Emission and structure of jets from the most distant Quasars

Illustration 1: Broadband spectral energy distributions (SEDs) fitted to a sample of 10 quasars at high redshifts. In each panel: the left hump originates from synchrotron emission produced by the movement of electrically charged, relativistic particles in a magnetic field. The right hump results from inverse Compton scattering of cosmic microwave background photons off the same electron-positron pairs that emit synchrotron radiation. The data points traversed by the left hump represent archival radio fluxes from the VLA interferometer, while those traversed by the right hump correspond to archival X-ray flux measurements from the Chandra Space Telescope. The downward-pointing arrows visible in some panels indicate upper limits on emission, applicable to observations that yielded non-detections. In particular, the downward-pointing arrows between the humps seen in several plots (e.g., for 1745+624 or GB1508+5714) represent archival upper limits on emission from the Hubble Space Telescope. (The Authors)

Among this 10% of active galaxies, only another 10% (meaning 1% of all galaxies) exhibit relativistic jets: extended outflows of plasma and magnetic fields whose emission is observed across almost the entire available electromagnetic spectrum, from long radio waves to energetic gamma rays. In a recent paper in The Astrophysical Journal, we investigated the properties of a sample of the 10 farthest known quasars exhibiting extended jets using data from the VLA interferometer (radio) as well as the Hubble (optical) and Chandra (X-ray) space telescopes.

The core of this work lies in applying a model of large-scale jets (extending over kiloparsecs—tens of thousands of light-years) in magnetohydrostatic equilibrium with a toroidal magnetic field, representing a state where magnetic forces balance plasma pressure gradients perpendicular to the jet axis. A toroidal magnetic field implies that the magnetic field lines form loops in the plane perpendicular to the outflow direction. Furthermore, we assume a cylindrical jet geometry to a good approximation and describe the hot jet fluid using the ultrarelativistic equation of state. Additionally, we consider emission from a relativistic electron-positron plasma. This allows us to parameterize the radial dependencies of the magnetic field induction and the fluid velocity in a very general manner, and thereby reconstruct the internal pressure of the jet in a fully consistent way. Having established a structural model dependent on several parameters, we pose the question: which properties of the jet can be recovered from broadband observations?

Equipped with a magnetohydrodynamic model representing the simplest, physically motivated description of a jet, we compiled a sample of 10 quasars with angularly resolved large-scale jets observed using the VLA radio interferometer in New Mexico and the Chandra X-ray Observatory. The latter, with a spatial resolution below one arcsecond, is particularly suitable for studying such objects. In several cases, we also utilized optical flux upper limits from the well-known Hubble Space Telescope. To the best of our knowledge, this constitutes an essentially complete sample of resolved jets above a redshift of 2.5, with the most distant object studied residing at a redshift of 6.1 -- meaning that the light from this source traveled to us for 12.9 billion years. At these vast distances, corresponding to such an early epoch in the Universe's history, the cosmic microwave background (CMB) was significantly hotter than today and served as the seed photon field for inverse Compton scattering: the prime suspect behind the X-ray emission (although for closer objects, alternative mechanisms have recently been sought!). Conversely, the radio emission originates from synchrotron radiation.

Using advanced statistical methods, Bayesian inference and Monte Carlo algorithms, we rigorously and quantitatively compared four variants of the aforementioned model across different magnetic field configurations, as well as two scenarios where the energy density of the emitting plasma constitutes a constant fraction of either the magnetic field energy density or the gas pressure. It turned out that the topology of the magnetic field is weakly constrained by broadband observations; however, normalizing the emitting particles to the gas pressure is statistically strongly favored. This is a key finding, as such a normalization is typically an assumption in modeling these objects, whereas in principle, the emitting plasma could reside separately from the bulk matter.