Do Low-Mass, Low-Luminosity AGNs Deviate from the Quasar Main Sequence?

TL;DR

This study characterizes low-mass, low-luminosity AGNs, revealing they deviate from the typical quasar main sequence by exhibiting distinct accretion and emission properties, and may represent a low-accretion state within narrow-line AGNs.

Contribution

It provides a detailed spectroscopic and variability analysis of low-mass AGNs, highlighting their unique position in the 4DE1 parameter space and suggesting a new low-accretion class.

Findings

Low Eddington ratios (median log R_Edd ≈ -0.68)

Weak FeII emission (median R_FeII ≈ 0.61)

Optical variability similar to Seyfert 1 galaxies

Abstract

We present a comprehensive spectroscopic and variability-based characterisation of a sample of low-luminosity active galactic nuclei (AGNs) hosting low mass black holes, identified by $H\beta$ full width at half maximum (FWHM) $< 2200$ km s$^{-1}$. While the narrow line widths are consistent with the formal definition of narrow-line Seyfert 1 (NLSy1) galaxies, the broader accretion and emission properties reveal key distinctions. The sample exhibits sub-Eddington accretion rates (median $\log R_{Edd} \approx -0.68$) and comparatively weak FeII emission (median $R_{FeII} \approx 0.61$), in contrast to the strong FeII strengths and high Eddington ratios characteristic of classical NLSy1s. Optical variability amplitudes, derived from Zwicky Transient Facility (ZTF) light curves, are similar to those typically seen in Seyfert 1 galaxies, with a median $\log(\sigma) \approx -0.68$, suggesting the AGN component's significant contribution to variability. In the optical plane of the 4D Eigenvector 1 (4DE1) parameter space, these sources occupy a distinct locus in the low-$R_{FeII}$, low-$R_{Edd}$ regime, suggesting a physically distinct accretion state. Our findings indicate that this population may represent a low-accretion analogue within the broader narrow-line AGN family, offering new insights into black hole growth at low mass scales.