Hamilton's Object -- a clumpy galaxy straddling the gravitational caustic of a galaxy cluster : Constraints on dark matter clumping

Understand

We report the discovery of a 'folded' gravitationally lensed image, 'Hamilton's Object', found in a HST image of the field near the AGN SDSS J223010.47-081017.8 ($z=0.62$).

• The lensed images are sourced by a galaxy at a spectroscopic redshift of 0.8200$\pm0.0005$ and form a fold configuration on a caustic caused by a foreground galaxy cluster at a photometric redshift of 0.526$\pm0.018$ seen in the corresponding Pan-STARRS PS1 image and marginally detected as a faint ROSAT All-Sky Survey X-ray source.
• The lensed images exhibit properties similar to those of other folds where the source galaxy falls very close to or straddles the caustic of a galaxy cluster.
• The folded images are stretched in a direction roughly orthogonal to the critical curve, but the configuration is that of a tangential cusp.