Steep Balmer Decrement in Weak AGNs May Not Be Caused by Dust Extinction: Clues from Low-luminosity AGNs and Changing-look AGNs

Abstract

The hydrogen Balmer decrement (e.g., Hα/Hβ) is widely adopted as an indicator of the internal reddening of active galactic nuclei (AGNs). This is challenged by some low-luminosity AGNs and changing-look AGNs (CLAGNs), which have steep Balmer decrement but without strong evidence for absorption. We compile a sample of normal AGNs and CLAGNs with a wider distribution of bolometric Eddington ratio (λ Edd = L bol/L Edd) and find a strong negative correlation between Hα/Hβ and λ Edd, which suggests that the Balmer decrement is also accretion-rate dependent. We further explore the Balmer decrement based on the photoionization model using the Cloudy code by considering spectral energy distribution from the accretion disk with different accretion rates (e.g., disk/corona and truncated disk at high and low Eddington ratios, respectively). Both the standard disk and truncated disk predict a negative correlation of Hα/Hβ–λ Edd, where the relation is steeper in the case of the truncated disk. The negative correlations are also explored in two single CLAGNs. The measured negative correlation of Hα/Hβ–λ Edd is mainly caused by the lower responsivity ( d log L line / d log L cont ) in Hα relative to that in Hβ due to the larger optical depth in the former. We propose that the steep Balmer decrements in low Eddington ratio AGNs (e.g., some Seyferts 1.5–1.9 and CLAGNs) are not simply caused by absorption but mainly caused by the relatively low flux of ionizing photons.

Unveiling Asymmetric Geometry of Broad-Line Region in Active Galactic Nuclei

“…2), and the results exhibit an identical relationship. This relationship further supports the notion that the BD values in the BLR are anti-correlated with the continuum fluxes, which is consistent with the prediction of the photoionization models [33][34][35].…”

“…The broad hydrogen Balmer decrements (BDs) are widely employed to probe the internal reddening of AGNs [31,32], where various values can be associated with different degrees of dust extinction. On the other hand, photoionization model simulations predict that the intrinsic BD should cover a range of values within a parameter grid [33][34][35]. Unfortunately, the actual dust and gas environments may vary substantially between individual AGNs, making it extremely challenging to differentiate the influences of these two physical mechanisms on the BD using statistical samples of AGNs.…”

Multi-Epoch Optical Spectroscopy Variability of the Changing-Look AGN Mrk 883

“…We report the estimations of the Balmer decrement (BD) and find that it also shows variability. This is an expected result in AGN with a low Eddington ratio (see Figure 2 in [27]).…”

The Weakness of Soft X-Ray Intensity: Possible Physical Reason for Weak-line Quasars

“…´- (Bentz et al 2013). The number density of the BLR cloud is roughly in the range of 10 8 -10 12 cm −3 (e.g., Guo et al 2020;Wu et al 2023), where the density cannot be too low (10 7 cm −3 ) due to the absence of forbidden broad lines (e.g., [O III] λ4363) or too high, which leads to more continuum thermal emission than emission lines (e.g., Korista & Goad 2000). In all Cloudy simulations, the grid steps in hydrogen density (n) and distance to the continuum source (r) are set to 0.2 dex.…”