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We investigate the mass structure of a strong gravitational lens galaxy at $z=0.350$, taking advantage of the milli-arcsecond (mas) angular resolution of very long baseline interferometric (VLBI) observations. In the first analysis of its kind at this resolution, we jointly infer the lens model parameters and pixellated radio source surface brightness. We consider several lens models of increasing complexity, starting from an elliptical power-law density profile. We extend this model to include angular multipole structures, a separate stellar mass component, additional nearby field galaxies, and/or a generic external potential. We compare these models using their relative Bayesian log-evidence (Bayes factor). We find strong evidence for angular structure in the lens; our best model is comprised of a power-law profile plus multipole perturbations and external potential, with a Bayes factor of $+14984$ relative to the elliptical power-law model. It is noteworthy that the elliptical power-law mass distribution is a remarkably good fit on its own, with additional model complexity correcting the deflection angles only at the $\sim5$ mas level. We also consider the effects of added complexity in the lens model on time-delay cosmography and flux-ratio analyses. We find that an overly simplistic power-law ellipsoid lens model can bias the measurement of $H_0$ by $\sim3$ per cent and mimic flux ratio anomalies of $\sim8$ per cent. Our results demonstrate the power of high-resolution VLBI observations to provide strong constraints on the inner density profiles of lens galaxies., 24 pages, 15 figures, 3 tables. Accepted in MNRAS

The projected density profiles of gravitational lens galaxies are often assumed to be consistent with a simple elliptical power-law. Previous studies at low angular resolution have found this description to be satisfactory, but the limitations of this smooth model assumption have not yet been observationally tested. Here we report the analysis of a gravitational lens system observed at milli-arcsecond angular resolution with very long baseline interferometry at 1.7 GHz using a state-of-the-art Bayesian lens modelling technique for interferometric data. An elliptical power-law mass distribution is found to be a remarkably good description of the data, but there remain source-plane light components that are misaligned by several milli-arcseconds. The addition of angular multipole perturbations gives a significantly better fit (Bayes factor of +4751), while the best-focused source model is recovered using pixellated corrections to the lens potential. The projected density in our best model deviates from an elliptical power-law by 2.4 per cent (RMS) inside a region within 17 mas of the lensed images. The assumption of a perfect elliptical power-law would bias the measurement of H_0 by +0.6 per cent, and the observed flux ratios by up to 10.3 per cent, for this lens. Our results demonstrate that low-level density perturbations exist in this massive elliptical galaxy. Their presence can significantly affect studies of galaxy formation via flux-ratio anomalies, but do not strongly affect time-delay measurements of the Hubble constant.

We report the localization of the X-ray emission from two strongly lensed AGN, CLASS B0712+472 ($z=1.34$) and CLASS B1608+656 ($z=1.394$). We obtain milliarcsecond X-ray astrometry by developing a novel method that combines parametric lens modelling with a Bayesian analysis. We spatially locate the X-ray sources in CLASS B0712+472 and CLASS B1608+656 within 11 mas and 9 mas from the radio source, respectively. For CLASS B0712+472, we find that the X-ray emission is co-spatial with the radio and optical emission. While, in CLASS B1608+656, the X-ray emission is co-spatial with radio, but displaced with respect to the optical emission at 1$\sigma$ level, which positions this source as an offset AGN candidate. This high astrometric precision improves on the limitations of existing X-ray instruments by two orders of magnitude. The demonstrated method opens a path to search for offset and binary AGN at $z>1$, and to directly test supermassive black hole formation models in a redshift range that has been mostly underconstrained to date., Comment: 16 pages, 7 figures. Accepted for publication in ApJ

We report on the direct observation of an extended X-ray jet in the $z$=6.1 radio-loud Active Galactic Nucleus PSO J030947.49+271757.31 from a deep Chandra X-ray observation (128 ksec). This detection represents the most distant kpc off-nuclear emission resolved in the X-rays to date. The angular distance of the emission is $\sim$4" (corresponding to $\sim$20 kpc at $z$=6.1), along the same direction of the jet observed at parsec scales in previous VLBA high-resolution radio observations. Moreover, the 0.5-7.0 keV isophotes coincide with the extended radio emission as imaged by the VLA Sky Survey at 3 GHz. The rest-frame 2-10 keV luminosity of the extended component is L$_{2-10keV}$=5.9$\times$10$^{44}$ erg s$^{-1}$, about 8% of the core: this makes it one of the most luminous jets resolved in the X-rays so far. Through Spectral Energy Distribution modelling we find that this emission can be explained by the Inverse Compton interaction with the photons of the Cosmic Microwave Background assuming that the jet's physical parameters are similar to those in the local Universe. At the same time, we find that the radiation produced by a putative population of high-energetic electrons through the synchrotron process observed at low redshift is quenched at high redshift, hence becoming negligible., Comment: Accepted for publication on A&A