The observed line intensity ratios of the Si II λ1263 and λ1307 multiplets to that of Si II λ1814 in the broad-line region (BLR) of quasars are both an order of magnitude larger than the theoretical values. This was first pointed out by Baldwin et al., 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 BLR calculations using several different atomic data sets 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 we also consider blending with other species. However, we find that none of the options investigated resolve the Si II disaster, with the potential exception of microturbulent velocity broadening and line blending. We find that a larger microturbulent velocity (~ 500km s-1) may solve the Si II disaster through continuum pumping and other effects. The CLOUDY models indicate strong blending of the Si II λ1307 multiplet with emission lines of O I, although the predicted degree of blending is incompatible with the observed λ1263/λ1307 intensity ratios. Clearly, more work is required on the quasar modeling of not just the Si II lines but also nearby transitions (in particular those of O I) to fully investigate whether blending may be responsible for the Si II disaster.
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S.L., C.A.R., and F.P.K. are grateful to STFC for financial support via grant ST/L000709/1. G.J.F. acknowledges financial support from the Leverhulme Trust via Visiting Professorship grant VP1-2012-025, and also support by the NSF (1108928, 1109061, and 1412155), NASA (10-ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153), and STSciI (HST-AR-13245, GO-12560, HST-GO-12309, GO-13310.002-A, HST-AR-13914, and HST-AR-14286.001).
Laha, Sibasish; Keenan, Francis P.; Ferland, Gary J.; Ramsbottom, Catherine A.; and Aggarwal, Kanti M., "Ultraviolet Emission Lines of Si II in Quasars—Investigating the "Si II Disaster"" (2016). Physics and Astronomy Faculty Publications. 419.