ALMA Imprint of Intergalactic Dark Structures in the Gravitational Lens SDP.81
Kaiki Taro Inoue, Takeo Minezaki, Satoki Matsushita, and Masashi Chiba

TL;DR
This study uses ALMA data of gravitational lens SDP.81 to detect perturbations caused by intergalactic dark structures, revealing their influence on flux ratios and astrometric shifts in lensed images.
Contribution
It demonstrates that intergalactic dark structures can explain observed anomalies in flux ratios and astrometric shifts in gravitational lensing.
Findings
A 10-20% perturbation in flux ratios at 2-3 sigma significance.
Detection of 0.01 arcsec astrometric shifts in source images.
Intergalactic dark structures with ~10^8 solar mass surface density can account for anomalies.
Abstract
We present an analysis of the ALMA long baseline science verification data of the gravitational lens system SDP.81. We fit the positions of the brightest clumps at redshift z=3.042 and a possible AGN component of the lensing galaxy at redshift z=0.2999 in the band 7 continuum image using a canonical lens model, a singular isothermal ellipsoid plus an external shear. Then, we measure the ratio of fluxes in some apertures at the source plane where the lensed images are inversely mapped. We find that the aperture flux ratios of band 7 continuum image are perturbed by 10-20 percent with a significance at 2 ~ 3 sigma level. Moreover, we measure the astrometric shifts of multiply lensed images near the caustic using the CO(8-7) line. Using a lens model best-fitted to the band 7 continuum image, we reconstruct the source image of the CO(8-7) line by taking linear combination of inverted…
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