The Excitation Mechanism of Strong FE II Emission from Quasars at High Redshift

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Session 24 -- AGNs/QSOs - High Luminosity
Display presentation, Tuesday, June 13, 1995, 9:20am - 6:30pm

[24.05] The Excitation Mechanism of Strong FE II Emission from Quasars at High Redshift

Jianfang Peng and Anil K. Pradhan (Department of Astronomy, The Ohio State University)

Photon pumping by UV continua and strong Hydrogen lines, coupled with collisional excitation, is investigated as the cause of anomalously strong Fe II emission by several strong high redshift quasars observed, with special focus on the relative stength of UV and optical lines. Line intensities of the multiplet 42 ($z^6P^0_J-a^6D_{J'}$) in high vs.low redshift quasars are observed to be differ considerably; these intensity ratios are studied theoretically. In our investigation we included a large number of Fe II fine structure levels, up to about 1 Rydberg where many excited levels pumped by the UV lines and continua are located. The Lyman series photons play a crucial role in pumping these levels, followed by cascades to the upper levels of multiplet 42. In our model more than 21,000 dipole transitions and 1,000 forbidden transitions are included, using a new set of recently computed radiative transition probabilities and collision strengths for FeII (Nahar 1994 and Zhang and Pradhan 1994, respectively). The effects of new atomic data on the model are pointed out. Observational tests of the possible excitation mechanisms are discussed and the implied abundance of Fe II is estimated. While the present model is applicable to optically thin sources, work is in progress on a complete Non-LTE model of Fe II with radiative transfer. \\ $\star$ Work supported by NSF grant PHY-9421898.

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