Changing-Look Active Galactic Nuclei in SDSS-V: Host-Galaxy Properties and Black-Hole Scaling Relations

TL;DR

Changing-look AGNs provide a unique laboratory to study the AGN–host connection because their spectra transition between AGN-dominated and host-dominated states within the same system. The authors present intermediate-resolution spectroscopy of 23 dimming CL-AGNs from SDSS-V with VLT/X-shooter and Gemini-N/GMOS, and analyze the Mg II emission to test the driver of variability, finding little support for variable obscuration. They show that the CL-AGN hosts largely follow the local BH scaling relations in the $M_{ m BH}$–$\sigma_{*}$ and $M_{ m BH}$–$M_{*}$ planes, with a median $M_{ m BH}/M_{*} \\approx 0.38\%$ and no strong evidence for different stellar populations compared to Type 2 AGNs. This supports the view that CL-AGNs are a phase of normal AGN activity in typical hosts and establishes CL-AGNs as useful probes of the AGN–host connection across spectral states.

Abstract

Changing-look active galactic nuclei (CL-AGNs) exhibit dramatic spectral variability on unexpectedly short timescales, challenging standard accretion flow models. Despite growing samples, the physical drivers of this extreme variability, and the potential link to host-galaxy properties, remain unknown. Regardless of the underlying mechanism, the transition between AGN-dominated and host-dominated spectra offers a unique opportunity to study relations between AGNs and their hosts within the same objects. We present intermediate-resolution spectroscopy of 23 CL-AGNs identified by the Sloan Digital Sky Survey V (SDSS-V), obtained with VLT/X-shooter and Gemini-N/GMOS. An analysis of the Mgii emission line observed in the spectra demonstrates that the majority of these sources cannot be driven by variable obscuration. Our CL-AGNs roughly follow the M_BH-sigma_* and M_BH-M_* relations of inactive galaxies, with a median black hole-to-stellar mass ratio of 0.38%, although they show hints of a shallower slope. We find no evidence that the stellar population properties of our CL-AGNs, including stellar mass, age, young stellar fraction, and star-formation rate differ from those of Type 2 AGNs in SDSS. These results suggest that CL-AGNs reside in typical AGN host galaxies and that their extreme variability is likely unrelated to host-galaxy environment, supporting the idea that CL-AGNs are not a distinct population, but rather represent a phase of normal AGN activity. This result, in turn, implies that CL-AGNs can serve as useful probes of the AGN-host connection, providing access to both AGN-dominated and host-dominated spectra of the same systems.

Figures (1)

• Figure 1: Left: The distribution of the Zeltyn24 sample of CL-AGNs on the luminosity-redshift plane, along with the subsample observed with VLT/X-shooter and Gemini-N/GMOS studied here (red circles). Middle:: The _ BH $M_{\rm BH}$-normalized density distributions of SDSS DR16 $z<0.6$ quasars WS22 and of the CL-AGNs analyzed in this work (red). The shape of the logarithmic BH mass function (BHMF) of broad-line AGNs, taken from Ananna22, is shown for reference with arbitrary scaling. Right: _ bol $L_{\rm bol}$ vs. _ BH $M_{\rm BH}$ distributions for the same sets of AGNs. Dashed diagonal lines indicate Eddington ratios of $L_{\rm bol} /L_{\rm Edd}= 0.01, 0.1$, and $1$. All _ BH $M_{\rm BH}$ values used in this figure were derived from the legacy SDSS spectra using the prescription of Cho23, based on the α H$\alpha$ line. For the two highest-redshift sources from our CL-AGN sample, _ BH $M_{\rm BH}$ was estimated from alternative lines (see Section \ref{['subsub:bh_mass_method']} for details). The _ bol $L_{\rm bol}$ values, likewise derived from the legacy SDSS spectra, were adopted from WS22 and Zeltyn24.