New Constraints on the Quasar Broad Emission Line Region

TL;DR

This paper introduces a novel near-infrared hydrogen emission line technique to determine the physical conditions of quasar broad line regions, emphasizing the importance of individual spectral fitting over composite spectra.

Contribution

It presents a new method using near-infrared hydrogen lines and cloud models to analyze quasar emission regions, highlighting the need for individual spectral analysis.

Findings

Low ionising fluxes are required to match observed line ratios.

High gas densities (>10^{12} cm^{-3}) are necessary for accurate modeling.

Composite spectra are inadequate for photoionisation modeling.

Abstract

We demonstrate a new technique for determining the physical conditions of the broad line emitting gas in quasars, using near-infrared hydrogen emission lines. Unlike higher ionisation species, hydrogen is an efficient line emitter for a very wide range of photoionisation conditions, and the observed line ratios depend strongly on the density and photoionisation state of the gas present. A locally optimally emitting cloud model of the broad emission line region was compared to measured emission lines of four nearby ($z\approx0.2$) quasars that have optical and NIR spectra of sufficient signal-to-noise to measure their Paschen lines. The model provides a good fit to three of the objects, and a fair fit to the fourth object, a ULIRG. We find that low incident ionising fluxes ($\phih<10^{18}$\cmsqs), and high gas densities ($\nh>10^{12}$\cmcu) are required to reproduce the observed hydrogen emission line ratios. This analysis demonstrates that the use of composite spectra in photoionisation modelling is inappropriate; models must be fitted to the individual spectra of quasars.