Changing-look Active Galactic Nuclei from the Dark Energy Spectroscopic Instrument. IV. Broad Emission Line Evolution Sequence Among H{\alpha}, Mg II, and H{\beta}

TL;DR

This study analyzes the evolution of broad emission lines in changing-look AGNs, revealing a stratified fading sequence of Hβ, Mg II, and Hα lines as luminosity declines, and shows black hole mass influences the suppression threshold.

Contribution

It provides the first statistical confirmation of a stratified broad line fading sequence in AGNs and links the phenomenon to accretion state and black hole mass.

Findings

Broad Hβ fades first, followed by Mg II and Hα as luminosity declines.

The fading sequence reflects a radially stratified broad line region.

Black hole mass affects the Eddington ratio threshold for line suppression.

Abstract

From a parent catalog of 561 changing-look active galactic nuclei (CL-AGNs) identified by Guo et al. (2025), we investigate the evolutionary sequence of broad emission lines using a redshift-selected subset (0.35 < z < 0.45) of 54 CL-AGNs whose Dark Energy Spectroscopic Instrument (DESI) spectra simultaneously cover the H{\alpha}, H\b{eta}, and Mg II emission lines. To provide a baseline for comparison, we construct a control sample of 19,897 normal Type 1 AGNs within the same redshift range from the DESI Year 1 data. Through stacked spectral analysis and line-continuum luminosity correlations, we identify a clear evolutionary sequence in all AGN where broad H\b{eta} fades first, followed by Mg II, and then H{\alpha}, as the AGN luminosity declines - consistent with expectations from reverberation mapping. This trend reflects a radially stratified broad line region (BLR), where each line's responsivity depends on its ionization potential and radial distance from the central engine. In addition, we find that more massive supermassive black holes (SMBHs) require lower Eddington ratios to fully suppress broad emission lines, suggesting that the critical accretion threshold for the CL phenomenon is mass-dependent. Our results present the first statistical confirmation of a stratified broad line fading sequence in AGNs, reinforcing the central role of accretion state in shaping BLR structure and visibility.