2008A&A...477..413B

Emission lines in quasars are believed to originate from a photoionized plasma. There are, however, some emission features that appear to be collisionally excited, such as the FeII multiplet bands. Shortward of Lyα, there are also a few permitted lines of species from low to intermediate ionization. Ton 34 (z_q=1.928) exhibits the steepest far-UV continuum decline known (F_ν∝ν^–5.3) shortward of 1050Å. This object also emits unusually strong low to intermediate-excitation permitted lines shortward of the Lyman limit.Using archive spectra of Ton 34 from HST, IUE, and Palomar, we measured the fluxes of all the lines present in the spectra and compared their relative intensities with those observed in composite quasar spectra. Our analysis reveals unusual strengths with respect to Lyα of the following low to intermediate-excitation permitted lines: OII+OIII (835Å), NIII+OIII (686-703Å), and NIII+NIV (765Å). We compared the observed line spectrum with both photoionization and shock models.Photoionization cannot reproduce the strengths of these far-UV lines. Shocks with V_s≃100km/s turn out to be extremely efficient emitters of these lines and are favored as an excitation mechanism.