe II strength to be unlikely based on the measured Fe II λ1786 emission and expected line ratios, but they explicitly retain uncertainty in the interpretation.

References

One complication in interpreting the broad N~{\small III}] profile is the potential presence of Fe~{\small II}~$\lambda1743$ emission. To test the potential impact of Fe~{\small II} on our fits, we simultaneously fit the N~{\small III}] profile with five narrow Gaussians and an additional Gaussian for the Fe~{\small II}~$\lambda1743$ component. We are able to recover the N~{\small III}] line profile, provided the Fe~{\small II}~$\lambda1743$ component is very strong, with a flux of $3.9\pm0.9\times10{-19}$ (GN-2) and $2.6\pm0.9\times10{-19}$~erg~s${-1}$~cm${-2}$ (GN-2004). In this scenario, the N~{\small III}] broad profile would be weaker. However, we consider this unlikely: the implied Fe~{\small II}~$\lambda1743$ emission would be well above the values expected given the measured flux of Fe~{\small II}~$\lambda1786$ and the typical $\lambda1743/\lambda1786$ flux ratios ($\sim0.01$ based on the ratio between Einstein A coefficients). We thus conclude the broad N~{\small III}] component is the most likely explanation for the observed profile, but we caution that there remains some uncertainty in its interpretation.

— 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 2.3.4, N III] emission discussion