Feeding frenzy in the mighty black holes: what we could learn from them?

TL;DR

This review highlights the central role of the Eddington ratio in governing AGN accretion histories and in standardizing the Radius–Luminosity relation for quasars as cosmological probes, addressing the Hubble tension. It presents a data-driven framework: a universal anti-correlation between optical variability and $\\lambda_{Edd}$ from ZTF data; the CLAGN population as evidence that rapid $\\lambda_{Edd}$ shifts drive state changes; a fast PRM pathway to measure accretion-disk sizes and BH masses at high redshift; and the development of accretion-dependent SEDs (notably xA templates) to accurately model extreme accretors. Together, these insights enable efficient mass measurements, improved BLR diagnostics, and refined cosmological applications of AGNs, particularly with upcoming LSST-era datasets. The work emphasizes the need for accretion-aware SEDs and observational strategies to exploit high-cadence surveys for robust AGN science across cosmic time.

Abstract

Eddington ratio is a paramount parameter governing the accretion history and life cycles of Active Galactic Nuclei (AGNs). This short review presents a multi-faceted view of the importance of the Eddington ratio spanning varied AGN studies. We find that the Eddington ratio is crucial for standardizing the Radius-Luminosity (R-L) relation - a necessary step for employing quasars (QSOs) as standardizable cosmological probes to help clarify the standing of the Hubble tension. In this data-driven era, we consolidated disparate aspects by developing novel relations borne out of large datasets, such as the robust, nearly universal anti-correlation between fractional variability and Eddington ratio derived from Zwicky Transient Facility (ZTF) data, which is vital for interpreting forthcoming high-cadence surveys like Rubin Observatory's LSST. Addressing the conundrum where JWST results suggest an overabundance of massive high-redshift black holes, we demonstrate that local AGNs offer clarification: Changing-Look AGNs (CLAGNs), driven by rapid Eddington ratio shifts, cluster in the low-accretion regime, a rate independently confirmed by our integral field spectroscopy and photoionization modeling of a well-known Seyfert 2 galaxy, rich in high-ionization, forbidden, coronal lines. Conversely, for the high-redshift, high-luminosity population where traditional reverberation mapping (RM) is highly impractical, photometric reverberation mapping (PRM) offers a rapid alternative to constrain accretion disk sizes, enabling efficient estimates of black hole masses and Eddington ratios. Finally, we developed tailored semi-empirical spectral energy distributions (SEDs) for extremely high-accretion quasars, successfully validating their characteristic extreme physical conditions.

Figures (6)

• Figure 1: Distribution of the fractional variability (F$_{\rm var}$) in the g-band ZTF lightcurves for the AQMES medium field monitored within the SDSS-V, versus the Eddington ratio. The latter is taken from the SDSS DR16 QSO catalogue Wu_Shen_2022ApJS..263...42W. The color axis depicts the distribution of redshift. The best-fit correlation after cleaning sources with insufficient F$_{\rm var}$ in g-band information: log $\lambda_{\rm Edd}$ = -0.61 log F$_{\rm var}$ - 1.38 ($\rho$ = -0.28; p-value = 2.8E-18).
• Figure 4: (Left:) Distribution of Eddington ratios in the sample from PandaSniegowska2024ApJS. We show the distributions for the earliest (in brown) and the latest (in purple) epochs for the sources in our sample. The median values for the two distributions (red = -1.99, blue = -1.685) are marked with vertical dashed lines. The magenta box marks the range of the Eddington ratio for NGC 5548, i.e., log $\lambda_{\rm Edd}$ = [-2.2, -1]. (Right:) The distribution of the emission line EW versus the AGN continuum luminosity. Here, we demonstrate the trend for two sources: SDSS J141324.27+530527.0 Wang_2018ApJ...858...49W with 72 spectral epochs over $\sim$15 years (5527 days), and NGC 5548 Bon_2018FrASS...5....3BPanda2022ANPanda2023BASBr with more than 750 spectral epochs over $\sim$25 years (9624 days). For the former source (SDSS J141324.27+530527.0), we have taken the spectral data from the homogeneous fitting in PandaSniegowska2024ApJS, which includes the Mg ii, H$\beta$, and H$\alpha$ emission lines and the corresponding AGN continua nearest to these lines (at 3000Å, 5100Å, and 6000Å), as shown in the panel. For NGC 5548, we show the dataset from Panda et al. (in prep.), which is an updated version of the dataset provided in Bon_2018FrASS...5....3B, where the authors analyzed the H$\beta$ region. The SDSS source shows a clear rise from a deep minimum to a high state in both Balmer lines. However, the Mg ii shows a rather flat behavior - reminiscent of the Baldwin effect, suggesting the difference in ionization and response to the changing continuum levels. NGC 5548 data has a wealth of data, but the change in the source is rather gradual, and hence, a clear spike in the trend is not that prominent.
• Figure 5: (Left:)[O iii]$\lambda$5007 emission image of ESO 138-G001 from the HST/WFPC2 HST2000ApJS..128..139F with a small inset showing the details of the central part, where the nucleus is marked with a plus sign; (Right:) [O iii]$\lambda$5007 emission image from SIFS after data treatment involving spatial re-sampling with quadratic interpolation followed by the Richardson-Lucy PSF deconvolution SIFS2024MNRAS. The green circles – with an aperture radius of 0.6 arcsec – denote the extraction regions of the spectra for the North-East (NE) knot (top left), the South-East (SE) blob (bottom left), and the nuclear region. The red circle denotes the PSF FWHM of 0.71 arcsec.
• Figure 6: (Top left:) Incident SEDs generated for our photoionization modeling. The incident continua for the two black hole mass cases are shown in dot–dashed. These are used for the modeling of the nuclear region. The corresponding transmitted continua from these models are shown in dotted lines. These latter distributions are used as incident continua for the SE blob. These SEDs have been made assuming a non-spinning black hole. The IPs for notable coronal lines along with the hydrogen ionization front at 13.6 eV are marked with vertical lines; (Top right:) Synthetic spectrum (blue) comparison with observed spectrum (in grey), for the nuclear region, generated for the SED corresponding to the M$_{\rm BH}$ = 10$^{5.5}$ M$_{\odot}$, and ionization parameter, log U = -1. (Bottom:) Heatmap for the same BH mass case, M$_{\rm BH}$ = 10$^{5.5}$ M$_{\odot}$, for the density law slope, $\alpha$ = -1, for the nuclear region. Each of the considered emission lines is normalized to the H$\beta$ emission line. The x-axis represents the range of ionization parameters considered in our models in each panel. The cases of ionization parameters for each line ratio that have the smallest residuals (modeled ratio–observed ratio) are highlighted with red boxes. Courtesy: SIFS2024MNRAS.
• Figure 7: Recovered $R_{\mathrm{BLR}}$--$R_{\mathrm{AD}}$ relation; Courtesy: PandaFrancisco2024ApJL. The solid black line represents the mean of the posterior probability distributions, while the shaded region denotes the corresponding $1\sigma$ confidence interval. The best-fit slope ($\alpha$), intercept ($\beta$), and intrinsic scatter ($\sigma$) are reported together with their $1\sigma$ uncertainties. The dashed lines indicate the mean model predictions at the upper and lower bounds when the intrinsic scatter is incorporated. The positions of the two studied sources, CT286 and CT406, which belong to the class of high-accreting sources, are marked by the blue and red circles, respectively.