Testing AGN outflow and accretion models with CIV and HeII emission line demographics in z=2 quasars

TL;DR

This study analyzes ultraviolet emission lines in 190,000 z=2 quasars to understand how black hole mass and accretion rate influence AGN outflows and accretion models, revealing dependencies on Eddington ratio and black hole spin.

Contribution

It provides the first large-scale empirical assessment of CIV and HeII emission line properties in relation to black hole parameters, testing and constraining AGN models with extensive SDSS data.

Findings

Large CIV blueshifts occur only at high mass and accretion rates.

HeII EW and X-ray strength trends align with low-spin QSOSED models.

A shift in emission properties at L/L_Edd<0.2 suggests changes in broad line region structure.

Abstract

Using 190,000 spectra from the seventeenth data release of the Sloan Digital Sky Survey, we investigate the ultraviolet emission line properties in z=2 quasars. Specifically, we quantify how the shape of CIV 1549A and the equivalent width (EW) of HeII 1640A depend on the black hole mass and Eddington ratio inferred from MgII 2800A. Above L/L_Edd>0.2, there is a strong mass dependence in both CIV blueshift and HeII EW. Large CIV blueshifts are observed only in regions with both high mass and high accretion rate. Including X-ray measurements for a subsample of 5,000 objects, we interpret our observations in the context of AGN accretion and outflow mechanisms. The observed trends in HeII and 2 keV strength are broadly consistent with theoretical QSOSED models of AGN spectral energy distributions (SEDs) for low spin black holes, where the ionizing SED depends on the accretion disc temperature and the strength of the soft excess. High spin models are not consistent with observations, suggesting SDSS quasars at z=2 may in general have low spins. We find a dramatic switch in behaviour at L/L_Edd<0.2: the ultraviolet emission properties show much weaker trends, and no longer agree with QSOSED predictions, hinting at changes in the structure of the broad line region. Overall the observed emission line trends are generally consistent with predictions for radiation line driving where quasar outflows are governed by the SED, which itself results from the accretion flow and hence depends on both the SMBH mass and accretion rate.