Nitrogen-Loud Quasars from the Dark Energy Spectroscopic Instrument. I. Sample Selection and Basic Properties

Shuo Zhai, Wei-Jian Guo, Yong-Jie Chen, Zhi-Qiang Chen, Haining Li, Jian-Min Wang, Gang Zhao

First listed 2026-09-11 | Last updated 2026-09-10

Abstract

We present the largest sample to date of nitrogen-loud (N-loud) quasars with strong broad N IV] $\lambda1486$ and/or N III] $\lambda1750$ emission lines over the redshift range $1.6 < z < 4.3$, selected from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. The final sample contains 1,993 N-loud quasars, corresponding to about 1.2% of the parent quasar sample. The $L_{1450}$ distribution of the N-loud quasars is broadly similar to that of the DESI parent sample, but their redshift distribution is distinct, with a stronger concentration around $z \sim 2.5$--3. Their composite spectrum displays a broadly similar UV continuum shape to that of the parent quasars, while showing significantly enhanced broad nitrogen emission features, including N V, N IV], and N III]. Other metal emission features also show a moderate enhancement. Relative to a control sample matched in redshift and UV continuum luminosity, the N-loud quasars show systematically narrower broad C IV and Mg II emission lines, lower single-epoch virial black hole masses, and higher Eddington ratios, suggesting that N-loud quasars may preferentially appear during a relatively rapid black hole accretion phase. The radio-loud fraction is 10.1%, with the highest fraction among objects exhibiting both N III] and N IV] emission. The catalog provides a statistical baseline for future studies of nitrogen enhancement and its physical origin.

Short digest

Zhai et al. construct the largest homogeneous catalog of nitrogen-loud quasars to date, selecting 1,993 DESI DR1 objects at 1.6 < z < 4.3 with strong broad N IV] λ1486 and/or N III] λ1750 emission from a 168,500-object parent sample. These quasars comprise about 1.2% of the parent population, cluster more strongly around z ~ 2.5–3, and retain a similar UV continuum while showing markedly enhanced N V, N IV], and N III] plus more moderate strengthening of other metal lines. Against redshift- and luminosity-matched controls, they have narrower C IV and Mg II, lower single-epoch virial black-hole masses, and higher Eddington ratios, consistent with preferential identification during rapid accretion; the catalog establishes a large baseline for separating overall metallicity from selective nitrogen enhancement in future line-ratio modeling.

Key figures to inspect

• Figure 1. This flowchart defines the statistically clean DESI selection, from the 168,500-object parent sample through local N III] signal-to-noise screening, equivalent-width cuts, and visual inspection to the final 1,993-object catalog. It is essential for interpreting the reported 1.2% incidence and the meaning of the N-loud classification.
• Figure 6. The composite-spectrum comparison is the paper’s clearest population-level spectral evidence: N-loud quasars have a broadly comparable UV continuum to parent DESI quasars but substantially stronger nitrogen features. It directly supports the claim that the sample is chemically and/or BLR-physically distinctive rather than simply continuum-selected.
• Figure 8. This comparison of C IV and Mg II equivalent widths and FWHM against the matched SDSS control visualizes the systematically narrower broad lines in the N-loud population. Those line-width offsets underpin the subsequent differences in virial mass and inferred accretion state.
• Figure 9. The black-hole-mass versus Eddington-ratio plane provides the conclusion-driving physical interpretation: DESI N-loud quasars occupy lower single-epoch virial masses and higher Eddington ratios than matched controls. It is the strongest figure for the proposed connection between nitrogen-loudness and a rapid black-hole-growth phase.
• Figure 10. The N-loud fraction under several continuum signal-to-noise thresholds tests whether the apparent redshift dependence is driven by spectral-quality selection. It provides an important observational-selection check on the catalog’s demographic interpretation.