Ultraviolet emission lines of Si II in quasars --- investigating the "Si II disaster"

TL;DR

This paper investigates the longstanding 'Si II disaster' in quasar broad line regions, testing atomic data, physical effects, and blending, and suggests microturbulence as a potential solution, highlighting the need for improved modeling.

Contribution

It provides a comprehensive analysis of the Si II line intensity discrepancy using updated atomic data and explores physical mechanisms, proposing microturbulence as a possible resolution.

Findings

Microturbulent velocity of ~500 km/s may resolve the discrepancy.

Blending of Si II 1307 Å with O I lines is significant but not fully explanatory.

Standard models cannot account for the observed line ratios.

Abstract

The observed line intensity ratios of the Si II 1263 and 1307 \AA\ multiplets to that of Si II 1814\,\AA\ in the broad line region of quasars are both an order of magnitude larger than the theoretical values. This was first pointed out by Baldwin et al. (1996), who termed it the "Si II disaster", and it has remained unresolved. We investigate the problem in the light of newly-published atomic data for Si II. Specifically, we perform broad line region calculations using several different atomic datasets within the CLOUDY modeling code under optically thick quasar cloud conditions. In addition, we test for selective pumping by the source photons or intrinsic galactic reddening as possible causes for the discrepancy, and also consider blending with other species. However, we find that none of the options investigated resolves the Si II disaster, with the potential exception of microturbulent velocity broadening and line blending. We find that a larger microturbulent velocity ($\sim 500 \rm \, kms^{-1}$) may solve the Si II disaster through continuum pumping and other effects. The CLOUDY models indicate strong blending of the Si II 1307 \AA\ multiplet with emission lines of O I, although the predicted degree of blending is incompatible with the observed 1263/1307 intensity ratios. Clearly, more work is required on the quasar modelling of not just the Si II lines but also nearby transitions (in particular those of O I) to fully investigate if blending may be responsible for the Si II disaster.