Studying the Variation of the Fine-Structure Constant Using Emission-Line Multiplets

As an extension of the method by Bahcall et al. (2004) to investigate thetime dependence of the fine structure constant, we describe an approach basedon new observations of forbidden line multiplets from different ionic species.We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [OIII], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies inthe low-z range 0.035 < z < 0.281. Each source and each multiplet isindependently analyzed to ascertain possible errors. Averaging over our sample,we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.However, our sample is limited in size and our fitting technique simplistic aswe primarily intend to illustrate the scope and strengths of emission linestudies of the time variation of the fine structure constant. The approach canbe further extended and generalized to a "many-multiplet emission line method"analogous in principle to the corresponding method using absorption lines. Withthat aim, we note that the theoretical limits on emission line ratios ofselected ions are precisely known, and provide well constrained selectioncriteria. We also discuss several other forbidden and allowed lines that mayconstitute the basis for a more rigorous study using high-resolutioninstruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A