The Eddington Ratio Distribution of Narrow Line Active Galactic Nuclei

TL;DR

This work addresses how optical narrow-line AGN activity, parameterized by the Eddington ratio $ ext{λ}$, depends on host galaxy properties in the local universe. Using MaNGA DR17 data, it identifies Seyfert-like nuclei via three-dimensional line-ratio diagnostics and rigorously accounts for non-detections with luminosity thresholds, then derives intrinsic distributions of narrow-line luminosities and $ ext{λ}$ by fitting Schechter functions and performing Bayesian inference. By employing multiple bolometric corrections and several $M_{ m BH}$–$\sigma$ calibrations, the study robustly constrains the AGN duty cycle $ ext{F}_{ m AGN}$ as a function of stellar mass and specific star-formation rate, revealing a nuanced dependence: $ ext{F}_{ m AGN}$ increases with mass for star-forming galaxies, decreases with mass for quiescent galaxies, and rises with $ ext{sSFR}$ at the high-mass end. This has significant implications for models of black hole growth and AGN feedback in galaxy formation, and provides a principled framework to correct for selection effects in future low-redshift AGN demographic studies, including DESI surveys.

Abstract

We measure the Eddington ratio distribution of local optical narrow line active galactic nuclei (AGN) as a function of host galaxy properties, as a potential test of supermassive black hole growth and feedback models in galaxy formation theory. We base our sample on integral field spectroscopy from the MaNGA data in SDSS-IV's DR17. Starting with MaNGA's calibrated row-stacked spectra, we produce new spectroscopic data cubes with minimal covariance between spaxels and higher resolution point spread functions (PSF), and then extract line fluxes for the central PSF. Using the line ratio diagnostic techniques of Ji & Yan (2020), we identify AGN galaxies and determine their H$β$ and [O III] line luminosities. For all galaxies not identified as AGN, we determine the threshold line luminosity they would have needed to be identified as AGN. These luminosity thresholds are essential to determine, because many star forming galaxies likely host AGN of significant luminosity that are unidentified because they are outshone by star formation related emission. We show that ignoring these selection effects when measuring the Eddington ratio distribution would lead to biased results. From the H$β$ luminosities and luminosity detection thresholds, accounting for selection effects, we measure the luminosity and Eddington ratio distributions of Seyferts as a function of specific star formation rate (sSFR) and stellar mass. Defining $F_{\rm AGN}$ as the occurrence rate of AGN above a fixed Eddington ratio of $10^{-3}$, we find that $F_{\rm AGN}$ is constant or increasing with stellar mass for star forming galaxies and declines strongly with stellar mass for quiescent galaxies. At stellar masses $\log_{10} M_\ast > 10.25$, the occurrence rate increases monotonically with sSFR. These patterns reveal a complicated dependence of AGN activity on galaxy properties for theoretical models to explain.

Figures (11)

• Figure 1: Stellar masses and sSFRs for the MaNGA sample used here. Red symbols show galaxies with AGN detections, black symbols show galaxies with AGN detection thresholds (which have all necessary line ratios detected), and blue symbols show galaxies with AGN detection thresholds (without all necessary line ratios detected). The boxes show divisions between stellar mass and sSFR samples we use to study the AGN luminosity and Eddington ratio distribution dependence on galaxy properties.
• Figure 2: Line ratio diagnostic diagram of ji20a, showing $P1$ and $P3$ for MaNGA galaxy central emission line ratios. The left hand panel shows the measured values and errors, with the distribution overlaid. The right hand panel shows one randomly selected galaxy at each location in the plane. The $r$-band image is shown as a grey scale, with the H$\alpha$ map contributing to the red channel and the [Oiii] $\lambda5007$ map contributing to the green channel. In both panels, the blue box shows our Seyfert galaxy classification.
• Figure 3: Same line ratio diagnostic diagram as in Figure \ref{['fig:p1p3']}, showing the same galaxies, illustrating our determination of $L$([Oiii]) detection thresholds for non-Seyfert galaxies. The blue region shows the line ratios for which we define Seyfert galaxies. The small grey points show each galaxy with all five lines detected at at least 2$\sigma$ significance (error bars are omitted for clarity). The large black points are a random set of non-Seyfert galaxies chosen to span a range of metallicities and ionization spectral shapes (i.e. $P3$ and $P1$ values). The lines emanating from each large black point show how the line ratios change with an addition of an AGN component of increasing luminosity. Note that some lines exist without a large black point; those lines are examples for which the host galaxy's lines are not all detected at 2$\sigma$, and the lines start at AGN luminosities which would be necessary to make the lines detectable. The lines are colored red for AGN luminosities high enough to be identified as AGN by our methods, and black otherwise.
• Figure 4: AGN-related dust-corrected H$\beta$ luminosities and detection thresholds for MaNGA galaxies, as a function of various host galaxy properties: stellar mass (upper left), stellar velocity dispersion $\sigma_v$ (upper right), SFR (lower left), and sSFR (lower right). The red points are detections, and the black upper limits are detection thresholds.
• Figure 5: Similar to Figure \ref{['fig:hb_lums_and_thresholds']}, for Eddington ratios and Eddington ratio detection thresholds. We use the greene06a$M_{\rm BH}$--$\sigma$ relation and the netzer19a bolometric corrections to calculate the Eddington ratios; the qualitative trends do not depend on these choices.