A research team led by Dr. ZHANG Ming from the Galaxy Cosmology Group at Xinjiang Astronomical Observatory (XAO), Chinese Academy of Sciences (CAS), detected and verified the existence of a secondarily-lensed Einstein arc caused by its companion galaxy in the CLASS lens B1152+199, by analyzing the Hubble Space Telescope (HST) optical-near-infrared tricolor images and the 8.4 GHz global very-long-baseline-interferometry (VLBI) data.

The results were recently published in the Monthly Notices of the Royal Astronomical Society.

Gravitational lensing is a powerful means to detect the distribution of dark matter, which is merely through gravitational effects and independent of dynamical methods. Currenlty, a dark cloud in the astrophysics sky is that the N-body numerical simulation based on the Lambda Cold Dark Matter (ΛCDM) cosmology has been successful in large-scale structures, but it predicts too many substructures small-scale structures at galaxy-level, while there is no observational evidence. Therefore, in the hunt for substructures in galaxy-scale lenses, there is a tendency to attribute all the observed anomalies that cannot be perfectly explained by the macromodel to the substructures.

B1152+199 is a double-image gravitational lens system, and the radio jet components in the primary image have been resolved in several lower-frequency VLBI observations, but in the secondary image, they have not been fully resolved. Thus, there was no enough radial observational constraints on the mass distribution of the lens galaxy for previous anaylses.

The jet profile in the secondary image seemed to have a curvature, which was also used as evidence for the existence of local substructural disturbances. Likewise, in the previous analysis of the HST I-band image, the companion galaxy was detected, but its role in the lensing macromodel was not emphasized.

At the same time, the vaguely-visible diffuse emission near the lensing galaxy was speculated to be part of the galaxy's spiral arm, but there is no more observational evidence for the existence of the galaxy's spiral arm ever since.

Through the analysis of the V, I, and H band images captured by the WFPC2 and NICMOS cameras on board the HST, the researchers found that the residual images at each band consistently show the Einstein arc, after deducting the photometric models of the foreground galaxies and background quasars, while the convergence of the arc is at the position of the companion galaxy. This lensing feature and the companion galaxy, which were overlooked by previous studies, will revolutionize our understanding of this lensing system.

In order to obtain the most stringent constraints from point source positions and fluxes for this lens system, the researchers successfully resolved the corresponding jet components in the secondary image using the latest 8.4 GHz global-VLBI observation. Together with the jet components of the primary image, the two-image three-component observational constraints on lens systems are presented.

By optimizing the macromodel of the host galaxy plus the companion galaxy through the VLBI observation data, the researchers found that the power-law slope of the host galaxy is steeper than that of the isothermal ellipsoid. Due to the presence of companion galaxies, the non-collimated alignment of jet components in the secondary image can be explained by the model without adding substructures near the secondary image.

At the same time, the researchers applied the optimized parametric lens macromodel to the HST three-color images, after the photometric models of the foreground lensed galaxy and the background quasar are optimized, the lens model is found to perfectly reproduce the observed Einstein arc, where the background source is the extended structure of the quasar host galaxy, thus the consistency of optical and radio observations on the lens model constraints is achieved.

This case study of B1152+119 showed that the detection of the substructure of lensed galaxies must be based on a full understanding of the lensing macromodel, rather than blindly adding the micromodel of substructures.