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 that appear to be collisionally excited, such as the Fe II multiplet bands. Shortward of Ly alpha, there are also a few permitted lines of species from low to intermediate ionization. Aims. Ton 34 (z(q) = 1.928) exhibits the steepest far-UV continuum decline known (F-nu proportional to nu(-5.3)) shortward of 1050 angstrom. 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, lUE, 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. Results. Our analysis reveals unusual strengths with respect to Ly alpha of the following low to intermediate-excitation permitted lines: O II+O III (835 angstrom), N III+O III (686 - 703 angstrom), and N III+N IV (765 angstrom). We compared the observed line spectrum with both photoionization and shock models. Conclusions. Photoionization cannot reproduce the strengths of these far-UV lines. Shocks with V-s=100 km s(-1) turn out to be extremely efficient emitters of these lines and are favored as an excitation mechanism.