SHARP - V. Modelling gravitationally-lensed radio arcs imaged with global VLBI observations

TL;DR

This study uses high-resolution VLBI observations to model the mass distribution of a gravitational lensing galaxy, revealing a steeper-than-isothermal density profile and highlighting the need for more complex models to explain observed image residuals.

Contribution

First detailed VLBI imaging of MG J0751+2716 at mas resolution constrains lens mass profile, showing deviations from smooth models and suggesting additional structure.

Findings

Mass-density slope steeper than isothermal, with gamma > 2.

Image residuals (~3 mas) exceed measurement errors, indicating complex mass structure.

Inner mass profile differs when including galaxy group environment.

Abstract

We present milliarcsecond (mas) angular resolution observations of the gravitationally lensed radio source MG J0751+2716 (at z=3.2) obtained with global Very Long Baseline Interferometry (VLBI) at 1.65 GHz. The background object is highly resolved in the tangential and radial directions, showing evidence of both compact and extended structure across several gravitational arcs that are 200 to 600~mas in size. By identifying compact sub-components in the multiple images, we constrain the mass distribution of the foreground z=0.35 gravitational lens using analytic models for the main deflector [power-law elliptical mass model; $\rho(r) \propto r^{-\gamma}$, where $\gamma=2$ corresponds to isothermal] and for the members of the galaxy group. Moreover, our mass models with and without the group find an inner mass-density slope steeper than isothermal for the main lensing galaxy, with $\gamma_1 = 2.08 \pm 0.02$ and $\gamma_2 = 2.16 \pm 0.02$ at the 4.2$\sigma$ level and 6.8$\sigma$ level, respectively, at the Einstein radius ($b_1 = 0.4025 \pm 0.0008$ and $b_2 = 0.307 \pm 0.002$ arcsec, respectively). We find randomly distributed image position residuals of about 3 mas, which are much larger that the measurement errors ($40$ $\mu$as on average). This suggests that at the mas level, the assumption of a smooth mass distribution fails, requiring additional structure in the model. However, given the environment of the lensing galaxy, it is not clear whether this extra mass is in the form of sub-haloes within the lens or along the line of sight, or from a more complex halo for the galaxy group.