Insights into physical conditions and magnetic fields from high redshift quasars

Abstract

We use archival WISE and Spitzer photometry to derive optical line fluxes for a sample of distant quasars at z ~6. We find evidence for exceptionally high equivalent width [OIII] emission (rest-frame EW ~400 {\AA}) similar to that inferred for star-forming galaxies at similar redshifts. The median Halpha and Hbeta equivalent widths are derived to be ~400{\AA} and 100~{\AA}, respectively, and are consistent with values seen among quasars in the local Universe, and at z ~2. After accounting for the contribution of photoionization in the broad line regions of quasars, we suggest that the OIII emission corresponds to strong, narrow line emission likely arising from feedback due to massive star-formation in the quasar host. The high [OIII]/Hbeta line ratios can uniquely be interpreted with radiative shock models, and translate to magnetic field strengths of ~8 microGauss with shock velocities of ~400km/s. Our measurement implies that strong, coherent magnetic fields were present in the interstellar medium at a time when the universe was < 1 billion years old. Comparing our estimated magnetic field strengths with models for the evolution of galaxy-scale fields, favors high seed field strengths exceeding 0.1 microGauss, the first observational constraint on such fields. This high value favors scenarios where seed magnetic fields were produced by turbulence in the early stages of galaxy formation. Forthcoming mid-infrared spectroscopy with the James Webb Space Telescope will help constrain the physical conditions in quasar hosts further.Comment: 11 pages, 6 figures, submitted to MNRA