The Highest Redshift Relativistic Jets

It is now well established that X-ray emission is a common feature of kiloparsecscale radio jets (see Harris & Krawczynski 2006, for a recent review and the associated website, http://hea-www.harvard.edu/XJET/). The spectral energy distributions (SEDs) of the powerful quasar jets are predominantly characterized as “optically faint”, with the spectra rising between the optical and X-ray bands. Current models for this ‘excess’ X-ray emission posit either inverse Compton (IC) scattering off CMB photons in a (still) relativistic kpc-scale jet or an additional high-energy synchrotron emitting component. In the simplest scenario, such models have diverging predictions at high redshift. Specifically, we expect a strong redshift dependence in the monochromatic flux ratio, fX/fr ∝ UCMB ∝ (1 + z) for IC/CMB, whereas in synchrotron models, we expect no such dependence, fX/fr ∝ (1 + z). As a first order test of this simple idea, our approach is to study the highest-redshift relativistic jets. Such jets probe the physics of the earliest (first ∼1 Gyr of the Universe in the quasars studied) actively accreting supermassive black hole systems and