ULTRAVIOLET EMISSION LINES OF Si II IN QUASARS-INVESTIGATING THE "Si II DISASTER"

作者：Laha Sibasish^*; Keenan Francis P; Ferland Gary J; Ram**ottom Catherine A; Aggarwal Kanti M

来源：Astrophysical Journal, 2016, 825(1): 28.

DOI：10.3847/0004-637X/825/1/28

• atomic processes
• quasars: emission lines

The observed line intensity ratios of the Si II lambda 1263 and lambda 1307 multiplets to that of Si II lambda 1814 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 (similar to 500 km s(-1)) may solve the Si II disaster through continuum pumping and other effects. The CLOUDY models indicate strong blending of the Si II lambda 1307 multiplet with emission lines of O I, although the predicted degree of blending is incompatible with the observed lambda 1263/lambda 1307 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.

• 出版日期2016-7-1