Active galactic nuclei at z ~ 1.5: I. Spectral energy distribution and accretion discs

TL;DR

This study investigates the spectral energy distributions of active galactic nuclei at z~1.5, using Bayesian models to estimate black hole parameters and assess the role of accretion disks in powering AGN.

Contribution

First comprehensive analysis of AGN SEDs at z~1.5 with Bayesian fitting of thin accretion disk models, revealing black hole spins and accretion rates across a diverse sample.

Findings

Thin accretion disks can fit most observed SEDs after reddening correction.

Black hole spins range from -1 to 0.998, with more massive BHs having higher spins.

Results support thin accretion disks as primary AGN power sources.

Abstract

The physics of active super massive black holes (BHs) is governed by their mass (M_BH), spin (a*) and accretion rate ($\dot{M}$). This work is the first in a series of papers with the aim of testing how these parameters determine the observable attributes of active galactic nuclei (AGN). We have selected a sample in a narrow redshift range, centered on z~1.55, that covers a wide range in M_BH and $\dot{M}$, and are observing them with X-shooter, covering rest wavelengths ~1200-9800 \AA. The current work covers 30 such objects and focuses on the origin of the AGN spectral energy distribution (SED). After estimating M_BH and $\dot{M}$ based on each observed SED, we use thin AD models and a Bayesian analysis to fit the observed SEDs in our sample. We are able to fit 22/30 of the SEDs. Out of the remaining 8 SEDs, 3 can be fit by the thin AD model by correcting the observed SED for reddening within the host galaxy and 4 can be fit by adding a disc wind to the model. In four of these 8 sources, Milky Way-type extinction, with the strong 2175\AA\ feature, provides the best reddening correction. The distribution in spin parameter covers the entire range, from -1 to 0.998, and the most massive BHs have spin parameters greater than 0.7. This is consistent with the "spin-up" model of BH evolution. Altogether, these results indicate that thin ADs are indeed the main power houses of AGN, and earlier claims to the contrary are likely affected by variability and a limited observed wavelength range.