The emission line spectrum of the UV deficient quasar Ton 34: evidence of shock excitation?

Context. Emission lines in quasars are believed to originate from a photoionized plasma. There are, however, some emission features which appear to be collisionally excited, such as the Fe ii multiplet bands. Shortward of Lyα, there also are a few permitted lines of species from low to intermediate ionization. Aims. Ton 34 (zq = 1.928) exhibits the steepest far-UV continuum decline known (Fν ∝ ν ) shortward of 1050 Å. This object also emits unusually strong low to intermediate excitation permitted lines shortward of the Lyman limit. Methods. Using archive spectra of Ton 34 from HST, IUE and Palomar, we measure the fluxes of all the lines present in the spectra and compare their relative intensities with those observed in composite quasar spectra. Results. Our analysis reveals unusual strengths with respect to Lyα of the following low to intermediate excitation permitted lines: O ii+O iii (835 Å), N iii+O iii (686–703 Å) and N iii+N iv (765 Å). We compare the observed line spectrum with both photoionization and shock models. Conclusions. Photoionization cannot reproduce the strengths of these far-UV lines. Shocks with Vs ≃ 100 kms −1 turn out to be extremely efficient emitters of these lines and are favored as excitation mechanism.