The preferentially magnified active nucleus in IRAS F10214+4724 - I. Lens model and spatially resolved radio emission

TL;DR

This study presents a new gravitational lens model and high-resolution radio and optical data for IRAS F10214+4724, revealing that the active nucleus is preferentially magnified, affecting interpretations of its star formation and AGN activity.

Contribution

It introduces a Bayesian MCMC lens modeling approach with multi-wavelength data, showing the active nucleus is more magnified than star-forming regions, challenging previous magnification estimates.

Findings

The active nucleus is preferentially magnified.

Magnification factors are lower than previously assumed.

Flux ratios are unreliable for estimating magnification.

Abstract

This is the first paper in a series that present a multi-wavelength analysis of the archetype Ultra-Luminous InfraRed Galaxy (ULIRG) IRAS FSC10214+4724, a gravitationally lensed, starburst/AGN at z=2.3. Here we present a new lens model and spatially-resolved radio data, as well as a deep HST F160W map. The lens modelling employs a Bayesian Markov Chain Monte Carlo algorithm with extended-source, forward ray-tracing. Using these high resolution HST, MERLIN and VLA maps, the algorithm allows us to constrain the level of distortion to the continuum spectral energy distribution resulting from emission components with differing magnification factors, due to their size and proximity to the caustic. Our lens model finds the narrow line region (NLR), and by proxy the active nucleus, is preferentially magnified. This supports previous claims that preferential magnification could mask the expected polycyclic aromatic hydrocarbon spectral features in the Spitzer mid-infrared spectrum which roughly trace the star-forming regions. Furthermore, we show the arc-to-counter-image flux ratio is not a good estimate of the magnification in this system, despite its common use in the IRAS FSC10214+4724 literature. Our lens modelling suggests magnifications of \mu ~ 15-20+-2 for the HST F814W, MERLIN 1.7 GHz and VLA 8 GHz maps, significantly lower than the canonical values of \mu = 50-100 often used for this system. Systematic errors such as the dark matter density slope and co-location of stellar and dark matter centroids dominate the uncertainties in the lens model at the 40 percent level.