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

Himanshu Sharma, Vivek Kumar Jha, Hum Chand, Swayamtrupta Panda

TL;DR

This study investigates whether low-mass, low-luminosity AGNs (LLAGNs) with an intermediate-mass black hole (IMBH) populate the quasar main sequence (QMS) and the 4D Eigenvector 1 (4DE1) framework or define a distinct accretion state. Using SDSS DR16 spectra and PyQSOFit, the authors derive key spectral parameters (e.g., $FWHM(H\beta)$, $R_{\mathrm{FeII}}$, $M_{BH}$) and bolometric properties ($L_{bol}=5L_{5100}$) for 315 LLAGNs with $FWHM(H\beta) < 2200$ km s$^{-1}$, complemented by ZTF-based optical variability modeled as a damped random walk. The LLAGNs exhibit sub-Eddington accretion ($\log R_{\mathrm{Edd}} \approx -0.68$) and relatively weak Fe II emission ($R_{\mathrm{FeII}} \approx 0.61$), occupying a distinct region in the 4DE1 plane and showing no strong $R_{\mathrm{FeII}}$–$FWHM(H\beta)$ trend, which suggests a different accretion state from luminous quasars. The results imply radiatively inefficient accretion in this low-mass regime and raise the possibility that LLAGNs represent seeds or faded remnants of SMBH growth, with significant host contamination and strong intrinsic variability that warrants further multi-wavelength follow-up to fully understand their role in black hole and galaxy co-evolution.

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 $Hβ$ 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 $\log(σ) \approx -0.68$, 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.

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

TL;DR

This study investigates whether low-mass, low-luminosity AGNs (LLAGNs) with an intermediate-mass black hole (IMBH) populate the quasar main sequence (QMS) and the 4D Eigenvector 1 (4DE1) framework or define a distinct accretion state. Using SDSS DR16 spectra and PyQSOFit, the authors derive key spectral parameters (e.g., , , ) and bolometric properties () for 315 LLAGNs with km s, complemented by ZTF-based optical variability modeled as a damped random walk. The LLAGNs exhibit sub-Eddington accretion () and relatively weak Fe II emission (), occupying a distinct region in the 4DE1 plane and showing no strong – trend, which suggests a different accretion state from luminous quasars. The results imply radiatively inefficient accretion in this low-mass regime and raise the possibility that LLAGNs represent seeds or faded remnants of SMBH growth, with significant host contamination and strong intrinsic variability that warrants further multi-wavelength follow-up to fully understand their role in black hole and galaxy co-evolution.

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 full width at half maximum (FWHM) km s. 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 ) and comparatively weak FeII emission (median ), 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 , 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-, low- 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.

Paper Structure

This paper contains 12 sections, 1 equation, 10 figures, 2 tables.

Table of Contents

  1. Introduction
  2. sample
  3. Analysis
  4. Results
  5. Spectral Properties and Derived Quantities
  6. Distributions of Parameters
  7. Correlation Analysis
  8. The 4DE1 picture
  9. Host Galaxy contribution and Optical variability
  10. Discussion
  11. Conclusions
  12. Example of poorly fitted sources.

Figures (10)

  • Figure 1: The distribution of continuum luminosity at 5100 Å ($L_{5100}$) as a function of redshift ($z$) for our final sample. The marginal histograms along the axes show the one–dimensional distributions of $L_{5100}$ and $z$.
  • Figure 2: An illustration of a good fit from PyQsoFit for the source J135159.16+025034.0, Top panel represents the host galaxy and iron template plus continuum subtracted fitted spectrum along with continuum and individual templates of iron and host galaxy represented by different colours. Bottom left shows the $H\beta$ fitted region and bottom right shows the $H\alpha$ fitted region. The blue line represents the emission line model, and the red lines represent the multi-Gaussians used to build the final emission line model. For fitting broad emission lines, we have used single Gaussian with the constraint of FWHM > 700 $km \; s^{-1}$, while for narrow line regions, we have used a single Gaussian with FWHM < 700 $km \; s^{-1}$. For fitting the tails of broad $H\beta$ line, we have also used a very broad single Gaussian with FWHM > 10,000 $km \; s^{-1}$ without assigning any physical meaning to it.
  • Figure 3: Distributions of FWHM $H\beta$, log($M_{BH}/M_\odot$) and log($L_{5100}$) for the final sample of 315 LLAGNs with red dashed lines mark the median values: 992.42 $km \; s^{-1}$, 5.67 and 42.40 respectively.
  • Figure 4: Distributions of parameters derived from spectral emission-line fitting, with median values of 255.54 $km \; s^{-1}$, –0.68, and 0.61 respectively, marked by red dashed lines. For visualisation, the $R_{{\mathrm{Fe~\textsc{ii}}}}$ histogram is shown with a cut at $R_{{\mathrm{Fe~\textsc{ii}}}} < 3$, excluding two sources, while the other parameters are plotted for the full sample.
  • Figure 5: Correlation plot between FWHM of $\;H\beta$ and $R_{\rm Fe~{\textsc{ii}}}$ constituting the quasar main sequence(QMS). QMS plot from our sample of IMBH. The coloured dots represent the IMBH sources while the semi-transparent grey points indicate the general main sequence of quasars taken from sdss_dr14q_cat. The thick horizontal Black dashed line at $4000$$km \; s^{-1}$ separates the two different populations named Pop A and Pop B, while the thin horizontal black dashed at $2000$$km \; s^{-1}$ indicates the limit of NLSy1s. Thick vertical Black dashed line at $R_{\rm Fe~{\textsc{ii}}} = 1$ separates the extreme Population A to the right. $R_{\rm Fe~{\textsc{ii}}}$ is truncated to < 3, with the exclusion of 2 sources, for better visualisation.
  • ...and 5 more figures